Quiz 2

• Attempt all questions in Part A and 1 of the 6 in Part B. Please write all answers to Part A on these sheets; extra paper is available if needed.
• With a few exceptions, Part B questions call for a paragraph of coherent English. Points will be deducted if sentences are incomplete and reduced to a list of items or a phrase or two. Do not, however, pad your answer by rewriting the question, by rewriting one sentence in several different ways, etc.
• Part B MUST be answered on separate sheets. Answers crammed on these pages of questions will not be graded.

Part A

A1.	Star X is twice as far away as Star Y. The parallax angle of Star X is
	a. half that of Star Y. b. the same as that of Star Y. c. twice that of Star Y. d. four times that of Star Y. e. (The answer cannot be determined from the information given.)
A2.	The determination of stellar parallax is important because it allows the direct determination of
	a. mass. b. distance. c. diameter d. velocity e. All of the above.
A3.	If it were possible to observe stellar parallax from a telescope in orbit about another planet, which planet listed below would provide the most accurate distances to the farthest stars?
	a. Venus b. Mars c. Jupiter d. Saturn e. Pluto
A4.	Identical red giants of spectral type K4III are observed at distances of 10, 20, and 100pc. If the brightness of the star at 10pc is taken as the standard, how many times fainter are the other stars?
A5.	Main sequence stars have masses between
	a. 0.001 and 1000 M. b. 1 and 2 M. c. 0.1 and 100 M. d. 1/1000 and 106 M.
A6.	Which equation best describes the mass-luminosity relation of main sequence stars?
	a. LM35 b. LM-35 c. LM40 d. LM4 e. ML4
A7.	The observed range of stellar luminosity (in units of solar luminosity) is
	a. 1 (they are all the same) b. 0.01 to 10 c. 0.01 to 100 d. 0.0001 to 1,000 e. 0.0001 to 108
A8.	Stellar surface temperatures range from
	a. 5,000 - 6,000 K b. 4,000 - 10,000 K c. 3,000 - 20,000 K d. 2,000 - 50,000 K e. 100 - 100,000 K
A9.	Draw and fully label an H-R diagram. Both axes must be labeled -- no abbreviations. (Draw this on a separate page.) Indicate on the diagram the location of: the main sequence, a red giant, a red supergiant, and a white dwarf.
A10.	Which of the following stars is the hottest? M5I, A0V, A5V, and G9III
A11.	Which of the stars in question A10 is appropriately called a red supergiant?
A12.	Are the following four statements true or false?
	a. Star A appears brighter than star B, as seen from Earth. Therefore, star must be closer to Earth than star B. b. Star A and star B have the same luminosity, but star B is twice as distant as star A. Therefore, star A appears 4 times brighter than star B. c. Differences among stellar spectra are mainly due to differences in composition. d. In a spectroscopic binary, the orbital motion of the component stars appears as variations in their radial velocities.
A13.	When the Sun begins to evolve to become a red giant,
	a. What element will be the most abundant at its surface? b. What element will be the most abundant at its center?
A14.	Once on the main sequence, gravity is no longer important in determining a star's internal structure. True or False?
A15.	Stars are spherical because of
	a. the high temperature in the interior. b. their large size. c. their hydrogen composition. d. gravity. e. their nuclear energy generation.
A16.	If the interior temperature of a star increases, the immediate response of the star will be to
	a. do nothing -- that is, remain the same. b. contract. c. expand. d. cease to exist as a star but cool off to become a planet.
A17.	Which of the following stars will evolve to become a supernova of Type II? the Sun _____ Yes _____ No a main sequence star of 20 solar masses _____ Yes _____ No a white dwarf _____ Yes _____ No Betelgeuse _____ Yes _____ No a brown dwarf _____ Yes _____ No
A18.	What is a brown dwarf?
A19.	Given that the Sun's lifetime is about 10 billion years, estimate the life expectancy of
	a. a 0.2 solar mass, 0.1 solar luminosity red dwarf b. a 3 solar mass, 30 solar luminosity star State your assumptions. Show all work.
A20.	A cluster of stars with a main sequence turn-off at spectral type F0 is _____ a cluster with its turn-off at B3.
	a. younger than b. older than c. the same age as d. more distant than e. less distant than
A21.	A red giant's size is that of
	a. the Sun b. the Earth c. the Earth's orbit d. the state of Kansas e. a typical city
A22.	A white dwarf's size is that of
	a. the Sun b. the Earth c. the Earth's orbit d. the state of Kansas e. a typical city
A23.	Why do massive stars run out of hydrogen in their cores faster than less massive stars?
	a. Their hydrogen fuses faster because of greater pressure. b. There is less hydrogen in their cores. c. The cores of less massive stars contain a greater percentage of helium, which slows hydrogen fusion. d. The cores of less massive stars contain a lesser percentage of helium, which slows hydrogen fusion. e. (The statement is false: more massive stars do not run out of hydrogen faster than stars of less mass.)
A24.	Why can't a white dwarf contract as it cools?
A25.	A planetary nebula is
	a. the vastly expanded shell of a dying star. b. a cloud of gas out of which stars form. c. a cloud of cold dust in space. d. the same as a white dwarf. e. a circular ring around a black hole.

