Monday, May 9, 2016

AST 115H Final Study Guide

Megan Purgahn
AST- 115 Final Study Guide
Sun and Stars
I.                    Theories of the Sun (what causes it to shine?)
a.       It is not on fire
b.      It is not contracting
c.       It is powered by nuclear energy
                                                               i.      E=mc^2
                                                             ii.      Nuclear potential energy(core)/luminosity=10 billion years
1.       Luminosity = total energy emitted per second
                                                            iii.      The sun shines because it is a giant nuclear reactor at its core
II.                  Gravitational equilibrium
a.       The sun is  a huge ball of hydrogen and helium gas held together by gravity
b.      The outward push/pressure from very hot expanding gases is balanced by the strong inward pull of gravity
c.       Pressure à
d.      Gravity ß
e.      Like a stack of pillows, the weight of the upper layers compresses lower layers
                                                               i.      This is what happens to the layers of hydrogen gas
                                                             ii.      Compressed gas = heat
f.        Role of gravity:
                                                               i.      Gravitational contraction:
1.       The contraction provided the energy that heated the core as the sun was forming
2.       Contraction stops when fusion begins
3.       When contraction stops the proto-sun becomes a star
                                                             ii.      Gravitational equilibrium:
1.       Energy provided by nuclear fusion of hydrogen to helium in the sun’s hot core maintains the very high temperature and creates a pressure balance against gravity
2.       Acts as a thermostat that keeps the fusion rate steady
g.       Our sun is about 5 billion years old and is expected to live for 10 billion years so it is “middle aged”
h.      The sun is plasma-charged particles (not solid or liquid anywhere)
III.                The Sun’s layers:
a.       The solar wind
                                                               i.      A flow of charged particles, protons and electrons, from the opaque “surface” of the sun
                                                             ii.      The wind causes the aurora when it hits the earth
                                                            iii.      We are in the solar wind right now
b.      Corona
                                                               i.      Outermost gaseous layer of the solar “atmosphere”
                                                             ii.      Very thin
                                                            iii.      Fast moving particles
                                                           iv.      Temperature: 1 million K
                                                             v.      Closer, outermost layer, very thin, we look right through it
c.       Chromosphere
                                                               i.      Middle layer of the solar atmosphere
                                                             ii.      Can be seen as pinkish in eclipse due to hydrogen emission
                                                            iii.      10^5 K
                                                           iv.      Where we see flares, appears pink when we see it with our eye
d.      Photosphere
                                                               i.      Visible “surface” of sun
                                                             ii.      Not really a surface in the normal sense
                                                            iii.      This is just as far into the sun as we can see
                                                           iv.      The gas becomes dense and opaque below this level
                                                             v.      6,000 K
                                                           vi.      Photo = light
                                                          vii.      So this is where light escapes the surface, but not a surface you can stand on
e.      Convection zone
                                                               i.      Hot, dense, very turbulent gas
                                                             ii.      Energy transported upward by uprising hot gas
                                                            iii.      Breaks through photosphere to cause a granulation appearance that reminds one of boiling water
                                                           iv.      Convection = movement of gas
f.        Radiation zone
                                                               i.      A much hotter and denser region
                                                             ii.      Energy transported upward by photons, not by mass motion of gas
g.       Core
                                                               i.      Extremely dense region, but still very gaseous
                                                             ii.      Energy generated by nuclear fusion
                                                            iii.      Temperature: 15 million K
                                                           iv.      Core = still plasma not solid
IV.                Nuclear Fusion
a.       2 kinds of nuclear reactions:
                                                               i.      Fission
1.       Big nucleus that splits into smaller pieces
2.       Ex. Nuclear power plants
                                                             ii.      Fusion
1.       Small nuclei stick together to make a bigger one
2.       Ex. Stars, sun, H-bomb
b.      Nuclear Fusion
                                                               i.      2 powerful forces
1.       The very high temperature enables nuclear fusion to happen in the core
2.       At low speeds electromagnetic repulsion prevents the collision of nuclei
3.       At high speeds nuclei come close enough for the strong force to bind them together
4.       Electrostatic repulsion between positively charged protons can be overcome if the nuclei get close enough for the strong force (a nuclear binding force) to take over
5.       We only have protons which usually repel but the sun is so hot that there is not time to repulse so they stick together
                                                             ii.      Basic process
1.       The sun releases energy by fusing four hydrogen nuclei (protons) into one helium nucleus (2 protons, 2 neutrons)
2.       Proton-proton-chain
a.       Step 1: 2 protons fuse to make a deuterium nucleus (1 proton, 1 nucleus), this step occurs twice in the overall reaction
                                                                                                                                       i.      (2 protons collide, 1 becomes a neutron)
b.      Step 2: the deuterium nucleus and a proton fuse to make a nucleus of helium-3 (2 protons, 1 neutron), this step also occurs twice

                                                                                                                                       i.      (the nucleus collide with another proton, gamma ray released)a.       Step 3: 2 helium-3 nuclei fuse to form helium-4 (2 protons, 2 neutrons) releasing 2 excess protons in the process
                                                                                                                                       i.      (isotope collides with proton and becomes 2p + 2n)
b.      Proton-proton chain is how hydrogen fuses into helium in the sun
2.       Total reaction:
a.       4 protons in
b.      Helium-4 nucleus out
c.       2 gamma rays out
d.      2 positrons out
e.      2 neutrinos out
f.        Total mass = 0.7% less
                                                                                                                                       i.      This mass goes to energy because E=mc^2
                                                                                                                                     ii.      (aka the extra  ass goes into the energy of gamma rays/light)
b.      How does the energy get out?
                                                               i.      Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons
                                                             ii.      The photons bounce off electrons and make a “random walk”
                                                            iii.      It can take over 100,000 years for a photon to reach the surface
                                                           iv.      At the top of the radiation zone the temperature has dropped to 2 million K
                                                             v.      Here photons are absorbed, not scattered
                                                           vi.      This creates rising plumes of hot gas (convection) that take energy to the surface
                                                          vii.      Granulation- bright blobs on the photosphere are where hot gas is reaching the surface
c.       How do we learn about the inside of the sun if we can’t directly observe it?
                                                               i.      Making mathematical models
1.       To predict the radius, temperature, luminosity and age of the sun from its mass and chemical composition
                                                             ii.      Observing “sun quakes”
1.       Core =  nuclear explosion that lasts 10 billion years
2.       This causes the sun to shake/ ring (sun quake)
                                                            iii.      Observing solar neutrinos
                                                           iv.      Ringing pattern
1.       The sun is like a bell
2.       Patterns of vibration on the surface tell us about what the sun is like inside
3.       Doppler shifts in the spectrum from different parts of the surface can be measured
4.       Results agree very well with mathematical models of the solar interior
5.       Helio-seismology uses “quaking” of the sun
d.      Neutrinos
                                                               i.      Neutrinos are very tiny nuclear particles with no electric charge created during fusion
                                                             ii.      They fly directly through the sun and escape
                                                            iii.      They have no charge so they do not react with matter
                                                           iv.      It is like a neutron but has no mass
                                                             v.      Observations of these solar neutrinos can tell us what is happening in the core
                                                           vi.      Neutrinos are remarkable because they are exceedingly small and have almost no interaction with anything
                                                          vii.      How to detect neutrinos:
1.       There are 3 types of neutrinos but the sun only has 1 type that we have seen
2.       We bury giant tanks of water in mines with cameras that detect light to detect neutrinos
II.                  Luminosity
a.       Everything we know about the sun is deduced from the light we receive
b.      Luminosity= amount of power(energy per second) a star radiates into space
c.       The units of power: 1 joule per second =  1 watt
d.      Apparent brightness= amount of starlight that reaches earth
                                                               i.      The energy per second per square meter
                                                             ii.      (how bright something appears from Earth)
                                                            iii.      So apparent brightness of a star depends on luminosity and distance
e.      Not all light goes straight to the earth, the light is more spread out the further you get
f.        How are luminosity and brightness related?
                                                               i.      Luminosity passing through each sphere of a diagram is the same
                                                             ii.      ^^^ that is conservation of energy
                                                            iii.      Area of a sphere = 4pi (radius)^2
                                                           iv.      So divide luminosity by area to get the brightness
                                                             v.      Brightness = luminosity/
                                                           vi.      Luminosity =  * brightness
g.       Parallax
                                                               i.      How we measure distance
                                                             ii.      This is how Greeks learned how to measure distance of stars without telescopes
                                                            iii.      In January the sky looks different than it does in July
                                                           iv.      We measure it in parsecs
1.       D (parsecs) = 1/p (arcsec)
a.       1 parsec is the distance that gives a parallax angle of 1 second of arc (= 3.26 lightyears = 206265 AU)
b.      Parsec = parallax arcsec (measures distance)
III.                How hot are stars?
a.       Laws of thermal radiation
                                                               i.      Hotter objects emit more light at all wavelengths
                                                             ii.      Hotter objects emit light at shorter wavelengths
                                                            iii.      If we measure the spectrum we can get temperature
b.      Spectra of stars fall into certain patterns
c.       Hottest stars = 50,000 K
d.      Coolest stars = 3,500 K
e.      Dark lines in a star’s spectrum(color) correspond to a spectral type that reveals its surface temperature
                                                               i.      (spectral type tells us the star’s surface temperature)
f.        Order of stellar spectra from hottest to coolest:
                                                               i.      O (oh)
                                                             ii.      B (be)
                                                            iii.      A (a)
                                                           iv.      F (fine)
                                                             v.      G (girl)
                                                           vi.      K (kiss)
                                                          vii.      M (me)
                                                        viii.      **our sun is a G star
IV.                2 Star Systems:
a.       Aka: gravitationally bound stars
b.      Aka: 2 stars orbiting each other
c.       Aka: binary stars
d.      Binary stars yield stellar masses
e.      About half of all stars are in binary systems
f.        Types of binary star systems:    
                                                               i.      Visual binary1.       We can directly observe the orbital motions of these stars
2.       We can determine the orbital period and projected size of the orbit (on the sky) directly
3.       We don’t know the incline of the plane of the orbit (so the orbital size is the “projected” size)
                                                             ii.      Eclipsing binary
1.       We see light from star A and star B then star B will pass behind or in front of star A causing us to see only some or no light from one of the stars
2.       Inclination of the orbit is essentially zero (edge-on), causing eclipses
3.       Periodic eclipses implies orbital period
4.       Duration gives radius
                                                            iii.      Spectroscopic binary
1.       It looks like one object because of large distance
2.       Looks like a eclipsing binary but turned on its side so we see both at all times
3.       Star B goes back and forth from approaching us (blue shifted) to receding (redshifted)
4.       We find the orbit (period and velocity) by measuring Doppler shifts
b.      Mass of stars
                                                               i.      We measure mass using gravity
                                                             ii.      Newton’s form of Kepler’s 3rd Law
                                                            iii.      Direct mass measurements are possible only for stars in binary star systems
                                                           iv.      Equation:
1.      
