Astronomy II: Test II (practice questions, quiz questions, test questions)
In what sense is a black hole like a hole in the observable universe? Define the event horizon and the Schwarzschild radius, and describe the three basic properties of a black hole.
A black hole is like a hole in the observable universe because it represents a part of the universe we can never observe and from which we could never return if we went in. The event horizon is the boundary between the inside of the black hole and the outside universe. The radius of the event horizon is called the "Schwarzschild radius." Black holes have only three measurable properties: mass, spin, and charge.
Describe the mass, size, and density of a typical neutron star. What would happen if a neutron star came to your hometown?
A neutron star packs a greater mass than the Sun into a ball about 10 kilometers in radius. Something so massive and compact is extremely dense: A paper clip made of neutron star material would weigh more than a mountain. If a neutron star came to my hometown, it would destroy my town and the entire Earth, crushing the planet into a shell no thicker than my thumb.
What is a nova? Describe the process that creates a nova and what a nova looks like.
A nova is the glow from the thermonuclear flash from the onset of fusion in a hydrogen shell on the surface of a white dwarf. The hydrogen shell comes from accretion when the white dwarf steals material from its companion. As the hydrogen builds up on the surface, the pressure and temperature rise, and eventually hydrogen fusion becomes possible.
What is a planetary nebula? What happens to the core of a star after a planetary nebula occurs?
A planetary nebula is a glowing shell of gas that was once the outer layers of a star. The gas glows because the hot core's ultraviolet light ionizes the gas. The core, now exposed, will cool with time and become a white dwarf
What is a protostellar disk? Describe how such a disk enables additional matter to accrete onto the protostar.
A protostellar disk is a spinning disk of gas around a protostar. Because of friction in the disk, material can lose its angular momentum and gradually spirals inward and eventually falls onto the protostar.
What do we mean by the singularity of a black hole? How do we know that our current theories are inadequate to explain what happens at the singularity?
A singularity is a mathematical concept where the math becomes undefined (like division by zero, or infinity). In this chapter, a singularity is the point where all of the mass of a black hole is crushed into a point. Unfortunately, general relativity (which describes gravity) disagrees with quantum mechanics (which describes the physics of the very small) at this extreme limit of tiny sizes but the Fg = ¥ at the center of a black hole, so our current theories are inadequate to describe what happens precisely at the singularity.
Describe the mass, size, and density of a typical white dwarf. How does the size of a white dwarf depend on its mass?
A typical white dwarf has about the mass of the Sun packed into a ball the radius of Earth. This compact object has a very high density: A teaspoon of a white dwarf would weigh as much as a small truck. Because the white dwarf is supported by degeneracy pressure, adding mass to the white dwarf causes it to shrink in radius.
What processed may cause a white dwarf supernova? Observationally, how do we distinguish white dwarf and massive star supernovae?
A white dwarf supernova occurs when the white dwarf gains enough mass that it exceeds the Chandrasekhar limit (1.4MSun) and this causes the star to begin carbon fusion. The fusion begins almost instantly throughout the star, so the entire star ignites and the white dwarf explodes completely. These supernovae, unlike the massive star supernovae, lack hydrogen lines in their spectra, allowing astronomers to tell the two types of supernovae apart. Also these types of SN decrease in brightness faster than massive star SN.
What happens to a low mass star after it exhausts its core helium? Why can't it fuse carbon into heavier elements?
After a low-mass star exhausts its core helium, fusion ceases and the star core contracts. Because degeneracy pressure halts the collapse before the carbon becomes hot enough to sustain carbon fusion, the core becomes inert. The layers about the core, where shells of hydrogen and helium both continue fusing, will keep generating heat, causing the star to expand further than in the red giant phase. But this fusion cannot last long, perhaps a few million years, and then the star is dead, leaving behind a white dwarf.
What is an accretion disk? Describe how an accretion disk can provide a white dwarf with a new source of energy.
An accretion disk is a disk of orbiting material that is falling toward a central body, like a white dwarf. We see these only in close binary systems (not isolated stars) because they require material to be transferred from one star to another. As the material falls onto the white dwarf, gravitational energy is turned into heat. The heat provides the white dwarf with a new energy source, allowing it to glow in the ultraviolet.
