A ___________________________________________
Last Name
First Name
___________________________________________
ASTR 2020, Spring 2015
Professor Jack Burns
Exam #2
March 18, 2015
Student ID Number
INSTRUCTIONS: Closed books, one page (2 sides) of notes allowed, calculators may
be used, strictly individual effort. WRITE your name and student ID number on this
page BEFORE you begin the exam.
The exam consists of 10 multiple choice questions worth 4 points each, 4 short answer
questions worth 15 points each, and an optional extra credit short answer question worth
5 points on the last page. Please allocate your time accordingly among these parts of the
exam.
Multiple Choice. In questions 1-10, choose the best answer (2 pts). Then explain your
reasoning in 1-2 complete sentences, including why these statements are correct or
incorrect (2 pts). So, a correct answer and correct explanation is worth a total of 4 pts.
1. Which of the following statements about light focusing is NOT true?
a. In a healthy eye, light is focused on the retina.
b. A camera should be placed at the focal plane.
c. If you try to look at an image that is not formed at the focal plane, it will
be blurry.
d. The focal plane of a reflecting telescope is always located within a few
inches of the primary mirror.
e. Light can be focused with a lens or a mirror.
A: d – For instance, a Newtonian telescope has the focus located to the side of the
secondary mirror.
2. What do we mean by the resolution of a telescope?
a. It is the maximum size to which any telescope can be built.
b. It describes the farthest distance to which the telescope can see.
c. It describes the maximum exposure time for images captured with the
telescope.
d. It is the smallest angle over which we can tell that two stars are distinct.
A: d – If two stars are closer to one another (by angle) than the resolution of the
telescope, they will appear as a single object.
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A 3. Which of the following is true about images constructed with the Chandra X-ray
telescope?
a. They are always displayed with the highest possible angular resolution.
b. They are always useful for seeing through things.
c. They are always displayed in false color.
d. They are always displayed with north pointing upward in the images.
A: c – We cannot see x-rays, so in order to make useful images, we must use false
colors.
4. Improvements in technology will eventually allow the entire electromagnetic
spectrum to be observed from high mountaintop observatories.
a. True
b. False
A: b (False) – There are portions of the electromagnetic spectrum which cannot
penetrate our atmosphere (e.g. x-rays), and so we can not observe them, even
from mountaintops.
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A 5. Why do sunspots appear dark in pictures of the Sun?
a. They are too cold to emit any visible light.
b. They actually are fairly bright but appear dark against the even brighter
background of the surrounding Sun.
c. They are holes in the solar surface through which we can see to deeper,
darker layers of the Sun.
d. They are tiny black holes, absorbing all light that hits them.
e. They emit light in other wavelengths that we can't see.
A: b – Sunspots are cooler than the rest of the photosphere. Cooler objects emit
less light than hotter objects. Thus the sunspots emit less light than the rest of the
photosphere.
6. What is the solar wind?
a. the uppermost layer of the Sun, lying just above the corona
b. the strong wind that blows sunspots around on the surface of the Sun
c. the wind that causes huge arcs of gas to rise above the Sun's surface
d. a stream of charged particles flowing outward from the surface of the Sun
e. low energy particles that flow from the interior of the Sun to the
photosphere
A: d – The Sun’s gravity isn’t strong enough to trap all of its particles. Through a
combination of already moving at high speeds due to the heat of the Sun, and
acceleration by magnetic fields, charged particles achieve escape velocity. These
particles stream out from the surface of the Sun.
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A 7. The relatively few craters that we see within the lunar maria
a. were formed by impacts that occurred before those that formed most of the
craters in the lunar highlands.
b. were formed by impacts that occurred after those that formed most of the
craters in the lunar highlands.
c. were created by the same large impactor that led to the formation of the
maria.
d. are volcanic in origin, rather than from impacts.
e. are sinkholes that formed when sections of the maria collapsed.
A: b – The impacts must have been after the maria was created, otherwise we
would not see them, as the creation of the maria would have destroyed the craters.
If option (a) were true, then the crater density would be the same (or greater)
within the maria as in the highlands.
8. Which of the following statement about the farside of the Moon is not true?
a. It has distinctly different geological features than the nearside including
fewer maria regions.
b. It has mountains or highlands on the farside of the Moon that are much
younger than those on the nearside.
c. It is an ideal location for a low frequency radio telescope to study the first
stars and galaxies in the early Universe.
d. It possesses the oldest impact crater, the South Pole Aitken Basin, which
was formed near the time life first developed on the Earth.
e. There are permanently shadowed craters near the poles on the farside that
contain water ice.
A: b – As can be deduced by the greater density of craters on the farside, the
farside is older than the nearside, and is especially older than the maria on the
nearside.
