PHYS 107 ASTRONOMY MIDTERM 2

• Question 1

  3 out of 3 points

  	On a Hertzsprung-Russell diagram, where on the main sequence would you find stars that have the greatest mass?
  	Selected Answer:    upper left Correct Answer:    upper left Response Feedback:  Correct, O stars have the largest mass

• Question 2

  3 out of 3 points

  	When we see a region of a planet that is not as heavily cratered as other regions, we conclude that
  	Selected Answer:    the surface in the region is younger than the surface in more heavily cratered regions. Correct Answer:    the surface in the region is younger than the surface in more heavily cratered regions. Response Feedback:  Correct, the regions that have been reformed via vulcanism, erosion, etc most recently have fewer craters

• Question 3

  3 out of 3 points

  	Hydrostatic equilibrium means that:
  	Selected Answer:   stars have a higher temperature and pressure in their centers to support the mass of their outer layers Correct Answer:  stars have a higher temperature and pressure in their centers to support the mass of their outer layers Response Feedback: Correct, the concept of hydrostatic equilibrium implies that a star must have a higher pressure in its center to support the weight of the outer layers.

• Question 4

  0 out of 3 points

  	Remembering that the Sun has a surface temperature of  approximately 5,800 K, a main sequence A star like Vega will have a temperature of approximately:
  	Selected Answer:   3,000 K Correct Answer:  10,000K Response Feedback: Incorrect, an A star is slightly hotter than the Sun, a star at ~50,000K is an O star (the hottest stars) and all of the other choices are as cool or cooler than the Sun so would not fit an A star.

• Question 5

  3 out of 3 points

  	The core of the Sun is
  	Selected Answer:    much hotter and much denser than its surface. Correct Answer:    much hotter and much denser than its surface. Response Feedback:  Correct, remember our discussion based upon the average density and the luminosity relationship with temperature and radius

• Question 6

  0 out of 3 points

  	Type II supernovae occur in evolved O and B stars because they:
  	Selected Answer:   form white dwarf stars in binary systems Correct Answer:  can fuse elements heavier than helium and thus have large iron cores Response Feedback: Incorrect, remember that iron has the highest binding energy of all elements and so a star cannot extract energy from fusing iron into heavier elements.

• Question 7

  3 out of 3 points

  	You observe a star cluster with a main-sequence turn-off point at spectral type G2 (the same spectral type as the Sun). What is the age of this star cluster?
  	Selected Answer:   10 billion years. Correct Answer:   10 billion years. Response Feedback: Correct, stars like the Sun live 10 billion years

• Question 8

  3 out of 3 points

  	Compared to stars like the Sun, B stars like RigeL have:
  	Selected Answer:   lifetimes much shorter than the Sun Correct Answer:  lifetimes much shorter than the Sun Response Feedback: Correct, hot, luminous massive stars like Rigel burn through there fuel much more quickly and thus have lifetimes much shorter than the Sun.

• Question 9

  3 out of 3 points

  	What happens to the core of a star after it ejects a planetary nebula?
  	Selected Answer:    It becomes a white dwarf. Correct Answer:    It becomes a white dwarf. Response Feedback:  Correct, this is the path stars like the Sun take in becoming white dwarfs

• Question 10

  3 out of 3 points

  	If we see a high mass Ostar in the Andromeda galaxy tonight, we know that:
  	Selected Answer:   it will have already gone (Type II) supernova Correct Answer:  it will have already gone (Type II) supernova Response Feedback: Correct, remember that an O star has lifetime of order 1 million years, and so even before the light from an O star reaches us from Andromeda (2 million light years away) the star has been born, evolved, and exploded as a Type II SN before any being in the MIlky Way can see it.

• Question 11

  0 out of 3 points

  	About how old is the Solar System?
  	Selected Answer:   5 trillion years Correct Answer:  5 billion years

• Question 12

  3 out of 3 points

  	You observe two stars over the course of a year (or more) and find that both stars have measurable parallax angles. Star X has a parallax angle of 1 arc-second. Star Y has a parallax angle of  1/4 of an arc-second. How do the distances to the two stars compare?
  	Selected Answer:   star Y is 4 times further away Correct Answer:  star Y is 4 times further away Response Feedback: Correct, the parallax angle is equal to (distance)-1, so the more distant star has a smaller parallax, and if the parallax is 4 times smaller then it is 4 times further away.

• Question 13

  0 out of 3 points

  	How thick are Saturn's rings from top to bottom?
  	Selected Answer:    a few hundred kilometers Correct Answer:    a few million kilometers Response Feedback:  Incorrect, the rings are incredibly thin

• Question 14

  3 out of 3 points

  	We currently know of approximately how many exo-planets?
  	Selected Answer:   900  Correct Answer:  900  Response Feedback: Correct, and increasing steadily.