2.       P = period
3.       A = average separation
4.       G = gravity
5.       M1 = mass 1
6.       M2 = mass 2
7.       **if you only have p and a then you can get M1+M2, but if you have the separate orbits then you can M1 and M2 separately
                                                             v.      The overall range of stellar masses runs from 0.08 times the mass of the sun, to about 150 times the mass of the Sun
1.       0.08 Msun is approximately 80 Mjupiter
2.       Objects less than 0.08 Msun = brown dwarf
II.                  Classifying Stars
a.       We classify stars according to their spectral types and luminosity class
b.      Mass and lifetime:
                                                               i.      Sun’s life expectancy: 10 billion years
1.       The sun will run out of hydrogen in its core eventually
                                                             ii.      Bigger stars use up hydrogen faster
                                                            iii.      Life expectancy of 10 Msun star:
1.       10 times as much fuel, but uses it 10^4 times as fast
2.       It will live 10 million years (10 billion * 10/10^4)
3.       Big star = hummer
                                                           iv.      Life expectancy of 0.1 Msun star:
1.       0.1 times as much fuel, uses it 0.01 times as fast
2.       It will live 100 billion years (10 billion * 0.1/0.01)
3.       Small star = prius
c.       Normal star
                                                               i.      High mass
1.       High luminosity
2.       Short-lived
3.       Large radius
4.       Blue
5.       Hotter
                                                             ii.      Low mass
1.       Low luminosity (dimmer)
2.       Long-lived
3.       Small radius
4.       Red
5.       Cooler
6.       **the sun is low mass
d.      Hertzsprung-Russel diagram
                                                               i.      Basically tell us everything in one picture
                                                             ii.      Plots the luminosity and temperature function of the many different kinds of stars
1.       Luminosity class tells us how much light it puts out (surface area)
                                                            iii.      Normal hydrogen-burning stars reside on the main sequence
                                                           iv.      Mass increases from lower right to upper left
1.       We use gravitational equilibrium for each mass
                                                             v.      Relative to the size of the sun, stars in the upper right are “giants,” those in the lower left are “dwarfs”
III.                Star clusters
a.       2 types:
                                                               i.      Open clusters
1.       Contain up to several thousand stars and are found in the disk of the Milky Way galaxy
                                                             ii.      Globular cluster
1.       Contain hundreds of thousands of stars, all closely packed together
2.       They are found mainly in the “halo” around our galaxy
b.      When star clusters form the cluster is full of all types of stars
c.       Measuring the age of a star cluster
                                                               i.      Because all of a cluster’s stars were born at the same time, we can measure a cluster’s age by finding the main-sequence turnoff
                                                             ii.      The clusters age is equal to the hydrogen-burning lifetime of the hottest, most luminous stars that remain on the main sequence (the stars that have not evolved into red giants)
IV.                **a stars mass is the most important property because its mass determines virtually everything that happens to it throughout its life

Star Birth
I.                    The life cycle of stars
a.       Stars like to recycle
b.      Star forming clouds
                                                               i.      Stars form in dark clouds of “dusty” gas in interstellar space
1.       Gas = hydrogen (70%) and helium (28%)
2.       Dust = tiny smoke-like grains of solid matter (about 1% of the gas by mass); often carbon and/or silicon
a.       The dust is important because you can see through the gas but not the dust
3.       The gas between the stars is called the interstellar medium
4.       A pink glow is from hot hydrogen gas heated by nearby stars
5.       Dark zones are due to obscuration by dust
c.       Molecular clouds
                                                               i.      Very empty but more crowded than regular space
                                                             ii.      Most of matter in star-forming clouds is in the form of molecules, not atoms
                                                            iii.      The dominant molecule is H2 with CO being a smaller fraction of the total, but a more easily detected molecule (CO is easy to see in radio eaves)
                                                           iv.      These molecular clouds have a temperature of 10-30 K and a density of about 300 molecules per cubic cm
d.      Interstellar dust
                                                               i.      Star forming region
                                                             ii.      Dark cloud of duct blocking the view of stars behind it
                                                            iii.      Tiny solid particles of interstellar dust block our view of stars on the other side of the cloud
                                                           iv.      Particles are <1 micron in size and made of elements like C, O, Si, Fe
                                                             v.      Stars viewed through the edges of the cloud look redder because dust blocks (shorter wavelength) blue light more effectively than (longer-wavelength) red light
e.      Infrared light
                                                               i.      Infrared light penetrates dark blobs in the sky
                                                             ii.      Glowing dust grains
1.       Dust grains that absorb visible light heat up and emit infrared light of even longer wavelength
2.       Long wavelength infrared light is brightest from regions where many stars are currently forming
3.       The longer infrared wavelengths are best detected from space
4.       Infrared colors are false in images, they represent intensity and not wavelength
                                                            iii.      First infrared camera was made in the mid-80s
                                                           iv.      False color
1.       Infrared wavelengths are not detected by the human eye, but our cameras can measure the brightness of an object and we can assign a color to the brightness
2.       Usually brighter parts of an infrared image are hotter
3.       Infrared brightness can be related to temperature (look at a cold blooded vs. warm blooded animal in infrared)
4.       Astronomy uses infrared because you can see through things or see gases/things that you can’t see in visible light
                                                             v.      IRAC flight unit = astronomer’s version of a digital camera
II.                  Why do stars form?
a.       Competition between gravity and thermal pressure
b.      Gravity can create stars only if it can overcome the force of thermal pressure in a cloud
c.       Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons
d.      Hot air clouds of interstellar gas wants to expand
e.      So it has to be cold enough for stars to form
f.        Mass of a star-forming cloud
                                                               i.      You need something that is 3000x the mass of the sun in order to have gravity be strong enough
                                                             ii.      A typical molecular cloud must contain at least a few hundred solar masses for gravity to overcome pressure
                                                            iii.      Emitted photons from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons (electromagnetic energy) that escape the cloud
                                                           iv.      CO is a very helpful “cooling” molecule
g.       Fragmentation of a cloud
                                                               i.      Gravity within a contracting gas cloud becomes stronger as the gas becomes denser                                                               i.      Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star
                                                             ii.      The random motions of different sections of the cloud cause it to become lumpy
                                                            iii.      Each lump of the cloud in which gravity can overcome pressure can go on to become a star
                                                           iv.      A large cloud can make a whole cluster of stars
                                                             v.      Much easier to make lots of small stars than one big star
                                                           vi.      (started off with the cloud spinning)
                                                          vii.      (this cloud contracts and breaks up into pieces that go on to form stars)
b.      Isolated star formation
                                                               i.      Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense
                                                             ii.      Such a cloud may make only a single star
c.       The first stars
                                                               i.      Elements like carbon and oxygen had not yet been made when the first stars formed
                                                             ii.      The big bang only produced hydrogen and helium
                                                            iii.      Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today’s molecular clouds
                                                           iv.      The first stars must therefore have been more massive then most of today’s stars, for gravity to overcome pressure
                                                             v.      Simulations of early star formation suggest the first molecular clouds never cooled below 100 K, making stars 100 Msun
II.                  Stages of star birth
a.       Trapping of thermal energy (slows the contraction of a star-forming cloud)
                                                               i.      As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape
                                                             ii.      Thermal energy then begins to build up inside, increasing the internal pressure
                                                            iii.      Contraction slows down, and the center of the cloud fragment becomes a protostar
b.      Conservation of angular momentum
                                                               i.      The rotation speed of the cloud from which a star forms increases as the cloud contracts
                                                             ii.      Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms
                                                            iii.      (rotation speeds up as cloud shrinks and flattens)
                                                           iv.      Conservation of angular momentum leads to disks around protostars
c.       Flattening
                                                               i.      Collisions between particles in the cloud cause it to flatten into a disk
                                                             ii.      Rotation also causes jets of matter to shoot out along the rotation axis
d.      From protostar to main sequence
                                                               i.      Protostar looks “star like” after the surrounding gas is blown away (by the stellar wind), but its thermal energy comes from gravitational contraction, not nuclear fusion
                                                             ii.      Gravitational contraction must continue until the core becomes hot enough for nuclear fusion
                                                            iii.      Collapses under gravity
                                                           iv.      Core starts getting hotter- allows it to shine
                                                             v.      Hot enough for nuclear fusion…
                                                           vi.      Contraction stops when the energy released by core fusion balances energy radiated from the surface- the star is now a main-sequence (adult) star
e.      Birth stages:
                                                               i.      Life track illustrates stars’ surface temperature and luminosity at different moments in time
                                                             ii.      Assembly of a protostar- Luminosity and temperature grow as matter collects into a protostar
                                                            iii.      Convective contraction- Surface temperature remains near 3,000 K while convection is main energy transport mechanism
                                                           iv.      Radiative contraction- luminosity remains nearly constant during late stages of contraction, while radiation is transporting energy through star
1.       Contraction = when a protostar is shrinking in on itself to become a normal star
                                                             v.      Self-sustaining fusion- core temperature continues to rise until star arrives on the main sequence
f.        Life tracks for different masses
                                                               i.      Models show that sun required about 30 million years to go from protostar to main sequence
                                                             ii.      High mass stars form faster
                                                            iii.      Lower mass stars form more slowly
g.       Fusion and Contraction
                                                               i.      Fusion will not begin in a contracting cloud is some sort of force stops contraction before the core temperature rises above 10^7 K
                                                             ii.      Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation
                                                            iii.      Electron degeneracy pressure can also stop contraction
                                                           iv.      Thermal pressure
1.       Main form of pressure in stars
2.       Depends on heat content
                                                             v.      Degeneracy pressure
1.       Particles can’t be in the same state in same place
2.       Doesn’t depend on heat content/temperature
3.       Extra electrons have higher energy
III.                Brown Dwarfs
a.       In between a star and Jupiter characteristics
b.      Degeneracy pressure halts the contraction of objects with <0.08 Msun before core temperature become hot enough for fusion
c.       Star-like objects not massive enough to start fusion are brown dwarfs
d.      A brown dwarf emits mostly infrared light, from residual heat left over from contraction. It fades gradually over time as it loses thermal energy
e.      Nuclear fusion never begins so they never shine
f.        Left over heat gives off some light but they eventually cool off with time
g.       Brown dwarfs are defined by mass
                                                               i.      Not massive enough for stable hydrogen fusion
1.       M<0.072 Msun
                                                             ii.      Massive enough to fuse deuterium
1.       M>0.013 Msun
                                                            iii.      Start part of the nuclear fusion process but doesn’t finish it
h.      Low temperatures (<2500 K) à complex spectra dominated by molecules
i.         Brown dwarf sets the lowest limits for stars
j.        They are considered the missing link between stars and planets
k.       They are also potentially very numerous
IV.                Mass continued…
a.       Radiation pressure
                                                               i.      Photons exert a slight amount of pressure when they strike matter
                                                             ii.      Very massive stars are so luminous that the collective pressure of photons drives their matter into space
b.      Limits
                                                               i.      Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart
                                                             ii.      Observations have not found stars more massive than 150 Msun
                                                            iii.      Stars more massive than 150 Msun would blow apart
                                                           iv.      Stars less massive than 0.08 Msun can’t sustain fusion
c.       Demographics of stars
                                                               i.      Our sun is not the most common size star
1.       They are usually 20-50% less than the size of our sun
                                                             ii.      Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars
V.                  Debris
a.       Material leftover from star formation creates a disk full of debris, planets are made from this debris left over from the star
b.      Material sticks together and grows
c.       Gravity plays a large role so some are rejected
d.      Takes less than 100 million years
Exoplanets and Astrobiology
I.                    History
a.       First discovery was in 1988, a planet found orbiting the binary star system Gamma Cephei was at the very limits of our technology, and took years to confirm
