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Ay 1 – Homework #3 Distributed on April 15, due by 5 pm on Friday, April 22, either to your section TA, or to your head TA’s mailbox (249 Cahill) Your name: ___________________________________________________ Your section: __________ Collaboration policy: you can discuss it among yourselves in general terms, but everyone has to write or derive their own solutions. Multiple choice questions have only one correct answer, worth 1 point; circle only one. 1. What phenomena would you expect to see near ISM glowing in Hα? A.
B.
C.
D.
Young, massive stars. A supernova remnant. A dense molecular cloud. Either A or B 2. Star formation happens… A.
B.
C.
D.
In cold molecular clouds. In warm ionized Hydrogen gas clouds. When a cloud is massive enough to collapse under its own gravity. When a cloud is hot enough for thermal pressure to balance gravitational pressure and stabilize. E. A and C F. B and D 3. Continuum light, after passing through a dust cloud, will appear: A.
B.
C.
D.
E.
F.
Bluer Redder Dimmer Same intensity B and D B and C 4. The full Moon appears red during a total lunar eclipse. The source of light is still the same – the Moon reflects sunlight, but we can still see it in the Earth’s shadow because our atmosphere refracts the sunlight. Why then does the Moon appear red during a lunar eclipse? A. The blue light in the sunlight is scattered away by the atmosphere before it can reach the Moon. B. The Earth’s shadow has a red tint. C. It’s an optical illusion – our eyes are more sensitive to red at night. D. None of the above. 5. In mid-­‐northern latitudes, which phase of the moon would you see if the moon were rising in the sky at 6pm? A.
B.
C.
D.
Full moon First quarter Third quarter New moon 6. True or false? All planets in the solar system have roughly the same chemical compositions in their atmospheres. A. True B. False 7. The reason[s] Pluto was reclassified from planet to dwarf planet include[s]… A. Although Pluto has cleared its orbit, there are many other Kuiper belt objects that are similar or larger in size. B. Pluto is not large enough for its self-­‐gravity to have made it roughly spherical in shape. C. Pluto’s orbit is much more eccentric and inclined to the ecliptic plane than the other eight planets. D. All of the above. 8. We know of the presence of the Oort cloud by… A. Direct observation of the comets in the Oort cloud which are spherically distributed around the outskirts of the Solar System. B. Indirect observation of its gravitational effects on the outer planets in the Solar System. C. Inferring its presence from the comets which pass through the Solar System on parabolic or highly elliptic orbits. D. All of the above. 9. Of all the phases of the Milky Way’s ISM, cold molecular clouds contribute the most: A.
B.
C.
D.
Mass Volume 21-­‐cm emission A and B 10. Two gas clouds are just barely able to start collapsing; one is 10 pc in diameter, and the other is 2 pc in diameter. What is true about the properties of these clouds, if they are the same temperature? A.
B.
C.
D.
The first cloud is less dense than the second. The first cloud is more dense than the second. The first cloud has less mass than the second. The first cloud has a longer free-­‐fall time than the second. E. A and D F. B and C Computational question: 11. (4 points) In lecture we discussed the collapse of protostellar clouds, which are normally supported by a balance between thermal and gravitational pressure. As heat escapes from the cloud, it cools down and shrinks, until the cloud is once more hot enough to support itself gravitationally. This process repeats, and the protostellar cloud slowly shrinks (this is known as the Kelvin-­‐-­‐-­‐Helmholtz timescale). Estimate the luminosity of a 1 Solar mass protostellar cloud, which collapses from an initial radius of 103 AU down to a Solar radius, over a timescale of 40 million years by radiation of its gravitational potential energy. Compare this to the luminosity of the Sun. Show your calculations (intermediate steps) here: Write your answer here: