Baker Lab 3: Philip and Aubrey

Baker Lab 3

Philip Goudeau & Aubrey Hormel

Abstract

In this lab, my partner and I were able to observe in detail two targets: the Moon and the star Arcturus. We noted that the portion of the moon within the telescope’s field of view was the southwest quadrant, and we were able to note several interesting characteristics, including craters, mountains, and maria. The moon was extremely bright, and it was sometimes easier to distinguish its surface features by partially covering the objective lens with a piece of paper. This darkened the image a little, and our eyes were able to note more detail. Our second target, Arcturus, was one of the brighter stars visible that night. When observing Arcturus through a telescope, the star’s light outshines other stars that appear nearby in the celestial sphere. With the naked eye, the star appeared red. Through the telescope we could not see light of this color, although a spectrum of visible colors was observed in the star’s corona.

Introduction

In this lab, students worked together to set up their telescope, focus it using a bright object in the sky, and locate two different targets in the sky to observe. Like the last lab at Baker, students were able to build skill in locating a target in their telescope’s field of view and focusing the image. Additionally, students were able to gain an appreciation for the difficulty involved in locating a tiny object in the celestial sphere and manipulating the direction of one’s telescope to view this object. Often, this included systematic searching of the nearby area; although sometimes even this seemed to fail, and some students would resort to randomly changing the direction of the telescope until something came into the field of view that “looked right.” This guessing and searching is helped somewhat by the use of StarChart, a similar app, or charts provided in the lab packets. Once targets were located, students noted distinguishing features and answered any questions asked in the lab manual.

Procedure

Materials needed: Telescope and associated “tackle box”, StarChart or similar app, Messier catalog

1. Students find a partner or group of peers to work with.
2. Lab partners get a telescope and its associated “tackle box”, or accessories kit. Partners mount and secure their telescope, attach eyepieces, and connect it to a power source.
3. Partners identify a bright object in the sky, such as Jupiter or the Moon, locate this in their field of view, and use the object to focus the telescope. To align the telescope, students should use either the finder scope or use their best judgement, then look in the eyepiece while using the hand paddle to locate the object. Students use the knob of the back of the telescope to focus their image.
4. Students observe how long the object takes to drift across the field of view, and make a note of why they are drifting.
5. Students choose two targets to observe, locate them in the field of view, and note distinguishing features.
6. For each target, students are directed to:
1. Sketch the target
2. Make notes of distinguishing features and properties
3. If the Moon is a target: identify surface features
4. Estimate the object’s brightness and the magnification of the eyepiece
5. Note whether you can see more stars with a telescope than with your eye and why.
6. If a star is your target: Determine why we cannot see the surface of stars using the concept of angular diameter.
7. If the target is changing in brightness, determine why.
7. At the end of the lab, students properly disassemble their telescopes and put all equipment away.

Results & Discussion

Focusing the telescope:

To focus our telescope, my partner and I located Jupiter. All four Galilean moons were visible that night, they orbited in the same plane, and Jupiter’s different colored bands appeared to be parallel to this plane. This makes sense, because the orientation and movement of material in these bands is determined by tidal forces produced by the Galilean moons.

Additionally, my partner and I timed how long it took the Moon to drift across our field of view. This time was approximately 2 minutes and 16 seconds. The Moon, as well as distant stars, appears to drift because Earth is continually rotating. This rotation changes our orientation with respect to the Moon and distant stars. While we are in centripetal motion in one direction, the stars and Moon appear to move in the opposite direction in the sky. Unlike distant stars, however, the view of the Moon is also affected by the Moon’s orbit around the Earth, causing it to drift faster than distant stars.

Target I: The Moon

1. Below is a sketch of the Northwest quadrant of the Moon as observed by my partner and I. Prominent features are identified and labeled. Note that the telescope flips the image, so what appeared as the Southwest quadrant in our eyepiece was actually the opposite side. This discrepancy was identified when trying to identify the features observed, and realizing the Northwest portion of the Moon matched our observations much better than the Southwest quadrant.