b.      First confirmed discovery was in 1992, a planet orbiting a pulsar
c.       First confirmed planet orbiting a main sequence star 51 Pegasus came in 1995
d.      There were no known exoplanets before 1988
e.      By 2015 there were hundreds of exoplanets
f.        Exoplanet = planets that orbit other stars
g.       Hot Jupiter = orbit their stars in only a few days, easier to detect
                                                               i.      Only 1% of stars have hot Jupiters but they are the easiest to detect
h.      Cold Jupiter = planets between 2/10 and 20x the mass of Jupiter
i.         Hot Neptune = mass of the earth up to 30x the size of the earth
j.        **earth is not alone or rare but these types of planets are harder to detect because of tis close distance to the sun and its small size
II.                  Methods for detecting (how to discover planets orbiting other stars)
a.       Direct Imaging (first success)
                                                               i.      First successful direct imaging of a extrasolar planet was the companion of a brown dwarf
1.       It was 4-5x the mass of Jupiter, and it orbits beyond the orbit of Neptune
                                                             ii.      Einstein’s theory of general relativity
                                                            iii.      Works through the detection of light
b.      Transits (most successful method)
                                                               i.      Best way
                                                             ii.      Can get radius and composition of planets
                                                            iii.      Indirect method that measures shadow of planet instead of actual planet
                                                           iv.      The transit of an earth in front of the sun produces a 100 ppm transit depth
                                                             v.      Jupiter has a transit depth of 1%
                                                           vi.      Transits detect the change in brightness of a star to tell if a planet is passing in front of it
                                                          vii.      First transit planet was found  using a telescope in a parking lot
1.       There used to be the problem of star’s twinkle due to earth’s atmosphere that makes stars brightness change without planets
2.       But now computers and other technology overcome that problem
                                                        viii.      During the transit, spectroscopy can be used to determine the planet’s atmosphere, and with precise observations, whether it has moons                                                               i.      Transits in front of and behind the parent star can be used to determine the planet’s radiation and its temperature
                                                             ii.      Kepler mission
1.       Uses transit method to scan over 150,000 stars
2.       Has detected over 1,000 planets as small as Mercury
3.       It’s the biggest camera sent into space
                                                            iii.      Subtract the brightness before and the brightness during transit to determine spectrum
                                                           iv.      On a data graph, jagged lines = sunspots, and from peak to peak = length of orbit
b.      Circum-binary planet
                                                               i.      Planet that orbits to stars
                                                             ii.      You would see 2 different sunsets from this planet
                                                            iii.      Kepler 16 AB b = 1st one discovered
c.       Kelper 452 b
                                                               i.      The 452nd confirmed planet from Kepler mission
                                                             ii.      Sits in the “goldilocks zone” so it is at the right distance that it could have liquid water and sustain life
                                                            iii.      Aside: validating all of the planets takes a long time because there could be false positives
                                                           iv.      50% chance of being a terrestrial planet or 50% chance of being gas
d.      General info
                                                               i.      M dwarf
1.       Smallest star
2.       Most common type of star
                                                             ii.      Our universe makes earth-size planets very easily
                                                            iii.      And also makes planets with similar composition to earth
                                                           iv.      Small planets have earth-like densities
                                                             v.      There are over 1 billion galaxies
                                                           vi.      Once you know the mass and radius of a planet you can know the density
1.       And then you can know what each planet is made of
2.       Kepler doesn’t tell us the masses of planets
e.      Radial velocities (aka wobble method)
                                                               i.      Use Doppler effect to measure mass because planets have stellar wobble
                                                             ii.      Radial velocity = speed/direction towards or away from us
                                                            iii.      Indirect method, don’t actually see the planet, we see the wobble of the star so it is a method used on the star not the planet
                                                           iv.      Doppler spectroscopy
1.       A periodic variation in the star’s velocity tells us that it has an unseen companion, possibly a planet
2.       The velocity change (K) gives the star’s orbital speed
3.       Combining K & P gives us the planet mass
4.       How long it takes to complete orbit + how much the velocity changes = mass
5.       Since planets orbit the center of mass, not the actual star, it causes the star to wobble
6.       Bigger mass = causes star to wobble more
f.        RV Method
                                                               i.      If a telescope was pointed at a person then the telescope could detect the speed that the person is moving
                                                             ii.      Accuracy of technology now = running speeds
g.       Gravitational microlensing
                                                               i.      Good for planets far away from its star
                                                             ii.      Found only about a dozen stars
                                                            iii.      Planet adds its own effect to gravitational microlensing of background stars
                                                           iv.      Requires the use of robotic telescopes and observations of a large number of stars
                                                             v.      Uses one star to act as a lens for another star
                                                           vi.      Similar to eclipse
h.      Pulsar timing
                                                               i.      Pulsars are leftover remnants of nuclear things
                                                             ii.      Pulsars are rapidly rotating neutron stars that emit radio waves at very regular intervals
                                                            iii.      A pulsar that is being tugged by orbiting planets will have anomalies in the timing of radio waves
                                                           iv.      Pulsar timing is very precise, and planets as small as 0.1 times Earth’s mass can be found
                                                             v.      First extrasolar planet found using pulsar timing
                                                           vi.      Pulsars aren’t rare, and remnants are supernovas
i.         The future
                                                               i.      TESS
1.       “NASA’s next planet hunter”
2.       Launches in 2018
3.       Will survey almost the entire sky and target 500,000 main sequence low mass stars, mostly M dwarfs
4.       Will identify 3,000 nearby transiting planet candidates that will need follow-up and characterization
5.       To follow of the Kepler telescope
6.       Going to find the nearest transitive planets
                                                             ii.      We are directly imaging exoplanets and measuring their atmospheres, and finding smaller and smaller planets
II.                  Habilitability
a.       For life to evolve, the parent star has to have a stable length of time in the main sequence
b.      Short lifetimes rule out O and B stars
c.       Spectral type A and F have 1-2 billion year lifetimes, and have more extensive habitable zones, but emit much more UV radiation
d.      Only 3% of stars are A or F
e.      1-2 billion years may not be enough for technological space-faring civilization to evolve
f.        The only form of life we know in a G-type star system is our own
g.       G spectral type stars make up  only 7% of all stars
h.      90% of all stars are spectral type K or M
i.         An M class star can have a main sequence of over 500 billion years but has an incredibly tiny habitable zone
III.                Drake Equation
a.       An equation of estimating number of habitable planets possibly out there
b.      N = R * Fs * Fp * Ne * Fl * Fi * Fe * L
c.       R – average rate of star formation
d.      Fs – fraction of good stars that have planetary systems
e.      Ne – number of planets around these stars within an “ecoshell”
f.        Fl – fraction of those planets where life develops
g.       Fi – fraction of living species that develop intelligence
h.      Fe – fraction of intelligent species with communication technology
i.         L – lifetime of the “communicative phase”
j.        **really advance civilizations are rare
k.       **another civilization would require a lot of infrared heat projected
l.         **earth- our energy has been exponentially increasing and is expected to keep growing
Death of High and Low Mass Stars
I.                    Life track after main sequence (main sequence/low mass/sun-like stars)
a.       A star remains on the main sequence (stable luminosity and temperature) as long as it can fuse hydrogen into helium in its core
b.      Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over
c.       When a star can no longer fuse hydrogen to helium in its core then the core will shrink and heat up
                                                               i.      The heat source (pressure support) disappears and gravity takes over; the helium core shrinks and heats up
d.      Broken thermostat
                                                               i.      Eventually, core is helium
                                                             ii.      As the He heats up, H begins fusing to He in a shell around the core
                                                            iii.      This new heat source pushes out the upper layers and the star swells and cools- becoming a Red Giant
                                                           iv.      Red giant layers: C (core) à He + C à H + He à H
                                                             v.      Luminosity increases because the core’s natural “thermostat” is broken- the increasing fusion rate in the shell does not stop the core from contracting
                                                           vi.      The contracting core heats up
                                                          vii.      Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion
                                                        viii.      Larger charge leads to greater repulsion
                                                           ix.      Fusion of 2 helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make Carbon
                                                             x.      When core temperatures rise enough for helium fusion to begin in a low-mass star, helium fusion rises sharply
1.       Degeneracy pressure is the main form of pressure in the inert helium core
2.       Degeneracy pressure is independent of temperature and thus the core does not automatically expand to reduce the rate of fusion
e.      Helium flash
                                                               i.      Thermostat is broken in low-mass red giant because degeneracy pressure supports core
                                                             ii.      Core temperature rises rapidly when helium fusion begins
                                                            iii.      Helium fusion rate skyrockets until the temperature is so high that ordinary thermal pressure takes over and expands core again
                                                           iv.      Helium burning stars neither shrink now grow because core thermostat is temporarily fixed
1.       Helium fusion into carbon in core
2.       Hydrogen burning shell
3.       Hydrogen burning shell à H fusing to He, Helium burning core à He fusing to C
f.        Life track after Helium flash
                                                               i.      A red giant should shrink and become less luminous after helium fusion begins in the core
                                                             ii.      Stellar winds increase during the red giant phase and stars can lose a lot of mass
                                                            iii.      Helium-burning stars are found in horizontal branch on the H-R diagram
                                                           iv.      Once the star’s core runs out of helium then the helium fuses in a shell around the core
g.       Double shell burning
                                                               i.      After core helium fusion stops, He fuses into carbon in a shell around the inert carbon core, and H fuses to He in  a shell around the helium layer
                                                             ii.      This double-shell burning stage never reaches equilibrium
                                                            iii.      Fusion rate periodically spikes upward in a series of thermal pulses
                                                           iv.      With each spike, convection dredges carbon up from core and transports in to the surface
h.      Planetary Nebulae                                                               i.      Double shell burning ends with a pulse that ejects the outer layers of hydrogen and helium into spaces as so-called Planetary Nebulae
                                                             ii.      The exposed core left behind is a White Dwarf
                                                            iii.      Basically it is the remnant of the exposed core of the star
b.      End of fusion
                                                               i.      Fusion progresses no further than carbon in low-mass star (like the sun) because the carbon core temperature never grows hot enough for fusion of heavier elements (like oxygen)
                                                             ii.      Degeneracy pressure supports the white dwarf (the carbon core remnant of the original star) against gravity
                                                            iii.      Stars die because they run out of hydrogen in their core
1.       Core = hydrogen + helium fusion
2.       Gravity pulls in
3.       Force of fusion pushes out
4.       = lots of pressure
5.       No more hydrogen protons = no more fusion
6.       No more pressure = core of star starts to contract
7.       Core shrinks and outer layers expand because of gravitational potential energy
8.       Collapse of core = star blows up 400x its size and becomes a red giant
9.       Core stops shrinking but  keeps getting hotter
10.   Star settles down
11.   Ran out of hydrogen, so
c.       Earths fate
                                                               i.      Sun’s luminosity will rise to 1,000 times its current level – too hot for life on Earth
                                                             ii.      Sun’s radius will grow to near the current radius of Earth’s orbit
II.                  Life of a high mass star
a.       High mass main sequence stars fuse to H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts
b.      Greater core temperature enables H nuclei to overcome greater repulsion
c.       Life stages of high mass stars are similar to those of low-mass stars
                                                               i.      Hydrogen fusion (main sequence)
                                                             ii.      Hydrogen shell burning (supergiant)
                                                            iii.      Helium core fusion (supergiant)
d.      CNO cycle can change C into N and O
                                                               i.      The process is called Helium Capture
1.       High core temperatures allow helium to fuse with heavier elements
2.       Helium capture builds C into O, Ne, Mg…
e.      Advanced nuclear burning
                                                               i.      Core temperature in stars >8 Msun allow fusion of elements as heavy as iron
f.        Multiple shell burning
                                                               i.      Advanced nuclear burning proceeds in a series of nested shells
                                                             ii.      Stops at iron
                                                            iii.      Iron is a dead end for fusion because nuclear reactions involving iron do not release energy
1.       Fe has the lowest mass per nuclear particle
g.       How does a high-mass star die?