What is degeneracy pressure, and how is it important to the existence of white dwarfs and neutron stars? What is the difference between electron degeneracy pressure and neutron degeneracy pressure?
Degeneracy pressure is a kind of pressure that arises when subatomic particles are packed as closely as the laws of quantum mechanics allow. Degeneracy pressure is important to neutron stars and white dwarfs because it is what allows them to resist the pull of gravity. In the case of white dwarfs, the degeneracy pressure is provided by electrons, so that version is called "electron degeneracy pressure." For neutron stars, it is the neutrons that provide the pressure, and this version of degeneracy pressure is therefore called "neutron degeneracy pressure.
What are gamma-ray bursts, and how do we think they are produced?
Gamma ray bursts are brief but incredibly energetic outbursts of radiation (primarily in the gamma rays) coming from locations well outside our own galaxy. We hypothesize that gamma-ray bursts may be caused by a supernova that forms a black hole, releasing many times more gravitational potential energy than the kind of supernova that forms a neutron star. It is also possible that some gamma-ray bursts are caused by neutron stars merging with each other.
Why can emission of gravitational waves lead to mergers of white dwarfs, neutron stars, and black holes? How do astronomers expect to be able to detect black hold mergers?
Gravitational waves carry energy away from the binary system - normally this is a tiny amount of energy and the orbit of the binary does not noticeably change. But in the case of very compact, close objects (like WD, NS, and especially BH), the amount energy carried away by gravitational waves is enough that it can cause the orbit to become closer and closer. The closer the objects get, the more energy is released in gravitational waves. Eventually the objects can merge. This question was put in the textbook before 2015, and so it is out of date. We HAVE detected gravitational waves from the merger of both BHs and NSs with the LIGO detector. Here's a link to a video showing how it was done.
Why does the H-R diagram of a globular cluster show a horizontal branch? What are the characteristics of the stars on the horizontal branch?
H-R diagrams for globular clusters show a horizontal branch because core helium-fusing stars all have about the same luminosity, but they vary in their surface temperature. Stars on the horizontal branch are all fusing helium in their cores.
In broad terms, explain how the life of a high-mass star differs from that of a low-mass star. How do intermediate mass stars fit into this picture?
High-mass stars go through their lives more quickly than low-mass stars. Part of this is because during their main-sequence lifetimes, they fuse hydrogens to create helium via the CNO cycle, which produces more energy. After their main-sequence lifetimes, high-mass stars begin fusion reactions involving a series of heavier and heavier elements in their cores. But eventually, high-mass stars reach a stage when they have iron cores and cannot fuse any elements together to produce more energy. When this happens, the star explodes as a supernova. Intermediate-mass stars have similar lives up through the phase where the cores create carbon and oxygen. At this point, the intermediate-mass stars can no longer fuse elements to produce energy and die as white dwarfs.
Explain how the presence of a neutron star can make a close binary star system appear to us as an X- ray binary. Why do some of these systems appear to us as X-ray bursters?
In a close binary, a neutron star can accrete matter from its companion. As this material falls down in the neutron star's intense gravity, its potential energy is converted to heat. This process makes the inner region of the disk so hot that it glows in the X rays. We call such a system an "X-ray binary." In some systems, we see bursts of X rays. These bursts are caused by a similar process to white dwarf novae, except that in this case it is helium fusion, not hydrogen fusion, that powers the burst. Because a steady stream of hydrogen pours onto the neutron star, the pressure and temperature at the bottom of the hydrogen shell are high enough for hydrogen fusion. Accretion builds up a layer of helium, which can eventually ignite, releasing a burst of X rays.
What is interstellar dust? How does it interact with visible light? What are the consequences for our view of the heavens, and how is that view different in infrared light?
Interstellar dust refers to tiny, solid grains of dust that make up about 1% of the mass of molecular clouds. These grains scatter or absorb virtually all of the light that enters molecular clouds so that the clouds appear black in our sky. At the edges of clouds, the grains cause the stars to appear redder than normal. Infrared light passes through the dust more easily so that we can see into and through the molecular clouds. We can also see the grains glow in the infrared because they are heated by stars in and near the cloud.
