29:50 Modern Astronomy
Second Hour Exam
October 29, 2003
Form A

Read through the question and all the answers before choosing. Check the end of the exam for figures and tables. Budget your time. There are 30 questions. No whining.

Walk with Ursus!

1. Many of the bright stars in the night sky are red in color. Are any of them red dwarfs? Choose the right answer and the right reason.
   (a) yes: most of the closest stars are red dwarfs, so they have small apparent magnitudes
   (b) no: red dwarfs are extremely rare stars, with the closest one being 250 parsecs away, so they cannot be bright
   (c) no: red dwarfs are so dim that even the closest ones have faint apparent magnitudes X
   (d) yes: they are the most numerous type of star, so the most luminous ones are prominent in the night sky
   (e) no: light from the red dwarfs is absorbed by interstellar dust, reducing them to faint magnitudes
2. Cepheid variables are important in astronomy because
   (a) they show a relationship between mass and luminosity
   (b) from observations of them, we can deduce their masses
   (c) all of the Cepheids in the universe formed at nearly the same time
   (d) from the period of the light curve, we can deduce a Cepheid's distance X
   (e) Cepheid variables are the oldest type of star known in the Milky Way galaxy
3. The Sun is in equilibrium, meaning that it neither shrinks nor grows. This equilibrium is due to
   (a) the force of gravity being offset by strong magnetic fields
   (b) the fact that there has been insufficient time for the Sun to shrink to a very small size
   (c) a balance between gravity and gas pressure X
   (d) quantum mechanical processes
   (e) the effect of dark matter on the evolution of stars
4. The Hertzsprung-Russell diagram is a graph of
   (a) stellar temperature versus apparent magnitude
   (b) stellar temperature versus absolute magnitude X
   (c) stellar mass versus luminosity
   (d) stellar age versus luminosity
   (e) color of star versus apparent magnitude
5. You observe a star in a telescope and it appears bluish white. It is too faint to be seen without a telescope. You are told that it is 3 parsecs away. What kind of star is it?
   (a) blue straggler
   (b) spectral class G main sequence star
   (c) spectral class O main sequence star
   (d) blue supergiant
   (e) white dwarf X
6. Which of the following types of object will the Sun become in the future?
   (a) protostar
   (b) molecular cloud
   (c) supernova
   (d) neutron star
   (e) planetary nebula X
7. The Main Sequence is an astronomical term for
   (a) the phase of a star's life before it begins to shine by its own light
   (b) a sequence of events that happen to a star after it has exhausted its nuclear fuel
   (c) a band in the Hertzsprung-Russell diagram that contains about 80 % of all stars X
   (d) the process by which the Sun and stars obtain energy
   (e) the mathematical relation between the parallax of a star and its distance
8. Which of the following responses most accurately completes the following sentence. The fuel used by the Sun and stars to power themselves is
   (a) strong magnetic fields that are found in sunspots and throughout the Sun.
   (b) matter falling at high velocities onto the surface of the Sun and stars
   (c) the matter of which they are composed X
   (d) antimatter trapped inside stars when they form
   (e) combustible chemicals which explode under the intense heat of the Sun and stars
9. The luminosity of a Main Sequence star
   (a) is primarily determined by the age of the star
   (b) increases with its distance from the center of the galaxy
   (c) decreases once it leaves the Main Sequence
   (d) depends strongly on the mass of the star X
   (e) depends weakly on the mass of the star
10. Given correct answers to the preceding two problems, one may deduce that (Watch Out: more than one of the following statements is true, but only one follows as a consequence of the answers to the preceding questions)
    (a) Stars form only in spiral arms of galaxies.
    (b) Older stars have smaller abundances of elements heavier than Helium.
    (c) Red stars are more luminous than yellow stars.
    (d) There are more red dwarf stars than spectral class O stars.
    (e) The Main Sequence lifetime decreases strongly with increasing stellar mass. X
11. You observe a star cluster with Cepheids possessing a period of 20 days. You observe them to have an apparent magnitude of +5. How far away is the star cluster?
    (a) 16 parsecs
    (b) 1000 parsecs X
    (c) 100 parsecs
    (d) 25 parsecs
    (e) 100000 parsecs
12. Which of the following observed properties of the Orion Nebula gives the most restrictive (or best) estimate of its age?
    (a) the fact that the larger of two molecular clouds has 100,000 solar masses
    (b) the fact that the molecular cloud has become opaque to visible light
    (c) the presence of the Main Sequence star $\theta_1 C$ Orionis, with a mass of 37 solar masses X
    (d) the presence of a red glow of excited hydrogen atoms throughout the nebula
    (e) the nearby presence of the red supergiant star Betelgeuse
13. The hottest Main Sequence stars are of spectral class
    (a) O X
    (b) B
    (c) A
    (d) G
    (e) M
14. If the spectrum of the Sun is measured in a laboratory in this building, is it the same as the spectrum of light emitted by the Sun? Choose the right answer and the right reason.
    (a) Yes. There is nothing to absorb or modify the light between us and the Sun.
    (b) Yes. Sunlight can be produced only by stars, and there are none between the Sun and the Earth.
    (c) No. The Earth is very distant from the Sun, and the spectrum becomes redder with distance from the Sun.
    (d) No. Gasses in the Earth's atmosphere absorb radiation at certain wavelengths. X
