29:50 Stars, Galaxies, and the Universe
Second Hour Exam
October 17, 2007
Form A

There are 21 questions worth 30 points. Note that different questions have different numbers of points. Read through each question and all the answers before choosing. Attached to this exam is figure from the textbook, which you may find useful.
Budget your time. No whining.

Walk with Ursus!

1. (2pts) Two stars have the same radius, but star A has a temperature which is twice that of star B. Which of the following statements is true?
   (a) Star B is a more massive star than A.
   (b) Star B will emit more of its radiation at shorter wavelengths than Star A.
   (c) The total amount of power (added up over all wavelengths) for Star B is greater than that for Star A.
   (d) If Star A is a spectral class G stars, Star B will be spectral class O.
   (e) Star A will emit more of its radiation at shorter wavelengths than Star B. $\ast$
2. (1 pt) A plot or graph which shows the intensity of electromagnetic radiation (light) emitted by an object versus the wavelength of the light is called a
   (a) spectrum $\ast$
   (b) wavelength
   (c) Hertzsprung-Russell diagram
   (d) spectral classification sequence
   (e) Dalitz plot.
3. (1pt) The Hertzsprung-Russell diagram is a plot or graph of
   (a) absolute magnitude versus spectral type of stars. $\ast$
   (b) apparent magnitude versus spectral type of stars.
   (c) temperature versus spectral type of stars.
   (d) absolute magnitude versus apparent magnitude of stars
   (e) age versus chemical composition of stars.
4. (3pts) One of the following responses correctly describes a scientific result or argument which led to our present understanding of the power source of the Sun and stars. Pick the best choice.
   (a) The Sun is made of hydrogen. The only way that energy can be released from hydrogen (a gas) is by interaction with the antimatter form of hydrogen. This understanding was not possible until antimatter had been isolated in laboratories in the 1920s.
   (b) If we multiply the mass of the Sun by an amount of energy per mass yielded in strong chemical reactions, the total amount of energy is far less than that which the Sun has radiated during geological history. This requires an energy source more powerful than chemical reactions. $\ast$
   (c) The Sun and solar system are known to be about 500 million years old. According to our present understanding of nuclear physics, the ``Triple Alpha Process'' only begins to yield energy after 1.2 billion years, so there are remaining fundamental problems in understanding how the Sun shines.
   (d) The Sun emits a continuous spectrum of electromagnetic radiation, which reaches a maximum (greatest amount of radiation emitted) in the ultraviolet part of the spectrum. This indicates that the Sun radiates by a process called synchrotron radiation.
   (e) The Sun is steadily contracting, which releases gravitational potential energy, which is converted to electromagnetic radiation. The Sun has so much gravitational potential energy that it can continue to shine for many times more than it has already existed.
5. (2pts) How can we know which chemical elements are in the Sun and other stars?
   (a) We apply Wien's Law to the spectra of the Sun and stars. The wavelength of light at which a star is brightest depends on its chemical composition.
   (b) We have direct samples of all stars within a distance of 55 light years, and we can carry out chemical analyses.
   (c) We look for absorption lines of those elements in the spectra of the Sun and stars. $\ast$
   (d) Modern astronomy has been unable to determine the chemical composition of stars.
   (e) We know the chemical composition of the Sun from direct samples, but we cannot know the exact composition of other stars.
6. (1pt) What type of star is the Sun?
   (a) A0V
   (b) M2I
   (c) K2III
   (d) G2V $\ast$
   (e) O3I
7. (1pt) The ``Main Sequence'' can be described as
   (a) a temporal sequence of stars as they become older.
   (b) a way of organizing stars and planets in a similar way.
   (c) a band of stars in a graph of absolute magnitude versus spectral type. $\ast$
   (d) a way of organizing giant and supergiant stars.
   (e) a 19th century way of thinking about how stars are formed.
8. (2pts) A star has an absolute magnitude of +10 and a surface temperature of 3000K. Which of the following statements about this star is true?
   (a) It is on the Main Sequence, and is more luminous than the Sun.
   (b) It is on the Main Sequence, and is less luminous than the Sun. $\ast$
   (c) It is not on the Main Sequence, and is a supergiant.
   (d) It is on the Main Sequence, and is a supergiant.
   (e) It is not on the Main Sequence, and is a white dwarf.
9. (2pts) In Friday questions, we decided (based on what had been discussed to that point) that more massive stars should shine longer than less massive stars. It turns out that this is not true. The reason is a scientific fact which had not been discussed until that point. That fact is
   (a) The luminosity of stars increases drastically with increasing mass. $\ast$
   (b) The luminosity of stars decreases drastically with increasing mass.