                                                               i.      Iron builds up in core until degeneracy pressure can no longer resist gravity
                                                             ii.      Core then suddenly collapses, creating a Supernova Explosion
h.      Supernova Explosion:
                                                               i.      Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos
                                                             ii.      Neutrons collapse to the center, forming a neutron star
                                                            iii.      Energy and neutrons released in supernova explosion enable elements heavier than iron to form including Au and U
i.         Supernova remnant
                                                               i.      Energy released by collapse of core drives outer layers into space
III.                Recap:
a.       High Mass stars:
                                                               i.      >8 Msun
                                                             ii.      High core temperature
                                                            iii.      Rapid fusion
                                                           iv.      More luminous
                                                             v.      Short-lived
                                                           vi.      Makes iron
                                                          vii.      Ends in supernova explosion
                                                        viii.      Death of high mass
1.       H fuses to He in core (main sequence)
2.       H fuses to He in shell around He core (red super giant)
3.       He fuses to C in core while H fuses to He in shell (helium core burning)
4.       Many elements fuse in shells (multiple shell burning)
5.       Supernova explosion leaves neutron star behind
b.      Low mass stars:
                                                               i.      <2 Msun
                                                             ii.      Cooler core
                                                            iii.      Slower fusion
                                                           iv.      Smaller luminosity
                                                             v.      Longer lifetime
                                                           vi.      Never fuse carbon nuclei
                                                          vii.      End as white dwarfs
                                                        viii.      Death of low mass
1.       H fuses to he (main sequence)
2.       H fuses to He in shell around He core (red giant)
3.       He fuses to C in core while H fuses to He in shell (helium core burning)
4.       H and he both fuse in shells (double shell burning)
5.       Planetary nebulae leaves white dwarf (exposed core behind)
c.       Intermediate stars
                                                               i.      Make elements heavier than carbon but end as white dwarfs
IV.                Crazy Phenomena: 2 stars in a binary system are close enough that matter can flow from one to another, star that is not a supergiant was originally more massive, as it reached the end of its life and started to grow, it began to transfer mass to its companion (mass exchange), now the companion star is more massive
White Dwarfs, Neutron Stars and Black Holes
I.                    White dwarf
a.       Remaining cores of dead stars like the sun (low mass stars)
b.      Size of the earth, made of carbon
c.       The outer layers of the original star have been blown into planetary nebula during the red giant phase
d.      The carbon core is very dense
e.      Electron degeneracy pressure balances gravity
f.        With no internal nuclear energy source, white dwarfs cool off and grow dimmer with time until they become black
g.       Typically diameter of earth, and mass of sun so very high density
h.      Cools off very slowly
i.         No more fusion to be done
j.        Produces its own pressure
k.       White dwarfs shrink when you add mass to them because their gravity gets stronger
                                                               i.      Quantum mechanics says that electrons in the same place cannot be in the same state
                                                             ii.      Adding mass to a white dwarf increases its gravity, forcing electrons into smaller space
                                                            iii.      In order to avoid being in the same state some of the electrons need to move faster
                                                           iv.      There is a limit to how much you can shrink a white dwarf
1.       Einstein’s theory of relativity says that nothing can move faster than light
2.       When electron speeds in a White Dwarf approach the speed of light, electron degeneracy pressure can no longer support the weight
a.       ^^When a white dwarf’s mass reaches 1.4 Msun (aka Chandrasekhar Limit)
l.         Binary star system:
                                                               i.      with one star more massive than the other, the more massive star is the first to become a red giant, eject its outer layers and end up as a white dwarf
                                                             ii.      Eventually, the second star of lower initial mass also swells up to become a red giant
                                                            iii.      If the stars are close enough, the second star swells up
                                                           iv.      An imaginary tear-drop shaped shell of equal gravity
                                                             v.      And dumps matter onto the White dwarf
                                                           vi.      Friction in the disk makes it very hot, causing it to glow
                                                          vii.      Friction removes angular momentum from inner regions of disk, allowing matter to sink onto the white dwarf
                                                        viii.      Hot hydrogen gas from the companion star accretes onto the white dwarf, building up in a shell on its surface
                                                           ix.      The gravity of the white dwarf is huge
                                                             x.      When the base of the shell gets hot enough, hydrogen fusion suddenly begins, leading to an explosive brightening of the white dwarf called Nova
1.       Nova
a.       The nova star system temporarily appears much brighter
b.      The phenomenon is regularly observed
c.       The explosion drives the accreted matter out into space
d.      The white dwarf settles down again and the build-up process starts over
                                                           xi.      When a white dwarf in a binary system happened to accrete enough matter to reach the 1.4 Msun Chandrasekhar limit, then it will explode
1.       This time the explosion is enormous so it is a supernova explosion
m.    Supernova
                                                               i.      2 types of supernova:
1.       Massive star supernova
a.       Iron core of massive exceeds white dwarf mass limit and collapses into a neutron star, causing a huge explosion of outer layers of the star
2.       White dwarf supernova
a.       Carbon fusion suddenly begins throughout the compact white dwarf in a binary system when accretion takes its mass above the limit
b.      There is no remnant, the white dwarf is destroyed
                                                             ii.      Nova vs. supernova
1.       Supernova are much much much more luminous than nova (about 10,000 times)
2.       Nova: H to He fusion of the accreted hydrogen layer on surface, white dwarf left intact
3.       Supernova: complete explosion of white dwarf, nothing left behind                                                               i.      Supernova type
1.       Both types of supernova can have peak luminosity about 10 billion times the sun
2.       Light curves differ- the way light fades with time
a.       Luminosity versus time
b.      One way to tell supernova types apart is with a light curve showing how luminosity changes with time
3.       Spectra differ- exploding white dwarfs don’t have hydrogen absorption lines because they are made of either carbon or helium
II.                  Neutron Stars
a.        A neutron star is the ball of neutrons left behind by a massive-star supernova
b.      Degeneracy pressure of neutrons supports a neutron star against gravity
c.       In the ultra-dense core of a massive star…
                                                               i.      The crushing force of gravity is enormous
                                                             ii.      Electron degeneracy pressure goes away because electrons are forced to combine with protons, making neutrons and neutrinos
                                                            iii.      Neutrons collapse to the center, forming a very compact neutron star
d.      A neutron star is about the same size as a small city (Springfield) but contains more mass than the sun
                                                               i.      A teaspoon of neutron material would weigh billions of tons on Earth
e.      Discovery of neutron stars
                                                               i.      Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky
                                                             ii.      The objects was called a pulsar
f.        Pulsar
                                                               i.      A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis
                                                             ii.      The radiation beams sweep through space, like lighthouse beams, as the neutron star rotates
1.       We miss lots of spinning neutron stars because the beam is not pointed in our direction
                                                            iii.      The beam of intense radiation looks like a pulse as it sweeps across the earth
                                                           iv.      The electromagnetic radiation is generated by electrons spiraling in a strong magnetic field
                                                             v.      Spin rate of pulsars – 1000 cycles per second
                                                           vi.      Surface rotation velocity- 60,000 km/s (20% speed of light)
                                                          vii.      (this is an enormous velocity for rotation, anything else would be torn to pieces)
                                                        viii.      Pulsars spin fast because the core’s rotation speeds up as it collapses from normal size to a neutron star: conservation of momentum
                                                           ix.      The change in radius is huge from original stellar core size down to the tiny neutron star size, and therefore the spin-up in enormous
g.       Binary System
                                                               i.      Matter falling toward a neutron star forms an accretion disk, just as in a white dwarf binary
                                                             ii.      Accreting matter adds angular momentum to neutron star, increasing spin rate
                                                            iii.      Metter accreting onto a neutron star can eventually become hot enough for helium fusion
                                                           iv.      The sudden onset of fusion produces bursts of x-rays
                                                             v.      Makes the system an x-ray binary
h.      The neutron star limit
                                                               i.      Quantum mechanics says that neutrons in the same place cannot be in the same state but…
                                                             ii.      Neutron degeneracy pressure can no longer support a neutron star against gravity if the neutron mass exceeds about 3 Msun
                                                            iii.      Beyond the neutron star limit, no known force can resist the crush of gravity
                                                           iv.      As far as we know, gravity crushes all the matter into a single point known as singularity
III.                Black Holes
a.       A black hole is an object whose gravity is so powerful that not even light can escape it
b.      The incredible density of matter required to do this warps space-time, creating what is effectively a bottomless pit from which there is no escape
c.       If you shrink an object, then the escape velocity will increase
d.      Calculating escape velocity
                                                               i.      Initial kinetic energy = final gravitational potential energy
                                                             ii.      m (escape velocity)^2 = (mG* (mass))/radius
                                                            iii.      once an object of mass M shrinks below R, light cannot escape it
e.      “Surface” of a black hole