    (e) No. The spectrum produced at the Sun is modified by its strong gravitational field.
15. In terms of stellar classification, the Sun is a
    (a) spectral class G Main Sequence star X
    (b) spectral class O Main Sequence star
    (c) red dwarf
    (d) white dwarf
    (e) horizontal branch star
16. The Main Sequence lifetime of the Sun is approximately
    (a) 1 billion years
    (b) 10 billion years X
    (c) 50 million years
    (d) 10 million years
    (e) 200 billion years
17. If the star Rigel ($\beta$ Orionis) were 10 parsecs away, what would its apparent magnitude be? How does that compare with other objects in the sky?
    (a) -6.8: barely visible to the naked eye in a dark sky location
    (b) +0.14: one of the brighter stars in the sky, but not as bright as several others
    (c) +0.14: brighter than any object other than the Sun
    (d) 250: much fainter than it really is
    (e) -6.8: brighter than any other star or planet X
18. The power source of the Sun is
    (a) nuclear fusion reactions in the solar core, in which helium atoms fuse to form carbon
    (b) release of gravitational potential energy
    (c) nuclear fusion reactions in the solar core, in which hydrogen atoms fuse to form helium X
    (d) nuclear fusion reactions, in which carbon and oxygen fuse to form neon
    (e) highly energetic chemical combustion reactions
19. You observe two star clusters. In star cluster A, the brightest Main Sequence star is a spectral class B star with about 500 times times the luminosity of the Sun. In star cluster B, the brightest Main Sequence star is a spectral class F star with 5 times the luminosity of the Sun. Which of the following statements about these star clusters is true?
    (a) Star cluster A is much older than star cluster B
    (b) Star cluster A is much more distant than star cluster B
    (c) Star cluster B is much older than star cluster A. X
    (d) Star cluster B is much more distant than cluster A.
    (e) Star cluster B never formed a complete Main Sequence
20. We can measure a type of radiation that gives a direct indication of conditions and physical processes at the center of the Sun. This type of radiation is
    (a) neutrinos X
    (b) radio waves
    (c) ultraviolet radiation
    (d) gamma rays
    (e) dark matter
21. Observations of binary stars provide a unique sort of information about stars. From observations of binaries we can measure
    (a) surface temperatures of stars
    (b) stellar masses X
    (c) stellar distances
    (d) ages of stars
    (e) absolute magnitudes of stars
22. Gravity is a force that exists between objects because they possess
    (a) diameter
    (b) mass X
    (c) electric charge
    (d) electrons
    (e) angular momentum
23. Consider regions where stars are forming. The stars are forming from supplies of matter. This matter is in the form of
    (a) atomic hydrogen
    (b) molecular gas X
    (c) primarily helium
    (d) ionized hydrogen
    (e) carbon and elements heavier than carbon
24. Our current view of star formation, based on observations as well as theoretical ideas, indicates that a ``protostar'' forms an accretion disk. Which feature of the present day Sun and solar system is an indicator that the Sun formed in this way?
    (a) the existence of sunspots
    (b) the fact that the surface temperature of the Sun is about 6000K
    (c) the difference between the Terrestrial and Jovian planets
    (d) the plane of the ecliptic X
    (e) the fact that the Terrestrial and Jovian planets orbit in different senses about the Sun
25. In astronomy, the ``Jeans Length'' is defined as
    (a) the size of a red giant star
    (b) the size of a compression in a gas when gravity dominates gas pressure X
    (c) the diameter of a typical molecular cloud
    (d) the distance from the Sun to the nearest supergiant star
    (e) a length at which repulsive electric forces keep apart a forming star
26. Which of the following statements about molecules in space is true?
    (a) Molecules have not been detected, and are not expected, given the hostile environment of space.
    (b) Three molecules have been detected in interstellar space, methane, molecular hydrogen, and acetone.
    (c) Approximately ten types of molecules have been found in space, exclusively far from regions of star formation.
    (d) Over 100 species of molecules have been detected, including those with as many as 13 atoms. X
    (e) The only molecules in space which have been found originate in the Earth's atmosphere.
27. The star Algol in the constellation of Perseus is an example of
    (a) a red giant
    (b) a Cepheid variable
    (c) a main sequence G star
    (d) a white dwarf
    (e) an eclipsing binary star X
28. When we look out in the night sky, we see regions where stars are obviously forming, or have formed recently. What kind of object is inevitably closely associated with these newly formed stars?
    (a) dark clouds X
    (b) globular star clusters
    (c) spiral galaxies
    (d) white dwarf stars
    (e) planetary nebulae
29. How do we detect the presence of molecules in interstellar space?
    (a) no scheme for detecting molecules has been successful, and it is believed that they do not exist in space
    (b) molecules emit x-rays which have been detected with the Chandra spacecraft
    (c) measuring spectral lines unique to a molecule, with wavelengths of about a millimeter X
    (d) we observe absorption lines in the visual light part of the spectrum
    (e) we observe a slight dimming of visible light emission lines of hydrogen, which is attributed to the presence of molecules
30. The Pleiades is an example of
    (a) an open star cluster X
    (b) a globular star cluster
    (c) an HII region
    (d) a molecular cloud
    (e) a spiral galaxy