• Question 15

  3 out of 3 points

  	What do asteroids and comets have in common?
  	Selected Answer:    Most are unchanged since their formation in the solar nebula. Correct Answer:    Most are unchanged since their formation in the solar nebula. Response Feedback:  Correct, both are left over from the formation of the Solar System

• Question 16

  3 out of 3 points

  	Why does the fusion of hydrogen release energy to power the Sun and stars?
  	Selected Answer: b.  fusion combines 4 hydrogen atoms into one helium which has less mass than 4 protons Correct Answer: b. fusion combines 4 hydrogen atoms into one helium which has less mass than 4 protons Response Feedback: Correct, this mass difference powers the stars, think Einstein's equation E = m c2, the energy released is proportional to the mass difference.

• Question 17

  3 out of 3 points

  	Consider two stars, A and B, with the following apparent and absolute magnitudes: m(A) = -2 (apparent magnitude) M(A) = 10 (absolute magnitude) m(B) = 3 (apparent magnitude) M(B) = 5 (absolute magnitude) which is more luminous?
  	Selected Answer: b.  A appears brighter, but B is actually brighter Correct Answer: c. A appears brighter, but B is actually brighter Response Feedback: Correct, A's apparent magnitude of -2 (a smaller number) means it appears brighter but B's absolute magnitude of 5 (versus 10 for A) means it is in intrinsically brighter.

• Question 18

  3 out of 3 points

  	Pluto is now classified as a dwarf planet because:
  	Selected Answer:   it is in the Kuiper belt Correct Answer:  it is in the Kuiper belt Response Feedback: Correct, Pluto has not "cleared" its orbit as it is surrounded by similar sized bodies.

• Question 19

  3 out of 3 points

  	A main sequence star's luminosity comes from:
  	Selected Answer: a.  fusion of hydrogen into helium in the core Correct Answer: c. fusion of hydrogen into helium in the core Response Feedback: Correct, the main sequence is when a star uses nuclear fusion reactions to convert hydrogen into helium in the core of the star.

• Question 20

  3 out of 3 points

  	The MK-spectral types OBAFGKM must represent a sequence of:
  	Selected Answer:   decreasing temperatures Correct Answer:  decreasing temperatures Response Feedback: Correct, the first letter of the spectral type is related to the effective surface temperature of the star.  Not that (for example) M stars can have a variety of radii and hence luminosities (as L=T4R2), and that a supergiant has a larger mass than a giant or main sequence M star.

• Question 21

  3 out of 3 points

  	Why do asteroids and comets differ in composition?
  	Selected Answer:    Asteroids formed inside the frost line, while comets formed outside. Correct Answer:    Asteroids formed inside the frost line, while comets formed outside. Response Feedback:  Correct, asteroids formed where the pre-Solar nebula was too hot for ices to form

• Question 22

  3 out of 3 points

  	If the Sun were to suddenly expand to be 10 times its current size of ~0.0047 AU but keep the same mass, the Earth would:
  	Selected Answer:   stay in the same orbit Correct Answer:  stay in the same orbit Response Feedback: Correct, the force on the Earth would be unchanged since the mass of the Sun is unchanged, so the orbit would not change. Note this would not be true if the Sun were to expand to be larger than the Earth's orbit of course. Also note that orbital distances are defined from both object's centers, so while the Earth in some sense is "closer" to the Sun's surface, its distance from the Sun's center and hence its orbital distance is unchanged.

• Question 23

  3 out of 3 points

  	How do the planets near the Sun differ from those farther out?
  	Selected Answer:   The ones nearest the Sun are generally smallest. Correct Answer:  The ones nearest the Sun are generally smallest.

• Question 24

  3 out of 3 points

  	Incandescent light bulbs have a temperature of ~2700 K, this means that their light is emitted mostly:
  	Selected Answer:   at infrared wavelengths Correct Answer:  at infrared wavelengths Response Feedback: Correct, the cooler temperature (relative to the Sun) means that these bulbs emit most of their light at longer - infrared - wavelengths.  This is why they are so inefficient.

• Question 25

  3 out of 3 points

  	A G2V star and a K2V star have the same:
  	Selected Answer:   none of the above Correct Answer:  none of the above Response Feedback: Correct, remember for main sequence stars the mass, radius, luminosity and temperature are all related (and all are lower as you get to later spectral types).

• Question 26

  3 out of 3 points

  	The Sun will most likely never become a nova or type Ia supernova because such events only happen to stars that
  	Selected Answer:   are in close binary systems Correct Answer:  are in close binary systems Response Feedback: Correct, novae and type Ia supernovae occur in binary systems where one member is a white dwarf.