                                                               i.      The “surface” of a black hole is the radius at which the escape velocity equals the speed of light
                                                             ii.      This spherical surface is known as the event horizon
                                                            iii.      There is no actual surface at this location, it is a boundary space
                                                           iv.      The radius of the event horizon is known as the Schwarzschild radius
                                                             v.      Example: light would not be able to escape earth’s surface if you could shrink earth’s mass into a volume of radius<1 cm
f.        Neutron stars vs. black holes
                                                               i.      Massive star supernova can make a black hole instead of a neutron star
                                                             ii.      The event horizon of a 3 Msun black hole is also about as big as a small city
g.       No escape
                                                               i.      Nothing can escape from within the event horizon because nothing can go faster than light
                                                             ii.      No escape means there is no more contact with something that falls in
                                                            iii.      Whatever falls in can increase the black hole’s mass, change the spin or charge of the black hole
                                                           iv.      The region of space-time within the event horizon is effectively cut-off from the rest of the universe
h.      Singularity
                                                               i.      Beyond the neutron star limit, no known force can resist the crush of gravity
                                                             ii.      We are not aware of some other kind of “degeneracy” pressure that can halt the collapse
                                                            iii.      Gravity crushes all matter into a single point known as singularity
                                                           iv.      Whether this happens or not may never be known as nothing can escape from within the event horizon
                                                             v.      If you add mass to the black hole then the radius of the event horizon will increase
i.         Visiting a black hole
                                                               i.      If the sun shrank to a black hole of equal mass, its gravity would be different only near the event horizon(3 km from the center)
                                                             ii.      There would be no change in the Earth’s orbit
                                                            iii.      Black holes don’t suck, they just pull due to gravity
1.       So it is possible to orbit
                                                           iv.      Space is being stretched near the hole
                                                             v.      Because the speed of light is constant, light waves take extra time to climb out of the deep hole in space-time
                                                           vi.      Photon frequency must decrease, leading to a gravitational redshift in the wavelength of the light
                                                          vii.      The stretching of space-time near the event horizon also means that time passes more slowly the closer you are to the black hole
                                                        viii.      Clocks tick slower
                                                           ix.      If you are approaching the black hole directly then you are not aware of this change
                                                             x.      Time seems to flow normally
                                                           xi.      You cross the event horizon and are lost forever
                                                          xii.      From afar, it seems to take you forever to reach the black hole
                                                        xiii.      Tidal forces (the change in gravity with distance) near the event horizon of a black hole would be lethal to humans
                                                        xiv.      The force is trillions of times that which raises the tides on the Earth
1.       Tidal forces would actually be much gentler near a supermassive black hole because its radius is much bigger
j.        Verification
                                                               i.      Need to measure mass (use orbital properties of companion, or measure velocity and distance of orbiting gas)
                                                             ii.      It’s a black hole if it is not a star and its mass exceeds the neutron star limit
                                                            iii.      Some x-ray binaries contain compact objects of mass exceeding 3 Msun which are very likely to be black holes
IV.                Gamma ray bursts
a.       They are very brief bursts of gamma rays coming from deep space
b.      They were first detected in the 1960s during the cold war by satellites designed to detect nuclear testing by the Soviet Union
c.       Gamma rays are already high energy
d.      To receive a high rate of these photons form galaxies far beyond the Milky Way = very powerful explosions
e.      Observations in the 1990s showed that many gamma ray bursts were coming from very distant galaxies
f.        To reach us, these energy bursts must be among the most powerful explosions in the universe (possibly witnessing the formation of a black hole)
g.       Observations show that at least some gamma ray bursts are produced by “hyper” supernova explosions in distant galaxies
h.      Some others may come from collisions between neutron stars in a binary system with 2 neutron stars
Our Galaxy/Cosmology
I.                    The Milky Way revealed
a.       Galileo was the first person to use a telescope to resolve the Milky Way into numerous individual stars
b.      A galaxy is any gravitationally-bound conglomeration of billions of stars
c.       Our galaxy is called the Milky Way and we can only see it from the inside
                                                               i.      Edge-on our galaxy looks like a flattened “disk” of stars with a central concentration or “bulge” of stars
                                                             ii.      The location of the sun is very far from the center
                                                            iii.      The disk is 1,000 lightyears thick, and its width is 100,000 lightyears
1.       So it is very thin, but large
                                                           iv.      Primary features: disk, bulge, halo, globular clusters
1.       Disk- thin and full of stars, contains the gas and dust of interstellar medium
2.       Halo- spherical region that surrounds the entire disk, contains very little gas
                                                             v.      If we could view the Milky Way from above the disk, we would see that it has spiral arms
d.      Stars in the disk all orbit in the same direction with little up and down motion (same plane and same direction)
e.      Orbits of stars in the bulge and halo have random orientations, even crossing through the disk plane
f.        Disk stars bob up and down because gravity of disk stars pulls toward disk
                                                               i.      The net gravitational effect of a large flat disk of mass is to pull objects vertically toward the disk
                                                             ii.      Objects with a slight motion out of the plane of the disk are pulled back
g.       Orbital velocity law
                                                               i.      The orbital speed (v) and radius (r) of an object on a circular orbit around the center of the galaxy tells us the mass (Mr) within that orbit
                                                             ii.      Equation: Mr = (r*v^2)/Ga.       The mass of the galaxy
                                                               i.      The sun’s orbital motion (radius and velocity) tells us the mass within the Sun’s orbit
1.       1.0*10^11 Msun
2.       Sun = 28,000 lightyears from the center
3.       Sun has a 230-million year orbit around the center
II.                  Galactic Recycling
a.       Star-gas-star cycle: gas from old stars is recycled into new stars
                                                               i.      High mas stars have strong stellar winds that blow bubbles of hot gas into the interstellar medium
                                                             ii.      Lower mass stars return gas to interstellar space through stellar winds and planetary nebula
                                                            iii.      X-rays from very hot gas in Supernova remnants reveal newly made heavy elements that get thrown back into interstellar medium
                                                           iv.      As the supernova remnant cools it begins to emit light as it expands
                                                             v.      New elements made by the supernova mix into the interstellar medium and we detect them using spectroscopy
                                                           vi.      Multiple supernova create huge hot bubbles that can blow out of the galactic disk
                                                          vii.      Gas clouds cooling in the halo can “rain” back down on the disk
                                                        viii.      Atomic hydrogen gas (H) forms as hot gas cools, allowing electrons to join with protons to make normal atoms
                                                           ix.      Molecular clouds form next, after the gas cools enough to allow atoms to combine into molecules; mostly H2
                                                             x.      Gravity forms stars out of the gas in the molecular clouds, thus completing the star-gas-star cycle
                                                           xi.      Radiation from newly formed stars is eroding these star-forming clouds
                                                          xii.      Summary:
1.       Stars make new elements by fusion
2.       Dying stars expel gas and new elements, producing hot bubbles
3.       Hot gas cools, allowing atomic hydrogen clouds to form
4.       Further cooling permits molecules to form making molecular clouds
5.       Gravity forms new stars (and planets) in  molecular clouds
b.      In 1 trillion years the gas will be locked into white dwarfs and low mass stars
c.       We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths from gamma rays to radio waves
                                                               i.      Radio waves show where gas has cooled and settled
                                                             ii.      Infrared emission shows where young stars are heating dust grains inside molecular clouds
                                                            iii.      X-rays are observed from very hot gas above and below the Milky Way’s disk
d.      Where do stars form in our galaxy?
                                                               i.      Ionization nebula are found around short-lived high-mass stars, signifying active star formation
                                                             ii.      Reflection nebula scatter the light from stars. The radiation from less massive young stars does not ionize the cloud
1.       Reflection nebula look bluer than the nearby stars because blue light (shorter wavelength) is “scattered” more than red light
2.       Halo: no ionization nebula, no blue stars, no star formation
3.       Disk: ionization nebula, blue stars, active star formation
                                                            iii.      Much of the star formation in the disk happens in the Spiral Arms
1.       Spiral arms = density waves, a bunching up of moving materials as it orbits the Galactic Center
2.       They are waves of star formation
a.       Gas clouds get squeezed as they move into spiral arms
b.      Squeezing of clouds triggers star formation
c.       Young stars flow out of spiral arms
d.      (think of traffic…cars on freeway bunch up as they approach a slow moving construction crew working along the hard shoulder, bunching up is the density wave, the wave keeps moving forward, and each car gets through eventually)
III.                History of the Milky Way
a.       Halo stars: contains only 0.02-0.2% heavy elements
                                                               i.      Only old stars, not much reprocessing by gas-star-gas cycle, globular clusters are found in the halo
b.      Disk stars: 2% heavy elements
                                                               i.      Stars of all ages, lots of reprocessing of material
c.       Conclusion: halo stars formed first then stopped; disk stars formed later and kept forming
d.      How did our galaxy form?