What do all low mass stars have in common? Why do they differ in their level of surface activity? What are flare stars?
Low-mass stars all have long lifetimes on the main sequence and go through the same basic life stages: main sequence, red giant with hydrogen shell fusion, helium flash, then white dwarf. They differ in details like the depth of their convective zone and their rotation rates. These two factors in turn affect how active the stars are. Flare stars are very-low-mass stars (M stars) with fast rotation rates and deep convection zones. Such stars have spectacular outbursts in X rays.
What features of molecular clouds make the conditions favorable for star formation?
Molecular clouds are favorable locations for star formation for two reasons: low temperature and high density. Their low temperature keeps their pressures about the same as other interstellar clouds, despite the higher density. But the higher density means that gravity is stronger in molecular clouds, so it is able to overcome the pressure in molecular clouds. This increased gravitational attraction allows collapse, leading to star formation.
Why do stars tend to form in cluster? Describe the process by which a single cloud gives birth to an entire cluster of stars.
Stars tend to form in clusters because more massive clouds are better able to overcome pressure and collapse. As these larger clouds collapse, it forms clumps because smaller and smaller parts of the cloud are able to collapse on their own, leading to fragmentation of the cloud. Each fragment becomes a star or a system of stars.
When a star exhausts its core hydrogen fuel, the core contracts but the star as a whole expands. Why?
The core of a red giant contracts because there is no more hydrogen fusion to heat the core and raise thermal pressure to resist gravity. However, the shell of hydrogen outside the core heats up to very high temperatures (hotter than the core during the main-sequence phase), and hydrogen fusion is occurring quickly. The star as a whole expands because the energy transport cannot keep up with the shell's increasing energy generation rate, so the thermal energy is trapped in the star and builds up, pushing the surface outward.
What is the maximum mass of a star? What kind of pressure limits how massive a star can be?
The maximum mass for stars is around 150 solar masses. This limit is set by radiation pressure, the pressure exerted by light. For stars larger than 150 solar masses, energy is generated so furiously that gravity cannot resist the force of radiation pressure and the extra mass is blown away into space.
What is the minimum mass of a star, and why can't objects with lower masses be true stars? What is a brown dwarf?
The minimum mass for a star is 0.08 solar mass. Below this mass, degeneracy pressure halts the collapse of the core before it gets hot enough to start fusion. A brown dwarf is an object in which the degeneracy pressure halted the collapse of the protostar before fusion began, making it a "failed star."
What is the helium fusion reaction, and why does it require much higher temperatures than hydrogen fusion? Why will helium fusion in the Sun begin with a helium flash?
The overall reaction involved in helium fusion is to combine three helium nuclei into one carbon nucleus. Because helium nuclei have two protons, and therefore twice the charge of hydrogen nuclei, they repel one another more strongly. Therefore, the nuclei must slam into one another at much higher speeds than is needed for fusion, requiring much higher temperatures. Before helium fusion begins, the core is supported by degeneracy pressure. This means that it does not expand as the core heats up, so that when the helium fusion begins, the core is very dense and very hot. This causes the helium fusion rate to rocket upward rapidly, resulting in the helium flash.
Describe some of the nuclear reactions that can occur in high-mass stars after they exhaust their core helium. Why does this continued nuclear fusion occur in high-mass stars but not in low mass stars?
The simplest sequences of fusion are helium capture reactions, where helium nuclei fuse with other nuclei. This builds carbon into oxygen, oxygen into neon, neon into magnesium, and so on. Also, at the high temperatures in the high-mass stars' cores, heavy nuclei can be fused together. So carbon can be combined with oxygen to form silicon, two oxygen nuclei can create sulfur, and so forth. These reactions all require high temperatures, which low-mass stars cannot produce. So low-mass stars are never able to use these reactions to power themselves like the high-mass stars do.
Why do we think that supernovae should sometimes form black holes? What observational evidence supports the existence of black holes?
We think that black holes should sometimes be formed by supernovae because models indicate that in some supernovae the outer layers of the star are not completely blown away. The extra mass can push the neutron star core over the mass limit for neutron stars and make it a black hole. We have strong evidence for black holes in X-ray binary systems where the mass of the unseen companions are larger than the mass limit for neutron stars. The only kinds of objects we currently know of that these companions could be are black holes. We also have evidence for supermassive black holes in the centers of many galaxies.