Part B

B1.	a.	Explain the terms 'visual binary', 'eclipsing binary', and 'spectroscopic binary'.
	b.	There are fewer eclipsing binaries known than spectroscopic binaries. Explain why.
	c.	Within 50 light years of the Sun, visual binaries outnumber eclipsing binaries. Why?
	d.	Which is easier to observe at large distances -- a spectroscopic binary or a visual binary? Why?
B2.	a.	Describe the H-R diagram for the sample of stars near the Sun. Label the axes -- no abbreviations. Indicate the approximate range of the quantities plotted.
	b.	Describe how and why the diagram drawn for part a differs from the H-R diagram constructed for stars belonging to a very young star cluster.
	c.	The H-R diagram at right shows the evolutionary track of a star. Is the radius of the star at the tip of the arrow larger/smaller/same as it was at the base of the arrow? Explain your answer.
B3.	a.	What is the mass-luminosity relationship for main sequence stars?
	b.	Using your knowledge of how stars evolve off the main sequence explain why you would or would not expect a mass-luminosity relationship for red giants.
	c.	Two cosmic engineers are asked to provide illumination for a certain region of space. Both are given 20 solar masses of hydrogen to make into stars -- all at the same distance from you. Engineer A decides to make a single 20 solar mass star. Engineer B decides to make 20 stars each of 1 solar mass.   Whose decision would provide you with the greater illumination? How many times greater? Whose decision would provide you with illumination of the longer time? How many times longer?
B4.	a.	Nuclear fusion reactions release energy inside a stellar core? In what forms is the energy released? Descriptions of two forms will suffice. ('Heat' is not an adequate answer.)
	b.	Suppose the proton-proton cycle in the Sun were to slow down suddenly and generate energy at only 90 percent of its current rate. Why would an observer on the Earth not see an immediate decrease in the Sun's brightness? Would she immediately see a decrease in the number of neutrinos emitted by the Sun? Justify your answers.
B5.	a.	Describe the evolution of a massive star from the main sequence to just prior to its terminal explosion. Include in your description an explanation of changes in temperature, density, and composition in the star's core.
	b.	Explain why an iron core marks the end of a massive star's life.
	c.	Betelgeuse is a massive star in the constellation of Orion. Why do you think it quite unlikely that Betelgeuse is now burning Si in its core?
B6.	a.	Describe the several steps in the argument that leads to the conclusion that the Sun must have a hot core (T~15 million K), and that it must currently be generating energy in the core. (It is not necessary to give me a description of the nuclear processes.)
	b.	Explain clearly why nuclear fusion occurs only in very hot gases.