Astronomy Test #3

24 July 2022
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question
What is the approximate chemical composition (by mass) with which all stars are born?
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3/4 Hydrogen, 1/4 Helium, no more than about 2% heavier elements.
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The total amount of power (in watts, for example) that a star radiates into space is called its?
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Luminosity
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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 from a factor of 9.
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Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
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Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
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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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Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 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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Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
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spectroscopic binary
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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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Question 13: 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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On an H-R diagram, stellar masses __________.
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can be determined for main sequence stars but not for other types of stars
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High-mass stars have __________ lifetimes than low-mass stars.
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much shorter
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On an H-R diagram, a red supergiant would be located in the __________.
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upper right
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What is the common trait of all main-sequence stars?
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They generate energy through hydrogen fusion in their core.
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Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
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Earth would be inside the supergiant.
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What is a white dwarf?
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It is the remains of a star that ran out of fuel for nuclear fusion
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Which of the following statements comparing open and globular star clusters is true?
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Stars in open clusters are relatively young, while stars in globular clusters are very old.
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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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What do we mean by the interstellar medium?
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the gas and dust that lies in between the stars in the Milky Way galaxy
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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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Which of the following types of molecule is the most abundant in an interstellar molecular cloud?
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H2
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Interstellar dust consists mostly of __________.
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microscopic particles of carbon and silicon
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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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infrared
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Suppose you look at a star that can be seen through the edge of a dusty interstellar cloud. The star will look __________ than it would if it were outside the cloud.
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dimmer and redder
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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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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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How are magnetic fields thought to affect star formation in molecular clouds?
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They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.
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Which of the following statements about the first stars that were born in the universe is expected to be true?
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They were made only from hydrogen and helium
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What is a protostar?
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a star that is still in the process of forming
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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 law explains why a collapsing cloud usually forms a protostellar disk around a protostar?
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conservation of angular momentum
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What can we learn about a star from a life track on an H-R diagram?
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the surface temperature and luminosity the star will have at each stage of its life
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When does a protostar become a main-sequence star?
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when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
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Approximately what core temperature is required before hydrogen fusion can begin in a star?
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10 million K
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Which of the following stars spends the longest time in the protostellar phase of life?
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a 1 solar-mass star
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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 statements about brown dwarfs is not true?
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Brown dwarfs eventually collapse to become white dwarfs.
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Which of the following stars will live longest?
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1 solar-mass star
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In the context of understanding stellar lives, "high-mass" stars have masses __________.
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more than about 8 times the mass of our 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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The primary source of energy for a star as it grows in size to become a red giant is __________.
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hydrogen fusion in a shell surrounding the central core
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The overall helium fusion reaction is __________.
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three helium nuclei fusing to form one carbon nucleus
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What is a helium flash?
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It is the sudden onset of helium fusion in the core of a low-mass star.
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An H-R diagram for a globular cluster will show a horizontal branch, which is a line of stars above the main-sequence but to the left of the subgiants and red giants. Which of the following statements about these horizontal branch stars is true?
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They generate energy through both hydrogen fusion and helium fusion.
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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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The ultimate fate of our Sun is to __________.
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become a white dwarf that will slowly cool with time
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Assume the following stars all have the same mass as our Sun. Which one does not have fusion occurring in its central core?
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a red giant
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How are low-mass red giant stars important to our existence?
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These stars manufactured most of the carbon atoms in our bodies.
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Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?
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helium and helium
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Which of the following stars will end its life in a supernova?
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a 10 solar-mass star
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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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To predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?
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its mass
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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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After a supernova explosion, the remains of the stellar core can be __________.
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either a neutron star or a black hole
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Why is Supernova 1987A particularly important to astronomers?
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It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
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Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
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The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
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A white dwarf is __________.
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what most stars become when they die
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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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The maximum mass of a white dwarf is ___________.
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about 1.4 times the mass of our Sun
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What is an accretion disk?
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a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
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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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What would happen if a white dwarf gained enough mass to reach the 1.4 solar-mass white dwarf limit?
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The white dwarf would explode completely as a white dwarf supernova.
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A neutron star is __________.
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the remains of a star that died in a massive star supernova (if no black hole was created)
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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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Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
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Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.
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Pulsars are thought to be __________.
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rapidly rotating neutron stars
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How is an x-ray burst (in an x-ray binary system) similar to a nova?
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Both involve explosions on the surface of stellar corpse.
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What is the basic definition of a black hole?
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an object with gravity so strong that not even light can escape
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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 event horizon of a black hole?
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It is the point beyond which neither light nor anything else can escape.
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Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). How would Earth's orbit change?
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It would not change; Earth's orbit would remain the same.
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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 as we expect from a system with an accretion disk, but the unseen star in the system is too massive to be a neutron star.
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The Schwarzschild radius of a black hole depends on __________.
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only the mass of the black hole
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Observational evidence indicates that at least some gamma ray bursts are produced by __________.
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the supernovas of very massive stars that leave behind black holes
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Suppose two neutron stars or two black holes are closely orbiting one another. What do scientists predict will eventually happen to them, and why?
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Their orbits will spiral inward until the two objects merge because of energy lost through gravitational waves.
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When did scientists first detect gravitational waves from mergers of compact objects (pairs of neutron stars or black holes)?
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2015
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According to current models, where does the element gold come from?
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mergers of neutron stars