2. With the Moon being so bright, it was somewhat difficult to distinguish between maria and the heavily-cratered highlands, but slight differences in color made it possible. It was also difficult to distinguish individual craters, but one that stood out (near the mountain ranges) appeared to be relatively young. We could see the stress cracks from the impact extending out around the crater. My partner and I were also able to see two features that at first seemed like rifts or valleys, but after studying a map of the Moon, we believe they are mountain ranges. Furthermore, we were able to observe the northernmost part of the visible side of the Moon. We noticed that this area was heavily-cratered, and from our vantage point, we could actually see some of the changes in elevation.

3. Of the features observed, we could identify several of them. Observable maria included Mare Tranquilitatis, Mare Serenitatis, Mare Imbrium, and Mare Frigoris. The two mountain ranges observed were the Apennine and Alps mountains, or Montes Apenninus and Montes Alpes. The two notable craters were harder to identify on map of the Moon, but we believe they are the Aristillus and Aristoteles craters.

4. When asked how bright the target is, this is a feature that is hard to quantify. However, the moon’s phase was a waxing gibbous, meaning it was nearly full. This made to the Moon so bright that it was uncomfortable to view it through the eyepiece without partially covering the objective lens with a piece of paper. My partner and I estimated the magnification of the telescope to be approximately 10 to 15 times the object's size as viewed from Earth.

5. Through the telescope, we can definitely see more stars than we see with our eyes. While the Moon was so bright and large that we were unable to see surrounding stars, when looking at other areas of the sky, many more become visible. This is because the more distant or less luminous stars are fainter and have a smaller angular size than closer or more luminous stars. Our eyes have an angular resolution of only about 1 arcminute and cannot detect these faint stars. However, telescopes improve our angular resolution. The angular resolution of a telescope depends on the diameter of the objective lens and is given by the equation R = (𝜆/D), where R is the resolution in radians, 𝜆 is the wavelength of incoming light, and D is the diameter of the objective lens.

6. The Moon did not change in brightness as we were observing it however, by blocking some of the light entering the telescope with a sheet of paper, we were able to make it appear more dim which made it less painful to observe.

The Moon

The Moon is Earth’s only natural satellite and the brightest object in the night sky. Despite common misconceptions, only one side of the Moon is ever visible from Earth, and in fact, the combinations of illuminated and dark areas of the Moon result from us seeing one side of a partially illuminated sphere. The Moon’s orientation with respect to the Earth and Sun give us the phases we see. For instance, the Moon’s phase is new when the Moon is between the Earth and Sun, because the side of the Moon that is visible to us is in darkness. The Moon’s phase is full when the Earth is between the Moon and the Sun because the entire visible side of the Moon is illuminated. We only ever see one side of the Moon because it is tidally-locked to the Earth. This means that the Moon’s orbital period is equal to its rotation period, resulting in the same side continuously facing the Earth. This phenomenon, as well as what the moon doesn’t do, is explained well in the following Youtube video:

https://www.youtube.com/watch?v=OZIB_leg75Q

Analysis has shown that the Moon is about 4.6 billion years old, and most likely formed early in the history of our solar system when a Mars-sized object collided with the Earth. Since both the Earth and the Moon have differentiated structures, with the densest material in the core, it is probable that the force of the impact liquified much of Earth’s material. Large amounts of debris escaped into space, and gravity eventually brought the pieces together to become the Moon. Evidence shows that the most heavily cratered areas of the moon, the highlands, are the oldest areas of the surface, more than 4 billion years old. The darker areas of the Moon, which happen to of lower elevation, relatively smooth, and contain less craters, are newer surfaces of about 2-3 billion years in age. These new surfaces were formed from volcanic activity on the Moon, where molten material escaped to the surface, settled into areas with low elevation, and covered or partially filled any existing craters. Any craters in the maria that do not appear to have been filled in were formed after those areas of the Moon had been resurfaced. Many of the visible mountain ranges are actually surviving rims of huge impact craters. This is evident in examining the shape and location of several mountain ranges. They often appear to form part of the circumference of a circle.