                                                               i.      Probably formed from a really giant gas cloud
                                                             ii.      Then gravity caused it to contract causing a burst of star formation
                                                            iii.      Simplified version:
1.       Halo stars formed first as gravity caused cloud to contract
2.       Remaining gas settled into a spinning disk
3.       Stars continuously  form in the disk as galaxy grows older
                                                           iv.      Detailed studies: halo stars formed in clumps that later merged to form a single, larger protogalactic cloud. (the early Milky Way suffered collisions and mergers)
                                                             v.      Carbon monoxide = really good at cooling off gas, today galaxy formation uses a lot of this but this didn’t exist when the universe was created
IV.                The mysterious galactic center
a.       The galactic center lies 28,000 lightyears from the sun, hidden by layers of dusty spiral arms
b.      Visible light cannot penetrate
c.       Infrared and radio emission allow us to see it
                                                               i.      Shows strange radio sources in galactic center
                                                             ii.      Swirling gas near center
                                                            iii.      Orbiting stars near center
                                                           iv.      Stars at galactic center: we can now map out the motions over many years, and use Kepler’s 3rd law to get orbits
                                                             v.      Stars appear to be orbiting something very massive but it is invisible
1.       Believed it be a black hole
2.       The orbits of stars indicate a mass of about 4 million Msun
3.       Further evidence
a.       X-ray flares from the galactic center suggest that the tidal forces of the suspected black hole occasionally tear apart chunks of matter that are about to fall in
V.                  Galaxies and cosmology
a.       Galaxies are the “building blocks” of the universe
b.      Our deepest images of the universe show a great variety of galaxies, some of them billion lightyears away
c.       Galaxies generally formed when the universe was young and have aged along with the universe
d.      A galaxy’s age, its distance, and the age of the universe are all closely related
e.      The study of galaxies is intimately connected with cosmology
                                                               i.      Cosmology = the study of the structure and evolution of the universe
f.        3 major types of galaxies:
                                                               i.      Spiral galaxy- have disk and spheroidal components
1.       Barred spiral galaxy- has a bar of stars across the bulge
a.       Looks stretched, with 2 arms
b.      How to determine barred:
                                                                                                                                       i.      How big the bulge is
                                                                                                                                     ii.      How tightly wound the arms are
                                                                                                                                    iii.      How much dust and gas is in it
2.       Lenticular galaxy- has a disk like a spiral galaxy but much less dusty gas
a.       Intermedisate between spiral and elliptical
                                                             ii.      Elliptical galaxy- all spheroidal component, virtually no disk component
                                                            iii.      Irregular galaxy
g.       Structure
                                                               i.      Old stars on the outside, new stars on the inside
                                                             ii.      Bulge (center, dense with stars)
                                                            iii.      Disk (outside of bulge, full of stars)
1.       Stars of all ages
2.       Many gas clouds
                                                           iv.      Halo (very outside, scattered stars, more of a glow)
                                                             v.      Spheroidal component
1.       Look at a galaxy horizontally
2.       Glowing sphere around the bulge
3.       Bulge and halo, old stars, few gas clouds
                                                           vi.      Blue-white color indicates ongoing star formation (hot stars/young stars)
1.       Ongoing star formation leads to a blue-white appearance because short-lived blue stars outshine others
                                                          vii.      Red-yellow color indicates older star population
h.      Hubble “tuning fork” diagram
                                                               i.      Edwin Hubble’s galaxy classes
                                                             ii.      Left side- spheroid dominates/rounder appearance
                                                            iii.      Right side- disk dominates/large bulge/less dusty gas/brighter spiral arms
i.         How our galaxies grouped together?
                                                               i.      Our galaxy is in the “local group”
                                                             ii.      Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)
                                                            iii.      Elliptical galaxies are much more common in huge clusters of galaxies (hundreds of thousands of galaxies)
VI.                Measuring galactic distances
a.       A star could be close but not very bright or really far away and really bright
b.      You need to know brightness and luminosity
c.       Distances to astronomical objects can be determined using a series of over-lapping steps in the Distance Ladder
                                                               i.      Step 1: determine size of solar system using radar
                                                             ii.      Step 2: determine distances of stars out to a few hundred light-years using parallax
1.       Relationship between apparent brightness and luminosity depends on distance
2.       We can determine a star’s distance if we know its luminosity and can measure its apparent brightness
3.       A standard candle is an object whose luminosity we can determine without measuring its distance
                                                            iii.      Step 3: the apparent brightness of a star cluster’s main sequence tells us its distance
1.       Then, knowing a star cluster’s distance, we can determine the luminosity of each type of star within it1.       Cepheid variable stars
a.       Very luminous and can therefore be seen over great distances
b.      These stars are excellent standard candles because of a relationship between their average luminosity and their average luminosity and their period of variation
c.       Thus… Polaris is pulsating
d.      Faint stars pulse faster; bright stars pulse slower
                                                             ii.      Step 4: observe the variations in apparent magnitude of Cepheid-type star and find its period. Read off the luminosity corresponding to this period. Calculate the distance.
1.       Cepheid variable stars with longer periods have greater luminosities.
2.       White dwarf supernova can also be used as standard candles
a.       Incredibly luminous
b.      Implies very large distances
c.       Same mass for all, same luminosity because they have reached their limit
                                                            iii.      Step 5: apparent brightness of white dwarf supernova tells us the distance to its host galaxy. This technique works for up to 10 billion lightyears.
1.       Tully-fisher relation
a.       Entire galaxies can also be used as standard candles because galaxy luminosity is related to rotation speed
b.      To summarize:
                                                               i.      We measure galaxy distances using a chain of interdependent techniques (aka the distance ladder)
                                                             ii.      When we see dwarf supernova + Cepheid variable combined = best technique
                                                            iii.      The distance-measurement chain begins with parallax measurements that build on radar ranging in our solar system
                                                           iv.      Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles
                                                             v.      We can measure distances greater than 10 billion lightyears using white dwarf supernova as standard candles
                                                           vi.      Measuring a galaxy’s distance and speed allows us to figure out how long the galaxy took to reach its current distance
II.                  Hubble’s law
a.       Hubble proved that galaxies lie far beyond the Milky Way
b.      The puzzle of “spiral nebula”
                                                               i.      Debate of whether “spiral nebula” were entire galaxies like the Milky Way or if they were smaller collections of stars within the Milky Way
                                                             ii.      Hubble measured the distance to the Andromeda Galaxy using Cepheid variables as standard candles -2.5 million light years- to settle the debate
c.       Redshift
                                                               i.      The spectral features of all galaxies beyond the local group are redshifted
                                                             ii.      This means they are all moving away from us
d.      Hubble’s law
                                                               i.      By measuring distances to galaxies, Edwin Hubble found that redshift (the amount by which spectral lines were shifted) and distance are related in a special way
                                                             ii.      The redshift gives velocity
                                                            iii.      The further the galaxy was from us, the faster they are moving away
                                                           iv.      Our universe is expanding
                                                             v.      Using galaxies of known distance (from Cepheid variables) one gets the slope of the line know as Hubble’s constant
                                                           vi.      Velocity =
                                                          vii.      Inverting Hubble’s law
1.       For an unknown distance, the spectrum redshift of a galaxy tells us its distance though Hubble’s law
a.       Distance = velocity/
2.       Distances of farthest galaxies are measured from redshifts
e.      Interpreting the Hubble law
                                                               i.      The more distant a galaxy, the faster it is receding from us
                                                             ii.      Hubble interpreted this as evidence for expansion of the universe
                                                            iii.      The universe has no center and no edge
f.        Analogy of expansion of our universe
                                                               i.      Something that expands but has not center or edge is the surface of a balloon
                                                             ii.      Fixed dots on the surface of a balloon
                                                            iii.      Dots are galaxies held together by gravity
                                                           iv.      Dots move further away from each other as the balloon expands
III.                Cosmological principle
a.       The universe looks about the same no matter where you are within it
                                                               i.      Matter is evenly distributed on very large scales in the universe
                                                             ii.      No center and no edges
                                                            iii.      Not proved but consistent with all observations to date
                                                           iv.      If you observe a galaxy moving away from you at 0.1 lightyears per year and it is now 1.4 billion lightyears away from you then it will have taken 14 billion years to get there. (t = D/V = 1.4 billion/0.1)
b.      Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies.
                                                               i.      Age = distance/velocity (1/)
c.       Distances between faraway galaxies change while light travels
d.      Astronomers think in terms of lookback time rather than distance
e.      Cosmological redshift
                                                               i.      Cosmological horizon
1.       Maximum lookback time of about 14 billion years limits how far we can see
2.       Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time
3.       Lookback time is easier to define than distance for objects whose distances grow while their light travels to Earth
IV.                Galaxy Evolution
a.       We observe the most distant galaxies
b.      We see galaxies back in time
c.       If a galaxy is 100 million lightyears away then we are looking at the galaxy 100 million years ago
d.      Old light from young galaxies
e.      Deep images (meaning a long exposure to reveal faint objects) show us very distant galaxies
f.        Observing galaxies at different distances shows how they age; this works because of the infinite speed of light and the enormous distances
g.       Irregular galaxies came first, then spirals and elliptical
h.      We still can’t directly observe the earliest galaxies, and we have not directly seen the first stars
i.         Our best models for galaxy formation assume:
                                                               i.      Matter originally filled all of space almost uniformly
                                                             ii.      Gravity of denser regions pulled in surrounding matter- clumps
                                                            iii.      With slight changes in the density of matter allows gravity to pull in the clumps
j.        Our universe was smaller billions of years ago
k.       Denser regions contracted, forming protogalactic clouds
l.         H and He gases in these clouds formed the very first stars
m.    Supernova explosions from first stars kept much of the gas from forming stars
n.      Leftover gas settled into spinning disk- conservation and  of angular momentum
o.      The youngest galaxies are more irregular and closer together
                                                               i.      Galaxies merge and change over time
V.                  The Lives of galaxies
a.       Spin: initial angular momentum of protogalactic cloud could determine size of resulting disk
b.      Density:
c.       Elliptical galaxies could come from protogalactic clouds that were able to cool and form stars before gas settled into a disk
d.      Distant red elliptical
                                                               i.      Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe
                                                             ii.      All stars in the galaxy are old (red) and any young blue stars have died
e.      Collisions
                                                               i.      Collisions take up to or more than 1 billion years long
                                                             ii.      Collisions were much more likely early in time, because galaxies were closer together, when everything was closer things collided more
                                                            iii.      Many of the galaxies we see at great distances (and early times)indeed look violently disturbed -evidence of merging
                                                           iv.      They look like they are undergoing collisions
                                                             v.      Antenae nebula
1.       Collisions between 2 spiral galaxies
                                                           vi.      The galaxy collision that we observe in nearby systems clearly trigger bursts of star formation- hot blue stars
                                                          vii.      Modeling such collisions on a computer shows that 2 spiral galaxies can merge to make an elliptical
                                                        viii.      Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together, as in a large cluster of galaxies
f.        Starbursts
                                                               i.      Starburst galaxies are forming stars so quickly they would use up all their gas in less than a billion years
                                                             ii.      These galaxies tend to be very bright in the infrared
                                                            iii.      The intensity of supernova explosions in starburst galaxies can drive galactic winds that expel material far above the galaxy
1.       X-ray images show individual supernova and bubbles of hot gas
2.       Most of the gas can be driven out of the galaxy
VI.                Quasars
a.       Quasar = quasi-stellar-object (associated with radio sources)
b.      If the center of a galaxy is unusually bright we call it an active galactic nucleus (core outshines the disk
c.       Quasars are the most luminous examples
d.      The highly redshifted spectra of quasars indicate large distances according to Hubble’s law
e.      From brightness and distance we find that luminosities of some quasars are 10x the entire Milky Way
f.        Quasars are only about the size of a solar system
g.       Redshift
                                                               i.      Redshift is defined by z = change in wavelength divided by original wavelength
                                                             ii.      Usually z = v/c and so z = 0.2 à 20% speed of light
                                                            iii.      If z = 6, that does not mean velocity is 6x the speed of light1.       Looks like it is moving away from us faster than the speed of light but the light has been stretched out 6 times
                                                             ii.      The redshift is cosmological – due to expansion of space – not due to the Doppler effect, so z is not equal to v/c
                                                            iii.      1/1+z is the size of the universe compared to now, z = 6 means that we see the object at a time when the universe was 1/7 of its present size
                                                           iv.      Quasar light is variable over a time of a few hours. All quasar energy must come from a region smaller than a solar system
                                                             v.      Very very very redshifted
                                                           vi.      Since they are vert redshifted then you can conclude they are generally very distant, they were more common early in time, galaxy collisions might turn them on, and nearby galaxies might hold dead quasars
                                                          vii.      Galaxies around quasars sometimes appear disturbed by collisions
                                                        viii.      Quasars powerfully radiate energy over a very wide range of wavelengths (emit all kinds of light), indicating that they contain matter with a wide range of temperatures
b.      Radio galaxies
                                                               i.      Contain active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed
                                                             ii.      Radio lobes are over 100,000 lightyears
                                                            iii.      But the lobes of some radio galaxies can extend over hundreds of millions of lightyears
                                                           iv.      An active galactic nucleus can shoot out blobs of plasma (ionized gas) moving at nearly the speed of light
                                                             v.      Speed of ejection suggests that a black hole is present
                                                           vi.      Quasars and radio galaxies are related
1.       Radio galaxies don’t appear as quasars because dusty gas clouds block our view of the hot accretion disk, but we can see the jet and the radio lobes
c.       Summary of properties of active galaxies
                                                               i.      Luminosity can be enormous
                                                             ii.      Luminosity can rapidly vary (comes from a space smaller than solar system)
                                                            iii.      Emit energy over a wide range of wavelengths (contain matter with wide temperature range)
                                                           iv.      Some drive jets of plasma at near light speed
                                                             v.      Quasars are the core of these galaxies
1.       Each quasar is a billion solar mass black hole
2.       The luminosity comes from the exceedingly hot accretion disk
II.                  Supermassive black hole
a.       A supermassive black hole is the power source for quasars and other active galactic nuclei
b.      Quasars are super massive black holes at the center of early galaxies
c.       Accretion of gas onto a supermassive  black hole appears to be the only way to explain all the properties of quasars
d.      Energy form a black hole
                                                               i.      Gravitational potential energy of matter falling into black hole turns into kinetic energy- the speeds are very high
                                                             ii.      Friction in the accretion disk turns kinetic energy into thermal energy (heat) – temperatures are very high (x-rays emitted)
                                                            iii.      Heat produces thermal radiation (photons)
                                                           iv.      This process can convert 10-40% of the rest-mass energy (E=mc^2) into radiation – hence the high luminosity
e.      Jets are thought to come from twisting of magnetic fields in the inner part of the accretion disk
f.        Evidence
                                                               i.      We have a dead quasar in the center of our galaxy (dead/not active)
                                                             ii.      The orbital speed and distance of gas orbiting the center of a galaxy indicate a black hole with a mass of 3 billion Msun (that’s enormous!)