Why do we think the very first stars were much more massive than the Sun?
We think that the first generation of stars must have been more massive than the Sun because there were no elements heavier than hydrogen and helium to form the molecules that cool molecular clouds. Since the clouds would have been warmer, the clouds would have had to form larger fragments to collapse. The larger fragments would have gone on to form larger stars.
What event initiates a supernova, and why is a neutron star or black hole left behind? What observational evidence supports our understanding of supernovae?
When high-mass stars reach the stage of iron cores, degeneracy pressure will briefly support the core against collapse. However, this situation cannot last as gravity pushes the electrons past the limits and degeneracy pressure fails. In a fraction of a second, the iron core shrinks from the size of Earth to a ball a few kilometers across. The contraction is halted by neutron degeneracy pressure. The contraction releases an enormous amount of energy that blows the outer layers away from the star in a supernova explosion. After the supernova, the core is left exposed. If the neutron degeneracy pressure is strong enough to resist gravity, a neutron star is left over. However, if even the neutron degeneracy pressure is insufficient to resist gravity, the core collapses into a black hole. Theoretical models reproduce the energy outputs of real supernovae, indicating that our understanding of supernovae is pretty good. Also, when Supernova 1987A occurred, we were able to look at the pre- supernova star and see that most of our predictions about the evolution of the star were pretty accurate.
According to our modern understanding, what is a nova? a. an explosion on the surface of a white dwarf in a close binary system b. the explosion of a massive star at the end of its life c. the sudden formation of a new star in the sky d. a rapidly spinning neutron star
a
How is an X-ray burst (in an X-ray binary system) similar to a nova? A) Both involve explosions on the surface of stellar corpse. B) Both typically recur every few hours to every few days. C) Both are thought to involve fusion of hydrogen into helium. D) Both result in the complete destruction of their host stars.
a
What effect are magnetic fields thought to have on star formation in molecular clouds? a. they can help to resist gravity so that more total mass is needed before the cloud can collapse to form stars b. they accelerate the star formation process c. they allow small stars to form in isolation within gas clouds d. none -- there are no magnetic fields in interstellar space
a
What happens when a low mass star exhausts its core hydrogen supply? a. its core contracts, but its outer layers expand and the star becomes bigger and brighter b. it contracts, becoming smaller and dimmer c. it contracts, becoming hotter and brighter d. it expands, becoming bigger but dimmer
a
What happens with a contracting cloud in which gravity is stronger than pressure and temperature remains constant? a) It breaks into smaller fragments. b) Thermal pressure starts to push back more effectively against gravity. c) It traps all the energy released by gravitational contraction.
a
What is the ultimate fate of an isolated white dwarf? A) It will cool down and become a cold black dwarf. B) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova. C) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova. D) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star. E) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.
a
What makes us think that the star system Cygnus X-1 contains a black hole? a. it emits x-rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star b. no light is emitted from this star system, so it must contain a black hole c. the fact that we see strong x-ray emission tells us that the system must contain a black hole
a
What prevents a brown dwarf from undergoing nuclear fusion? a. degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion b. there is not enough mass to maintain nuclear reactions in a self-sustaining way c. the surface temperature never raises hot enough for the radiation to be trapped and heat their interior to the temperatures required for nuclear fusion
a
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 to appear in your hometown? a. the entire mass of Earth would end up as a thin layer, about 1 cm thick, over the surface of the neturon star b. it would rapidly sink into the center of Earth c. it would crash through Earth, creating a large crater, and exit Earth on the other side d. the combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole
a
Which of the following is closest in size (radius) to a white dwarf? a. Earth b. a small city c. a football stadium d. the sun
a
Which of the following statements about degeneracy pressure is not true? a. degeneracy pressure varies with the temperature of the star b. degeneracy pressure can halt gravitational contraction of a star even when no fusion is occurring in the core c. degeneracy pressure keeps any protostar less than 0.08 solar mass from becoming a true, hydrogen-fusing star d. degeneracy pressure arises out of the idea of quantum mechanics
a
Which of these colors of light passes most easily through interstellar clouds? a) red light b) green light c) blue light
a
Which of these objects has the smallest radius? a) a 1.2 MSun white dwarf b) a 0.6Msun white dwarf c) Jupiter
