Download 1 - Physics

Astronomy A105 Midterm 1
Spring 2007
______________________________________ Name
Multiple Choice (3 points each):
Please circle your answers as they will be your only record of the answers you have given
on the test. Please then mark the answer in the appropriate bubble on the scan sheet. Be
sure to include your name. Use only a #2 pencil. If your pencil breaks, or you do not
have one, a pencil will be provided at the front of the room.
1. What happens to the passage of time very near to the “event horizon” of a black hole?
• A) nothing
• B) it speeds up
• C) it slows down
• D) it depends on the “spin” of the black hole
2. Why are black holes “black”?
• A) they only emit light in wavelengths we cannot see
• B) they emit no light
• C) black holes don’t exist
• D) black holes are not black, just misnamed
3. Why does fusion generate energy in the cores of stars?
• A) The loss of mass energy releases the energy.
• B) The release of gravitational energy.
• C) Fusion does not generate energy.
• D) The release of Kinetic Energy of colliding particles releases the energy.
4. If our sun were to be replaced with a black hole of identical mass, what would happen
to the earth?
• A) The earth would be sucked into the black hole.
• B) Time would slow down on the earth.
• C) The earth would be tossed out of the solar system.
• D) Nothing would happen to the orbit of the earth.
5. Why do large stars tend to use the CNO cycle to fuse protons into Helium while
smaller stars rely on the proton-proton chain?
• A) The temperatures are higher in the cores of larger stars.
• B) The temperatures are lower in the cores of larger stars.
• C) Larger stars have a higher percentage of Carbon in their cores.
• D) Smaller stars have no carbon in their cores.
6. Which takes longer to form, stars the mass of the sun or stars ten times the mass of the
sun?
• A) ten times the mass of the sun
• B) both take the same length of time to form
• C) mass of the sun
• D) the formation rates will depend on the rotation of the gas clouds
7. Of the following, which is the most important factor that causes fusion in the cores of
stars to be highly dependant on the temperature of the core?
• A) because there is a low abundance of fusible material
• B) because atoms will run into each other more often
• C) because of the difficulties in getting the fusion reaction to occur
• D) fusion in not very highly temperature dependant
8. How dense is the sun?
• A) about the density of air
• B) about the density of water
• C) about the density of lead
• D) more dense than any material on earth
9. What is the difference between the diameter of our galaxy and the diameter of our sun?
(Hint: how many stars are there in our galaxy?)
• A) about a factor of 100
• B) about a factor of 10,000
• C) about a factor of a million
• D) about a factor of a trillion
10. When stars are formed, how and in what amounts are they formed?
• A) usually by themselves and only a few
• B) usually by themselves but in large clusters
• C) usually in multiple star systems but with only a few systems
• D) usually in multiple star systems and in large clusters of systems
11. The sun’s corona has a temperature of millions of degrees. If you flew a space craft
to the corona and shielded the craft from the rest of the sun, then what impact would the
corona have on the spacecraft?
• A) fry it
• B) crush it
• C) disintegrate it
• D) nothing
12. When is a star considered “born”?
• A) when the star starts to emit light
• B) when the cloud of dust and gas the star is formed from is fully removed
• C) when planets start to form
• D) when fusion starts in the core of the star
13. Is the sun a “star”?
• A) yes
• B) no
14. What is the expected lifetime of a 10 solar mass star?
• A) 100 million years
• B) 10 billion years
• C) 1 trillion years
• D) a 10 solar mass star dies before it gets fully formed
15. Red dwarfs are very dim compared to the sun. What does this tell you about the
energy generated at the core of a red dwarf?
• A) red dwarfs generate energy more slowly
• B) the energy takes longer to reach the surface
• C) the energy has a harder time escaping from the red dwarf
• D) this tells you nothing
16. Which of the following values for a photon increase if the energy of the photon is
decreased?
• A) wavelength
• B) frequency
• C) velocity
• D) none of these increase
17. What happens most of the times when two protons move directly towards each other
in the core of a star?
• A) a fusion reaction
• B) a fission reaction
• C) a chain reaction
• D) they are repelled
18. Which of the following produces the most of the energy in the cores of main
sequence stars?
• A) fission
• B) fusion
• C) the burning of molecules
• D) gravitational contraction
19. How does a neutron star keep itself from collapsing?
• A) nuclear fusion
• B) electron degeneracy pressure
• C) neutron degeneracy pressure
• D) all of the above
20. How does core of the sun keep itself from collapsing during its current part of its
lifetime?
• A) the sun has no gravity
• B) convection
• C) gas and radiation pressure
• D) neutron degeneracy pressure
21. Which of the following does NOT emit light waves?
• A) dentist X-ray machine
• B) an outgoing radio tower
• C) the night side of Pluto
• D) all of these emit light
22. A 3000K continuum source is shined through a 10,000K gas which is “thin”. What
type of spectrum will this produce?
• A) continuum
• B) emission
• C) absorption
• D) none
23. What type of spectrum will a cold blackbody produce?
• A) continuum
• B) emission
• C) absorption
• D) this object will not emit light of any type
24. When we look at the sun we get an absorption spectrum. What does this tell us about
the top part of the photosphere?
• A) the top of the photosphere is cooler than the bottom of the photosphere
• B) the top of the photosphere is made of mostly Hydrogen and Helium
• C) the top of the photosphere is thin
• D) all of the above
25. How does a white dwarf keep itself from collapsing?
• A) nuclear fusion
• B) electron degeneracy pressure
• C) neutron degeneracy pressure
• D) all of the above
26. What will our sun become when it dies?
• A) nothing (it will blow itself apart in a fiery supernova that destroys everything)
• B) a black hole
• C) a neutron star
• D) a white dwarf
27. What is a black hole?
• A) an object so cool that it does not emit light
• B) the kind of star our sun will become when it dies
• C) an object where the escape velocity exceeds the speed of light
• D) a fictitious type of object which does not actually exist
28. What kind of object is this (the large dark zone pictured above)?
• A) black hole
• B) giant molecular cloud
• C) a flaw in the image
• D) a region which contains nothing
29. The nebular pictured above is an example of what type of nebula?
• A) planetary nebula
• B) stellar nebula
• C) supernova
• D) giant molecular cloud
30. What do stars do while on the “main sequence”?
• A) fission Helium into Hydrogen
• B) fuse Hydrogen into Helium
• C) fuse Helium into Carbon
• D) fuse all elements into Iron
31. Mark A
A) Mark A
B) Mark A
C) Mark A
D) Mark A
SHORT ANSWER QUESTIONS:
Each are worth 15 points. Be sure to answer all parts of all questions fully (I can’t give
points if you don’t answer).
1) What happens to the core of a star once it starts to form Iron? Why?
The core collapses because the core can no longer generate energy from fusion (fusing
Iron takes energy and does not generate energy). Therefore gravity starts to win and the
core shrinks.
2) Why do stars greater than 8 times the mass of the sun supernova at the end of
their lifetimes? What two possible kinds of objects can remain?
Their cores collapse suddenly which releases a lot of energy. The two possible objects
are neutron stars and black holes.
3) How does the spin of a gas cloud affect the formation of stars? Give a least 2
reasons.
The spin fractures the initial giant cloud into smaller clouds which then form their own
star systems. Each system then has spin which causes an accretion disc to form out of
which you can form planets.
4) How can we use the absorption spectrum of the sun to determine what the sun is made
of? What is the sun mostly made of?
The sun is mostly made of Hydrogen and Helium. The specific energies that are
absorbed in the absorption spectrum give us the fingerprint of the elements that absorbed
the energy, which in this case are H and He.