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A 9. The Cosmic Origin Spectrograph on the Hubble Space Telescope
a. Operates in the ultraviolet part of the spectrum which cannot be viewed
from the Earth’s surface.
b. Studies gas in the intergalactic medium between the Earth and distant
quasars.
c. Both a and b.
d. Produces clear, high angular resolution images of distant galaxies because
it is located in space.
e. Both a and d.
A: c – Hubble observes in the UV and IR, so option (a) is true. One of its key
projects was option (b). Option (d) is incorrect because spectrographs do not
produce images.
10. Magnetic fields on the Sun are the cause of much of the solar storm activity that
produces Sunspots and Coronal Mass Ejections.
a. True
b. False
A: a (True) – For instance, sunspots are created when a magnetic field loop lifts
off from the Sun’s surface. Sunspots and coronal mass ejections follow the same
periodic variation as the solar magnetic fields.
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A Short Answer Questions 11-14: Please answer the following questions in a few
sentences. Be sure to write legibly. Also, use sketches, if helpful, in addition to the text.
Please be brief. Literacy and clarity count! Each short answer is worth 15 points.
11. Comparing the properties of telescopes. Let’s look at two space telescopes: the
first is the James Webb Space Telescope with a primary mirror diameter of 6.5
meters operating at an infrared wavelength of 10 microns, and the second is a
next-generation space telescope with a primary mirror diameter of 10 meters
operating at a visible wavelength of 0.5 microns. Remember that 1 micron = 10-6
meters.
a. Calculate the RATIO of the light gathering areas of the Webb telescope
relative to the next-generation telescope.
A: The light collecting area is the area of a circle: 𝐴 = 𝜋 𝑑 2 ! . So the ratio of
the light collecting areas is:
!
𝐴!"#$ 𝜋 𝑑!"#$ 2
𝑑!"#$
=
=
!
𝐴!"
𝜋 𝑑!" 2
𝑑!"
!
=
6.5 m
10 m
!
= 0.4225
b. Calculate the RATIO of the resolutions of the Webb relative to the nextgeneration telescope.
A: The resolution is: 𝜃 = 206,265 arcseconds × 𝜆 𝑑, so the ratio of the
resolutions is:
𝜃!"#$ 𝜆!"#$ 𝑑!"#$ 10 microns 6.5 m
=
=
≈3
𝜃!"
𝜆!" 𝑑!"
0.5 microns 10 m
Thus the resolution of JWST has a resolution that is three times worse than the next
generation visible light telescope.
c. Which telescope would be the most effective in studying cooler objects
such as molecular clouds in the Milky Way? Why?
A: Cooler objects give off infrared light, so even though JWST has a worse
angular resolution, it would be the better choice over the next generation visible
light telescope.
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A 12. Discuss how and why the optics of the Chandra X-ray Observatory are different
from that of the Hubble Space Telescope. Drawing pictures may be helpful as
part of your explanation.
A: Hubble is a traditional reflecting telescope. Light bounces roughly
perpendicularly off the primary and secondary mirrors, and onto the detector.
Chandra, on the other hand, is a grazing incidence telescope. Light comes in
nearly parallel to the mirrors and grazes of their surfaces.
Traditional reflecting telescope (Cassegrain):
Grazing incidence telescope: 7
A 13. Discuss the sources of radiation in interplanetary space. Why are these harmful to
future astronauts who will travel long distances between the Earth and Mars? In
your answer, discuss the effects of radiation on the human body.
A: There are two main types of space radiation: solar wind and Galactic cosmic rays.
While here on the surface, Earth’s magnetosphere protects us from most of the
radiation, that is not the case in interplanetary space. Over the course of a trip to
Mars, astronauts would be exposed to many times the levels of radiation which we on
Earth are exposed to over our entire lifetimes. Damage is done through the breaking
of DNA. Such a dose of radiation could be life-threatening, due to increased risk of
cancer, damage to the central nervous system, and acute radiation syndrome.
14. Describe how the Moon formed. Include in your discussion, the relationship with
Earth, the age of the Moon, and evidence from core samples returned by the
Apollo missions for this model.
A: During the formation of the Earth, a roughly Mars-sized proto-planet collided with
Earth. This ripped up much of the outer layers of the early Earth. The debris both
accreted back onto the Earth and coalesced to form the Moon. This happened roughly
30-50 million years into the formation of the solar system (4.5 billion years ago).
Rock samples returned from the Moon by Apollo have compositions which, although
similar, are not identical with Earth rocks. This fits with the proposed model due to
the different formation locations of the proto-planet impactor and Earth.
Extra Credit (5 pts). Describe the participants in and goals for NASA’s Commercial
Orbital Transportation Services (COTS).
A: NASA has contracted with Orbital Sciences and SpaceX through COTS for resupply
services to the ISS. Orbital Sciences does this through their Antares rocket and Cygnus
spacecraft. SpaceX likewise has their Falcon rocket and Dragon spacecraft.
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