a
Which of these stars does not have fusion occurring in its core? a) a red giant b) a red main-sequence star c) a blue main-sequence star
a
Which of these stars has the hottest core? a) a white main-sequence star b) an orange main-sequence star c) a red main-sequence star
a
Why do we think the first generation of stars would be different from stars born today? a. without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse b. without heavy elements, the nuclear reactions at the center of stars would be very different c. without heavy elements, there was no dust in the clouds and they collapsed faster
a
4) What kind of pressure supports a white dwarf? A) neutron degeneracy pressure B) electron degeneracy pressure C) thermal pressure D) radiation pressure E) all of the above
b
By mass, th interstellar medium in our region of the Milky Way consists of: a. 70% hydrogen, 30% Helium b. 70% Hydrogen, 28% Helium, 2% Heavier elements c. 50% Hydrogen, 30% Helium, 20% heavier elements
b
From a theoretical standpoint, what is a pulsar? A) a star that alternately expands and contracts in size B) a rapidly rotating neutron star C) a neutron star or black hole that happens to be in a binary system D) a binary system that happens to be aligned so that one star periodically eclipses the other E) a star that is burning iron in its core
b
From an observational standpoint, what is a pulsar? A) a star that slowly changes its brightness, getting dimmer and then brighter with a period of anywhere from a few hours to a few weeks B) an object that emits flashes of light several times per second or more, with near perfect regularity C) an object that emits random "pulses" of light that sometimes occur only a fraction of a second apart and other times stop for several days at a time D) a star that changes color rapidly, from blue to red and back again E) a star that rapidly changes size as it moves off the main sequence
b
How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf? A) It has a larger radius. B) It has a smaller radius. C) It has a higher surface temperature. D) It has a lower surface temperature. E) It is supported by neutron, rather than electron, degeneracy pressure.
b
How many helium nuclei fuse together when making carbon? a. 2 b. 3 c. 4 d. varies depending on the reaction
b
If a star is extremely massive (well over 100 solar masses), why isn't it likely to survive for long? a. it explodes as a supernova after just a few dozen years b. it may blow itself apart because of radiation pressure c. it eventually divides into two lower-mass stars
b
Molecular clouds stay cool because their molecules emit photons. Which of these molecules produces the largest number of photons in a molecular cloud? a) molecular hydrogen (H2) b) carbon monoxide (CO) c) water (H2O)
b
Scientists have detected thousands of gamma-ray bursts. The evidence suggests that most or all of these bursts a. have occurred in the central regions of the Milky Way b. have occurred in distant galaxies c. come from the same types of close binary systems that produce X-ray bursts d. come from the Oort cloud surrounding the sun
b
The typical density and temperature of molecular clouds are: a. 100 molecules per cubic centimeter, 10-30 Kelvin b. 300 molecules per cubic centimeters, 10-30 Kelvin d. 100 molecules per cubic centimeter, 100-300 Kelvin e. 300 molecules per cublic centimeter, 100-300 Kelvin
b
What is the eventual fate of a brown dwarf? A) It remains the same forever. B) It gradually cools down and becomes ever dimmer. C) It gradually contracts and heats up until nuclear fusion ignites in its interior and it becomes a faint star. D) It becomes ever denser and hotter until it becomes a white dwarf. E) Gravity ultimately "wins" and it becomes a small black hole.
b
What would you be most likely to find if you returned to the solar system in 10 billion years? a) a neutron star b) a white dwarf c) a black hole
b
Where does gold come from? a. it is produced by mass transfer in close binaries b. it is produced during the supernova explosions of high-mass stars c. it is produced during the late stages of fusion in low-mass stars d. it was produced during the Big Bang
b
Which kind of pressure prevents stars of extremely large mass from forming? a) thermal pressure b) radiation pressure c) degeneracy pressure
b
Which of the following is closest in size (radius) to a neutron star? A) Earth B) a city C) a football stadium D) a basketball E) the Sun
b
Which of the following statements about an open cluster is true? a. all stars in the cluster are approximately the same color b. all stars in the cluster are approximately the same age c. all stars in the cluster have approximately the same mass
b
Which of these neutron stars must have had its angular momentum changed by a binary companion? a) a pulsar that pulses 30 times per second b) a pulsar that pulses 600 times per second c) a neutron star that does not pulse at all
b
Which of these stars has the hottest core? a) a blue main-sequence star b) a red supergiant c) a red main-sequence star
b
Why are the very first stars thought to have been much more massive than the Sun? a) The clouds that made them were much more massive than today's star-forming clouds. b) The temperatures of the clouds that made them were higher because the clouds consisted entirely of hydrogen and helium. c) Star-forming clouds were much denser early in time.