Target II: Arcturus

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The star Arcturus initially caught our attention because at one point it was the brightest object in that section of the sky. The red light that it gave off also served to draw our attention, and was very helpful when it came time to locate it with the use of a telescope.

Below is a view of the star from a star chart mobile app that was used to identify the star. Following is a sketch made of our view of the star through our telescope.

1. Arcturus is a red giant which glows orange, so the lack of color seen in the telescope could have been due to the intense light emitted or the fact that the human eye does not observe color well in darkness. What we did see, however, was a visible spectrum created from the light produced by the star. This spectrum, which surrounded Arcturus, began with violet visible on the inside and red wavelengths on the outside.
2. Arcturus was bright enough in the telescope to block the light from other stars in the field of view. We estimate the magnification to be approximately 10-15x.
3. We can see more stars through a telescope than with our eyes because, in short, the large diameter of an objective lens compared to the diameter of the lens in our eye increases angular resolution and light-gathering power, allowing us to see smaller and fainter objects. The angular resolution of a telescope is given by r = (𝜆/d), where r is the resolution in radians, 𝜆 is the wavelength of incoming light, and d is the diameter of the objective lens.
4. We cannot see this star’s surface, only the light surrounding it. This is due to the fact that the angular diameter of the star is small, and our eyes have an angular resolution of about 1 arcminute. The angular diameter of an object is given by the formula r = (d/D), where r is the resolution in radians, d is the actual diameter, and D is the distance to the object. The average distance to Arcturus is 36.66 light years and its diameter is 36 million km. To calculate the angular diameter of Arcturus, convert radians to arcseconds and convert light years to kilometers. Use the equation: r = (3600*180/π)(36,000,000/(36.66(9.461*10^12)) = .0214 arcseconds. Therefore, the angular diameter of Arcturus is too small for our eyes to resolve.
5. The only observable variation in brightness is the twinkling caused by turbulence in Earth’s atmosphere.

Arcturus

“The red giant Arcturus is roughly 25 times the diameter of our sun. It is not the largest of the red giant, however [...] . Because of its larger size, in visible light Arcturus radiates more than 100 times the light of our sun. If you consider infrared and other forms of radiant energy, Arcturus is about 200 times more powerful than the sun. Its mass is hard to exactly determine, but may be slightly greater than that of our sun (1.1-0.4+0.6 solar mass).

The reddish or orange color of Arcturus signifies its temperature, which is about 7,300 degrees Fahrenheit. That makes it several thousand degrees cooler than the surface of the sun” - Larry Sessions and Deborah Byrd.

Arcturus is the brightest star in Earth’s Northern half of the celestial sphere. One interesting characteristic is Arcturus’ large degree of proper motion, or lateral motion, across the sky. In fact, among bright stars in the sky, only Alpha Centauri has a higher degree of proper motion. From this proper motion, we can infer that Arcturus is moving at an approximate rate of 122 km/s, and is not moving with other stars in the disk of our galaxy. Instead, it cuts through the disk at a near perpendicular angle, and therefore has an irregular orbit compared to stars in the disk. Arcturus is a red giant and is thought to be an old star, and this coincides with knowledge of halo stars, which tend to be older and possess orbits different than the stars in the disk. Arcturus is type K0 III, and may be a good model for what our Sun will look like when it reaches the red giant phase.

If you want to learn more about the star Arcturus you can click the link here:

http://earthsky.org/brightest-stars/bright-orange-arcturus-use-the-big-dipper-to-find-it

Conclusion

Through the course of this lab, partners were able to observe interesting targets. I believe our choices in targets were beneficial because they were a combination of an object we are knowledgeable about (the Moon) and an object which was unfamiliar (Arcturus). In focusing our telescope, locating the targets, and making observations of them, we were able to build practical skills useful to astronomers. This lab, along with other labs at the observatory, are the most rewarding, because we could sit in classroom and answer questions any time, but to observe distant objects with our own eyes is authentic learning.