   (c) Massive stars don't shine for the same reasons that smaller mass stars do.
   (d) Massive stars are made of different elements than less massive stars.
   (e) The most massive stars are made of antimatter rather than ordinary matter.
10. (1pt) The most abundant element in the Sun and other stars is
    (a) helium
    (b) silicon
    (c) hydrogen $\ast$
    (d) oxygen
    (e) carbon
11. (1pt) The power source of the Sun and other stars is
    (a) nuclear fission reactions in the cores of stars.
    (b) a highly powerful chemical reaction (like an explosion) which occurs in the convective part of the stellar interior.
    (c) the conversion of matter to energy via matter-antimatter collisions.
    (d) the conversion of gravitational potential energy to heat energy.
    (e) nuclear fusion reactions in the cores of those stars. $\ast$
12. (2pts) In class, I gave a number for the radius of the Sun. How do we know this number, in other words, how can we measure its radius or diameter?
    (a) We measure its color, and use Kirchoff's 3rd law to give us the diameter.
    (b) We measure the Sun's angular size. Knowing its distance gives us its radius. $\ast$
    (c) We measure the orbital speed and size of the Earth's orbit. The equations of gravity give us its radius.
    (d) Knowing the spectral type of the Sun immediately gives us its density. From that, we can calculate its diameter.
    (e) Careful observations give the gravitational redshift of the Sun, which is dependent only on the radius.
13. (2pts) A star has an absolute magnitude of -5 and a surface temperature of 7000K. Which of the following statements about this star is true?
    (a) It is on the Main Sequence, and is intrinsically brighter than the Sun.
    (b) It is on the Main Sequence, and is intrinsically fainter than the Sun.
    (c) It is not on the Main Sequence, and is a supergiant. $\ast$
    (d) It is on the Main Sequence, and is a supergiant.
    (e) It is not on the Main Sequence, and is a white dwarf.
14. (2pts) In class, I gave a number for the mass of the Sun. How do we know this number, in other words, how can we measure the mass of the Sun?
    (a) We measure its color, and use Kirchoff's 3rd law to give us the mass.
    (b) We measure the Sun's angular size. Knowing its distance gives us its mass.
    (c) We measure the orbital speed and size of the Earth's orbit. The equations of gravity give us its mass. $\ast$
    (d) Knowing the spectral type of the Sun immediately gives us its density. From that, we can calculate its mass.
    (e) Careful observations give the gravitational redshift of the Sun, which is dependent only on the mass.
15. (1pt) Which of the following is a famous equation of physics which is crucial in understanding the power source of stars?
    (a) $E=mc^2$ $\ast$
    (b) $I=\frac{V}{R}$
    (c) $P=\sigma T^4$
    (d) $\nabla \times \vec{E} = -\frac{1}{c}\frac{\partial \vec{B}}{\partial t}$
    (e) $PV=nRT$
16. (1pt) Virtually the only way to measure the mass of a star other than the Sun is to
    (a) measure the gravitational redshift of the star.
    (b) measure the spectral type of a star, then apply a formula presented in class to get the mass.
    (c) measurable the gravitational pull of the star on all the other stars in the Milky Way galaxy.
    (d) measure the modes of vibration of a star, and use those modes to give the mass.
    (e) measure the properties of a binary star. $\ast$
17. (1pt) In class I discussed a solar phenomenon called granulation. The existence of granulation demonstrates that
    (a) the interior of the Sun is cooler than the surface.
    (b) several billion years ago, the Sun was more massive than it is now.
    (c) the interior of the Sun is hotter than the surface. $\ast$
    (d) several billion years ago, the Sun was less massive than now.
    (e) there are planets orbiting the Sun.
18. (1pt) The outermost layer of the Sun's atmosphere, visible at times of total solar eclipse, is called the
    (a) photosphere
    (b) chromosphere
    (c) corona $\ast$
    (d) magnetosphere
    (e) stratosphere
19. (1pt) Two phenomena which occur on the Sun are explosive in nature, are more common at solar maximum, and can affect conditions here on Earth. These phenomena are
    (a) solar rotation and convection.
    (b) the generation and resistive diffusion of the solar magnetic field.
    (c) primary and secondary eclipses of the Sun by its companion star.
    (d) The Evershed Effect and the Hanle Effect.
    (e) flares and coronal mass ejections. $\ast$
20. (1pt) The ``fuel cycle'' which is responsible for the Sun shining is called the
    (a) CNO cycle
    (b) triple-alpha cycle
    (c) Wannecker cycle
    (d) proton-proton cycle $\ast$
    (e) magnetic reconnection cycle
21. (1pt) In class and in the book, we discussed three main classes of binary stars. These are
    (a) visual, eclipsing, and spectroscopic $\ast$
    (b) visual, gravitational, and radiational
    (c) eclipsing, radiation-dominated, and electromagnetic
    (d) giant, bright giant, and supergiant
    (e) spectroscopic, neutronic, and Alfvénic