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)
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
* 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 binary
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
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
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
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
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)/G
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 it
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 light
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:
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
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.
Facts
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)
No comments:
Post a Comment