• Question 27

  3 out of 3 points

  	Which of the following is not a characteristic of the inner planets?
  	Selected Answer:    They all have substantial atmospheres. Correct Answer:    They all have substantial atmospheres. Response Feedback:  Correct, only Venus and Earth have significant atmospheres

• Question 28

  3 out of 3 points

  	Imagine a star which has the same luminosity as the Sun but which has a surface temperature that is twice that of the Sun. You would then expect that a planet identical to Earth orbiting at 1 AU from this star would:
  	Selected Answer:   be approximately the same temperature as the Earth Correct Answer:  be approximately the same temperature as the Earth

• Question 29

  3 out of 3 points

  	On a Hertzsprung-Russell diagram, where would you find red giant stars?
  	Selected Answer:    upper right Correct Answer:    upper right Response Feedback:  Correct, red giants are cool (to the right) and bright (toward the top)

• Question 30

  3 out of 3 points

  	Remembering that the surface temperature of the Sun is ~6000K, a red giant would have a temperature of:
  	Selected Answer:   3000 K Correct Answer:  3000 K Response Feedback: Correct, a red giant like Arcturus is ~1/2 the (surface) temperature of the Sun

• Question 31

  3 out of 3 points

  	Type-II supernovae occur when:
  	Selected Answer: d.  a massive star collapses after running out of fuel Correct Answer: c. a massive star collapses after running out of fuel Response Feedback: Correct, a Type II supernova is when the core of a massive star collapses, releasing energy that explodes the rest of the star

• Question 32

  3 out of 3 points

  	After the collapse began were all parts of the Solar Nebula the same temperature, and if not which part was hottest?
  	Selected Answer:   Not all the same temperature, and the inner parts were the hottest. Correct Answer:  Not all the same temperature, and the inner parts were the hottest.

• Question 33

  3 out of 3 points

  	A constellation is
  	Selected Answer: a.  stars at varying distances in the same part of the sky Correct Answer: a. stars at varying distances in the same part of the sky Response Feedback: Correct, the stars in a constellation are not necessarily members of a single star cluster.  stars in a single cluster are approximately the same age and distance.

• Question 34

  3 out of 3 points

  	A star like the Sun will:
  	Selected Answer: a.  become a white dwarf Correct Answer: a. become a white dwarf Response Feedback: Correct, after it uses all of its hydrogen fuel a star like the Sun will expand into a red giant, then eventually after running out of its core helium become a planetary nebula and then a white dwarf

• Question 35

  3 out of 3 points

  	Approximately how many Earth's fit inside the Sun:
  	Selected Answer: b.  1,000,000 Correct Answer: a. 1,000,000 Response Feedback: Correct, the volume of the Sun is ~1,000,000 times that of the Earth.

• Question 36

  3 out of 3 points

  	An M-star which is 1/1000th the luminosity of the Sun an 1/10th its mass should live approximately
  	Selected Answer: c.  1 trillion years Correct Answer: b. 1 trillion years Response Feedback: Correct, since the amount of energy it puts out is 1/1000th that of the Sun but it has only 1/10th the fuel, it should last 100 times longer- since the Sun lives 10 billion years an M star should live 1 trillion years.

• Question 37

  3 out of 3 points

  	Your mass would be approximately (          ) times larger than it is here on Earth if you were on a 1 Solar mass white dwarf:
  	Selected Answer:   the same Correct Answer:  the same Response Feedback: Correct, your mass is always the same.  Note that the weight increases as the gravitational force increases.

• Question 38

  3 out of 3 points

  	We can determine the age of a globular cluster by:
  	Selected Answer:   finding the spectral types of the most luminous main sequence stars. Correct Answer:  finding the spectral types of the most luminous main sequence stars. Response Feedback: Correct, the most massive/luminous main sequence stars left are the ones whose lifetime is less than the age of the cluster, so the cluster must be just older than that age. All of the stars with lifetimes less than the age of the cluster will have evolved off the main sequence already.

• Question 39

  3 out of 3 points

  	What was the material out of which the Solar System formed primarily made of?
  	Selected Answer:   Mostly hydrogen and helium with a small amount of dust grains Correct Answer:  Mostly hydrogen and helium with a small amount of dust grains Response Feedback: Correct, the Universe is still mostly hydrogen and helium (from the Big Bang)

• Question 40

  3 out of 3 points

  	Open clusters contain approximately ( ) stars:
  	Selected Answer:   1,000 Correct Answer:  1,000 Response Feedback: Correct, open clusters typically contain fewer stars (1,000) than the globular clusters (~100,000-1,000,000) which formed early in the galaxies history.