ASTR 101: TEST 3
66 Cards in this Set
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What is the approximate chemical composition (by mass) with which all stars are born?
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three quarters hydrogen, one quarter helium, no more than 2% heavier elements
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According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
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Its apparent brightness will decrease by a factor of 9.
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If star A is closer to us than star B, then Star A's parallax angle is _________.
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larger than that of Star B
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Ten parsecs is about _________.
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32.6 light-years
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Star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5. Which star is brighter in our sky?
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Star A
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From hottest to coolest, the order of the spectral types of stars is _________.
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OBAFGKM
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Our Sun is a star of spectral type _________.
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G
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Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
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The star is a member of a binary star system.
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The axes on a Hertzsprung-Russell (H-R) diagram represent _________.
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luminosity and surface temperature
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What can we infer, at least roughly, from a star's luminosity class?
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its size (radius)
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On an H-R diagram, stellar radii _________.
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increase diagonally from the lower left to the upper right
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Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
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Spectral type M2, luminosity class I
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Which of the following statements comparing open and globular star clusters is not true?
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Open and globular clusters each typically contain a few hundred stars.
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What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
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It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.
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The interstellar clouds called molecular clouds are _________.
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the cool clouds in which stars form
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Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
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thermal pressure
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Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
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Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
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What kind of gas cloud is most likely to give birth to stars?
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a cold, dense gas cloud
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Which of the following phenomena is not commonly associated with the star formation process?
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intense ultraviolet radiation coming from a protostar
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What is the approximate range of masses that newborn main sequence stars can have?
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0.1 to 150 solar masses
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The vast majority of stars in a newly formed star cluster are _________.
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less massive than the Sun
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Which of the following lists the stages of life for a low-mass star in the correct order?
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protostar, main-sequence star, red giant, planetary nebula, white dwarf
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What happens when a main-sequence star exhausts its core hydrogen fuel supply?
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The core shrinks while the rest of the star expands.
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What is a helium flash?
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The sudden onset of helium fusion in the core of a low-mass star
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What is a planetary nebula?
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Gas ejected from a low-mass star in the final stage of its life
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What is the CNO cycle?
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a set of steps by which four hydrogen nuclei fuse into one helium nucleus
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Why is iron significant to understanding how a supernova occurs?
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Iron cannot release energy either by fission or fusion.
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A typical white dwarf is _________.
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as massive as the Sun but only about as large in size as Earth
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If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh _________.
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as much as a truck
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According to our modern understanding, what is a nova?
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an explosion on the surface of a white dwarf in a close binary system
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Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit?
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The white dwarf will explode completely as a white dwarf supernova.
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A typical neutron star is more massive than our Sun and about the size (radius) of _________.
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a small asteroid (10 km in diameter)
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If you had something the size of a sugar cube that was made of neutron star matter, it would weigh _________.
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about as much as a large mountain
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Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _________.
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3 solar masses
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What do we mean by the singularity of a black hole?
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It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
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What makes us think that the star system Cygnus X-1 contains a black hole?
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It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
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The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that ______.
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Vega is nearer than 10 parsecs from Earth.
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Which of the following statements about spectral types of stars isnot generally true?
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The spectral type of a star can be used to determine its distance.
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To calculate the masses of stars in a binary system, we must measure their _________.
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orbital period and average orbital distance
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You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's _________.
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mass
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The approximate main-sequence lifetime of a star of spectral type O is _________.
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3 million years
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The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
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The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
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Our Sun is considered to be a _________.
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low-mass star
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low-mass star
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A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
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We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
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Brown dwarfs would outnumber all ordinary stars.
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Where would a brown dwarf be located on an H-R diagram?
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below and to the right of the lowest part of the main sequence
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Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?
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H-R diagrams of globular clusters
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Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?
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Fusion reactions are producing energy at a greater rate in the red giant.
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Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen-16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?
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At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.
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Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?
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Planetary nebula formation is more common.
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You discover a binary star system in which one star is a 15 MSunmain-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?
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The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.
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The more massive a white dwarf, the _________.
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smaller its radius
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Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
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Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit.
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The white dwarf that remains when our Sun dies will be mostly made of ______.
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carbon
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Which statement about accretion disks is not true?
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The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
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According to present understanding, a nova is caused by _________.
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hydrogen fusion on the surface of a white dwarf
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Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
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No, because it is not orbited by another star.
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Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your home town?
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The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
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How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
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The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
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Which statement concerning black hole masses and Schwarzschild radii is not true?
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In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
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Which of statement below about black holes is not true?
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A spaceship passing near a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main-sequence star.
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When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would rule out the possibility of a black hole?
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intense X-ray bursts
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Which of the following statements about gamma ray bursts is nottrue?
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The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
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Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSunblack hole. The black hole is surrounded by an accretion disk. Sometime within the next milli…
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the red giant will probably supernova within the next million years
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One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiat…
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X rays from the accretion disk
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Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book from that civilization arrives on your desk; it is entitled Iguoonos: How We Evolved. In the first chapter, you learn that these beings evolved from organisms that lived 5 billio…
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They evolved on a different planet in a different star system, and moved to their current location.
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Study Guide ASTR 101