                                                            iii.      Many nearby galaxies- perhaps all of them- have supermassive black holes at their centers
1.       We believe this because of observations of stars and gas clouds orbiting at the centers of galaxies
                                                           iv.      These black holes seem to be dormant active galactic nuclei
                                                             v.      All galaxies may have passed through  a quasar-like stage earlier in time
1.       The earliest galaxies were bigger (more merging)
2.       Their black holes are much bigger (quasars)
                                                           vi.      The mass of a galaxy’s central black hole is closely related to the mass of its bulge of stars
                                                          vii.      The larger the bulge mass the greater the black hole mass
                                                        viii.      The development of the central black hole must somehow be related to galaxy evolution
g.       Intergalactic gas clouds between a quasar and earth absorb some of a quasar’s light
h.      We can learn about protogalactic clouds by studying the absorption lines they produce in quasar spectra
Dark Matter, Dark Energy, and the Big Bang
I.                    We don’t know what 95% of our galaxy is made of
II.                  In the last two decades we have come to realize that the dominant source of gravity in the universe is a dark form of matter that differs from fundamentally from atoms that make people, stars, and galaxies (not on the periodic table)
a.       Dark matter: an undetected form of mass that emits light but whose existence we infer from its gravitational influence
                                                               i.      The unseen mass whose gravity governs the observed motion of stars and gas clouds
1.       Responsible for:
a.       Rotation curve of galaxies
                                                                                                                                       i.      Galaxies are spinning too fast to explain
b.      Formation of galaxies
c.       Structure of our universe
d.      Decides the fate of our galaxies
e.      Clusters of galaxies (very important)
                                                                                                                                       i.      We can measure their masses 3 ways
                                                                                                                                     ii.      Dark matter holds the galaxies together
                                                                                                                                    iii.      Amount of bending is proportional to mass
III.                In addition, there appears to be a dark form of energy throughout the universe that counteracts gravity and is driving the expansion
a.       Dark energy: an unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate
                                                               i.      Whatever might be causing the expansion of the universe to accelerate
1.       The energy of empty space can explain dark energy
2.       More empty space = more energy = faster acceleration of expansion
IV.                Contents of the Universe
a.       Normal matter = 4.4%
                                                               i.      Normal matter inside stars = 0.6%
                                                             ii.      Normal matter outside stars = 3.8%
b.      Dark matter = 23%
c.       Dark energy = 73%
V.                  Evidence
a.       We measure the mass of the solar system using the orbits of the planets
b.      Mass = (r*v^2)/G
c.       Rotation curve
                                                               i.      A plot of orbital velocity versus orbital radius
                                                             ii.      Solar system’s rotation curve declines because the sun has almost all the mass
                                                            iii.      Example: merry-go-round, the rotation curve rises with radius
                                                           iv.      The rotation curve of the Milky way stays flat with distance instead of raising or declining
1.       Mass must be more spread out than in the solar system
2.       Mass in the Milky Way is spread out over a larger region than the stars, and it does not glow like stars or nebula
3.       Most of the Milky Way’s mass seems to be dark matter
4.       Mass with in sun’s orbit = 1.0 * 10^11 Msun, but total mass = 10^12 Msun (there is 10x as much matter in the Milky Way as we can see in stars)
5.       The visible portion of a galaxy lies deep in the heart of a large sphere of dark matter
6.       We can measure the rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H
a.       Here we do not rely on stars but use the abundant interstellar hydrogen gas itself
7.       Spiral galaxies all tend to have flat rotation curves, indicating large amounts of dark matter in all of them, and that most of their matter lies outside their visible regions
8.       Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
a.       These galaxies also have dark matter, about 10x the amount of stars
                                                             v.      We can also measure the velocities of galaxies in a cluster from their Doppler shifts
1.       The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars within the galaxies
2.       We now know that galaxy clusters contain large amounts of X-ray emitting hot gas (there is gas between galaxies)
3.       The temperature of hot gas (particle speeds) tells us cluster mass:
a.       85% dark matter
b.      13% hot gas
c.       2% stars
                                                           vi.      Gravitational lensing- the bending of light rays by gravity, can also tell us a cluster’s mass
a.       Mass can bend light
b.      We see multiple images of the same galaxy
c.       Causes curved streaks
d.      Maps out distribution of matter in distant galaxies
e.      Gravitational lensing reveals much more mass than we can “see” in starlight from these galaxies, more evidence for dark matter
                                                          vii.      All three methods of measuring cluster mass indicate similar amounts of dark matter (galaxy motions, temperature of hot gases, and gravitational lensing = 3 methods)
d.      Our options
                                                               i.      Dark matter really exists and we are observing the effects of its gravitational attraction
                                                             ii.      Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter
                                                            iii.      Because gravity is so well tested, most astronomers prefer option #1
                                                           iv.      2 basic options
1.       Ordinary dark matter
a.       (MACHOS)
b.      Protons, neutrons, etc, as usual, but in very dim objects
c.       Massive compact halo objects:                                                                                                                                       i.      Dead or failed stars in halo of galaxies
b.      MACHOS occasionally make other stars appear brighter through lensing, which has been observed…but not enough lensing events to explain all the dark matter
2.       Extraordinary day matter
a.       (WIMPS)
b.      Leading candidate to explain dark matter
c.       Not protons, neutrons or ant form of normal matter
d.      Weakly interacting massive particles:
                                                                                                                                       i.      Mysterious neutrino-like particles (but not neutrinos because they are too fast)
e.      Why believe in WIMPs?
                                                                                                                                       i.      There’s simply not enough ordinary matter
                                                                                                                                     ii.      WIMPs could be left over from the big bang
1.       There are plausible models of such weakly interacting particles generated under the Big Bang conditions
                                                                                                                                    iii.      Models involving WIMPs explain how galaxy formation works
1.       These particles do not feel the electromagnetic force nor the strong nuclear force; they feel  only gravity and the weak force
2.       They can’t radiate energy away and thus can’t collapse                                                       
II.                  Structure formation
a.       The gravity of dark matter is what caused protogalactic clouds to contract early in time
b.      WIMPs can’t contract to center because they don’t radiate away their orbital energy
                                                               i.      This explains the rotation curves of galaxies
c.       Dark matter is still pulling things together
                                                               i.      After correcting for Hubble’s law, we can see that galaxies are “flowing” toward the densest regions of space, which is where the dark matter is denser
                                                             ii.      Maps of galaxy positions reveal extremely large structures: superclusters and voids on a scale of millions of lightyears
1.       Galaxies seem to be distributed in gigantic chains and sheets that surround great voids
                                                            iii.      Models show that gravity of dark matter pulls mass into denser regions- so the universe grows lumpier with time
1.       Universe began smooth with a high degree of symmetry- it has a very low entropy- as time progressed (under gravity) it becomes disordered and entropy increases. This may provide the “arrow of time
2.       Structures in galaxy mas look very similar to the ones found in models in which dark matter is WIMPs
III.                The fate of the universe
a.       Does the universe have enough kinetic energy to escape its own gravitational pull?
b.      The fate of the universe depends on the amount of dark matter
c.       We can calculate how much mass or density is needed to stop the expansion
d.      Amount of dark matter is 25% of this critical density suggesting that the fate is eternal expansion
e.      But observations show that the expansion appears to be speeding up
                                                               i.      This means that there must be something working against gravity (dark energy)
                                                             ii.      Our universe would be coasting but instead it is accelerating
f.        Estimated age depends on both dark matter and dark energy
g.       Brightness of distant white-dwarf supernova tells us how much universe has expanded since they exploded
h.      Evidence
                                                               i.      Distance from Hubble’s law and the distance from luminosity of white dwarf supernova don’t agree yet both are solid methods
                                                             ii.      H must have been smaller in the past
                                                            iii.      White dwarfs supernova are fainter than expected
                                                           iv.      Brightness of distant white dwarf supernova tell us how much the universe has expanded since they exploded.
                                                             v.      Dark matter and normal matter create gravity and should slow down expansion
                                                           vi.      Dark energy is like coiled up spring releasing energy
                                                          vii.      The accelerating universe is best fit to supernova data
1.       Einstein’s cosmological constant- a repulsive energy associated with the vacuum of empty space, is a strong candidate for the dark energy
IV.                The Big Bang
a.       We observe the universe to be expanding, so it must have started out much smaller
b.      If you go all the way back to when the universe wan an infinitesimal object – that moment was the beginning of space and time, the Big Bang
c.       The universe must have been smaller, hotter, and denser earlier in time
d.      The early universe must have been extremely hot and dense, with emphasis on the word extreme
e.      Photons converted into particle-antiparticle pairs and vice-versa
f.        The early universe was full of particles, anti-particles, and radiation because of its very high temperature
                                                               i.      Particles can become other particles by slamming into each other
g.       The beginning…
                                                               i.      The Planck Era
1.       (all theory)
2.       Before this time (10^-43 second) we don’t know what happened because we have no theory of quantum gravity
3.       So small and so dense- less than the size of a nucleus of an atom
                                                             ii.      The forces of nature
1.       Strong force: binds nuclei- nuclear force, like a restraining spring that prevents the 3 quarks that make up protons and neutrons from escaping, nuclear fusion
2.       Electromagnetism: binds atoms and molecules, 1 single force
3.       Weak force: responsible for radioactive decay
4.       Gravity: holds planets together, longest distance time scale, it is a weak force
5.       Observations at giant particle accelerators have proved that these forces unite at high energy
a.       Scientists believe the first three forces merge around 10^29 K
b.      Superstring theory is based on the idea that everything is made from tiny ribbons of energy. Strings are much smaller than we can detect
h.      Gut era (grand unified theory)
                                                               i.      Lasts from Planck time to end of gut force (10^-38)
                                                             ii.      Perhaps the simplest elementary particles form
                                                            iii.      This is mostly theory but has some evidence
                                                           iv.      Something caused our universe to grow enormous in size
i.         Electroweak era
                                                               i.      Has evidence
                                                             ii.      Lasts from 10^-38 to 10^-10 seconds
                                                            iii.      Intense radiation fills all of space
                                                           iv.      Particles create and annihilate
                                                             v.      The universe is expanding and cooling. Eventually it cools to 100 million times the core of the sun. Suddenly weak and electromagnetic forces separate. From this time/temperature onwards we have direct experimental evidence.
                                                           vi.      Very very very hot
j.        Particle era
                                                               i.      Photons convert into all sorts of matter
                                                             ii.      Amounts of matter and antimatter nearly equal
                                                            iii.      Roughly 1 extra proton for every billion proton-antiproton pairs
                                                           iv.      We don’t know why there was such an excess, but it is the reason we are all here now
                                                             v.      Creation of protons and anti-protons stopped (so matter and anti-matter annihilated itself), the extra protons were left
k.       Era of nucleosynthesis
                                                               i.      Begins when matter annihilates remaining antimatter at 0.001 sec
                                                             ii.      Nuclei began to fuse, temperature is now a few billion degrees
                                                            iii.      A lot of protons + very hot = stars
l.         Era of nuclei
                                                               i.      Helium nuclei form at age 3 minutes
                                                             ii.      Universe has become too cool to blast helium apart
                                                            iii.      Expansion and cooling continue
                                                           iv.      As the universe cooled, particle production stopped, leaving matter instead of antimatter
                                                             v.      75% H and 25% He
1.       Matching the composition of the oldest hydroelectric clouds
                                                           vi.      But it is still a hot opaque plasma
                                                          vii.      For 3 minutes, frantically fused helium then stopped
m.    Era of atoms
                                                               i.      Rate of cooling starts slowing down
                                                             ii.      Neutral atoms form at age of 380,000 years
                                                            iii.      Radiation traveled freely after formation of atoms (no more electron scattering)
                                                           iv.      At 3,000 K (surface temp of a star) the mix becomes transparent to radiation, instead of being an opaque plasma
                                                             v.      Radiation streams to every part of the cosmos, establishing a kind of background glow – we detect this today as the cosmic microwave background
1.       Can detect matter/glow from when the universe was 1st created
                                                           vi.      Atoms are formed in this stage
                                                          vii.      Goes from a plasma to a gas
n.      Era of galaxies
                                                               i.      Galaxies form at age 1 billion years
                                                             ii.      In principle we could see this happen using the next generation of big telescopes in space and on the ground
                                                            iii.      Microwave photons are flying all around us
V.                  Evidence
a.       Hubble expansion = evidence
b.      We have detected the leftover radiation from the Big Bang- the cosmic microwave background
c.       The big bang theory correctly predicts the abundance of helium and other light elements (deuterium, lithium)
d.      The cosmic microwave background
                                                               i.      The radiation left over from the Big bang- was detected in 1965
                                                             ii.      Background radiation from big bang has been freely streaming across the universe since the atoms formed at a temperature of 3,000 K                                                               i.      Thermal spectrum: visible/IR photons with wavelength of 1 millionth of a meter
                                                             ii.      Background has perfect thermal radiation spectrum at temperature
1.       The young universe had a thermal spectrum
2.       Expansion of the universe has redshifted thermal radiation from that time to 1000 times longer wavelengths
3.       The photons now have mm wavelengths (these are microwaves)
                                                            iii.      Out universe has grown 1000 times since this
b.      Patterns of structure observed by WMAP (Wilkinson microwave anisotropy probe satellite) tells us “genetic code” of universe, and has been mapping the cosmic microwave background
                                                               i.      Evidence:
1.       Expansion
2.       Lack of old stars
3.       Microwave background
c.       Predictions match up
                                                               i.      The reaction in the suns core
1.       Protons and neutrons combined to make long-lasting helium nuclei when universe was 3 minutes old because the rapidly dropping temperature was just right
                                                             ii.      The big bang theory prediction: 75% H, 25% He (by mass), no free neutrons around, all bound in atoms
                                                            iii.      Predictions line up with how much matter there is in the form of WIMPs
                                                           iv.      More detailed evidence- the deuterium (heavy hydrogen) abundance was established
1.       Abundance of other light elements agree with Big Bang model having 4.4% normal matter
2.       We make predictions of how much deuterium and helium there is
3.       Helium- heated up and stored carbon process but created lithium instead
                                                             v.      Radiation leftover from the big bang is now in the form of microwaves
d.      Mysteries needing explanation
                                                               i.      Where does structure come from that eventually leads to galaxy formation?
1.       The structure begins with tiny, tiny fluctuations in energy at the quantum level
                                                             ii.      Why is the overall distribution of matter so uniform?
1.       How can distant parts of the universe be almost identical in temperature?
                                                            iii.      Why is the density of the universe so close to the critical density? Or why does space look flat?
                                                           iv.      Quantum ripples in space-time
1.       Structure comes from ripples
2.       Size of ripple before inflation = size of atomic nucleus
3.       Size of ripple after inflation = size of solar system
4.       Our universe must have gone through a phase change/stretch during the gut era to cause these ripples
5.       Inflation is a process that can make all the structure by stretching tiny quantum ripples to enormous size
6.       Space-time expands much faster than the speed of light, which is okay because it is not matter
7.       These ripples in density then become the seeds for all structures
                                                             v.      Space-time diagram
1.       How can microwave temperature be nearly identical on opposite sides of the sky?
a.       Regions now on opposite sides of the sky were once very close before inflation pushed them part apart
                                                           vi.      Geometry of the universe
1.       The overall geometry of the universe is closely related to total density of matter and energy
2.       Einstein’s general theory: matter tells the universe how to curve and the curvature tells matter how to move
3.       Space-time can have curvature
4.       Flat universe
a.       Density = critical
b.      180 degrees
5.       Closed universe
a.       Sphere
b.      Density>critical
c.       180 <
6.       Open universe
a.       saddle
b.      Density<critical
c.       180 >
                                                          vii.      After expansion we have concluded our universe is flat
1.       Inflation of the universe flattens the overall geometry, like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density, that is the geometry of space-time is flat
2.       Observable universe became smooth before inflation, when it was very tiny
3.       Inflation flattened the curvature of space, bringing expansion rate into balance with the overall density of mass-energy
4.       Patterns of structure observed by WMAp show us the “seeds” of universe/shows us ripples (about 1 degree in size)
5.       Observed patterns of structure in universe agree with “seeds” that inflation would produce
                                                        viii.      “seeds” inferred form CMB
1.       All pieces “fit” (still a lot of info missing)
2.       Overall geometry is flat
a.       Totally mass + energy has critical density
3.       Ordinary matter = 4.4% of total
4.       Total matter = 27% of total
a.       Dark matter = 23% of total
b.      Dark energy = 73% of total
5.       Age is 13.7 billion years
II.                  Observing the big bang
a.       Olber’s parallax
                                                               i.      Analogy is with a forest of trees so dense with more or less identical trees that every line of sight is blocked
                                                             ii.      Night sky is dark because the universe changes with time
                                                            iii.      As we look out in space, we can look back to a time when there were no stars
1.       As we look further we start seeing less
                                                           iv.      The universe started out with not stars and galaxies 13.7 billion years ago
                                                             v.      It is not infinite, eternal, or unchanging
1.       ^^^strong evidence for big bang
                                                           vi.      If the universe were eternal, unchanging, and everywhere the same, the entire night sky would be covered with stars
III.                Factsa.       The sun is a thermonuclear furnace
b.      The sun will not go supernova, instead it will become a red giant star and then a dense white dwarf the size of the earth
c.       Heavier stars will explode and become ultra-compact spinning objects made entirely of neutrons, or if even heavier, they will collapse to become black holes
d.      There is evidence for black holes forma few solar masses to billions of solar masses
e.      The universe began 13.7 billion years ago in a big bang event that caused the enormous expansion of space we see today
f.        There are at a minimum of 4-80 billion roughly earth-sized planets in orbits around sun-like stars at the right distance such that, if they have surface water, it will be liquid and capable of supporting life
g.       63,000 AU = 1 lightyear
h.      Nearest star = 4.3 lightyear
i.         Solar system = 100 AU, so stack 630 solar systems to = 1 lightyear
j.        Titan (one of Jupiter’s moons) rains methane, like a cold version of the earth
k.       200 little stars for every big star
l.         High mass stars = blue (brighter)
m.    Low mass = red
n.      Luminosity = radius^2
o.      Luminosity = temperature^4
                                                               i.      Star A is 2x brighter, but same radius as star B
                                                             ii.      Star A has a higher luminosity by 16 (2^4)

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