b
Why does a star grow larger after it exhausts its core hydrogen? a. the outer layers of the star are no longer gravitationally attracted to the core b. hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward c. helium fusion in the core generates enough thermal pressure to push the upper layers outward d. helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward
b
A teaspoon of neutron star material on earth would weight a. about the same as a teaspoon of Earth-like material b. a few tons c. more than Mt. Everest d. more than the moon
c
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly a. 0.5 solar masses b. 1.4 solar masses c. 3 solar masses d. 10 solar masses
c
Compared to the star it evolved from, a red giant is A) hotter and brighter. B) hotter and dimmer. C) cooler and brighter. D) cooler and dimmer. E) the same temperature and brightness.
c
If we see a nova, we know that we are observing a) a rapidly rotating neutron star b) a gamma-ray emitting supernova c) a white dwarf in a binary system
c
If you wanted to observe stars behind a molecular cloud, in what wavelength of light would you most likely observe? a. ultraviolet b. visible c. infrared d. x-ray
c
Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova? A) A massive-star supernova is brighter than a white-dwarf supernova. B) A massive-star supernova happens only once, while a white-dwarf supernova can repeat periodically. C) The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white- dwarf supernova does not. D) The light of a white-dwarf supernova fades steadily, while the light of a massive-star supernova brightens for many weeks. E) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.
c
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? A) The white dwarf will collapse in size, becoming a neutron star. B) The white dwarf will undergo a nova explosion. C) The white dwarf will explode completely as a white dwarf supernova. D) The white dwarf will collapse to become a black hole.
c
Suppose the star Betelgeuse (the upper left shoulder of Orion) were to become a supernova tomorrow (as seen here on Earth). What would it look like to the naked eye? a. because the supernova event destroys the star, Betelgeuse would suddenly disappear from view b. we'd see a cloud of gas expanding away from the position where Betelgeuse used to be c. Betelgeuse would remain a dot of light but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime
c
Viewed from a distance, how would a flashing red light appear as it fell into a black hole? a) It would appear to flash more quickly. b) Its flashes would appear bluer. c) Its flashes would shift to the infrared part of the spectrum.
c
What do we mean by the singularity of a black hole? A) There are no binary black holes—each one is isolated. B) An object can become a black hole only once, and a black hole cannot evolve into anything else. C) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions. D) It is the edge of the black hole, where one could leave the observable universe. E) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole.
c
What happens to the core of a star after a planetary nebula occurs? a. it contracts from a protostar to a main-sequence star b. it breaks apart in a violent explosion c. it becomes a white dwarf d. it becomes a neutron star
c
What happens to the rotation of a molecular cloud as it collapses to form a star? a. the rotation rate remains the same and results in stellar rotation b. the rotation dissipates and any residual is left in small overall rotation of the star c. the rotation rate increases and results in a disk of material around a protostar d. the rotation rate increases and results in fast rotation of the star
c
What is a planetary nebula? a. a disk of gas surrounding a protostar that may form into planets b. what is left of the planets around a star after a low-mass star has ended its life c. the expanding shell of gas that is no longer gravitaitonally held to the remnant of a low-mass star d. the molecular cloud from which protostars form
c
What is interstellar reddening? a. interstellar dust absorbs more red light than blue light, making stars appear redder than their true color b. interstellar dust absorbs more blue light than red light, making stars appear bluer than their true color c. interstellar dust absorbs more blue light than red light, making stars appear redder than their true color d. interstellar dust absorbs more red light than blue light, making stars appear bluer than their true color
c
What is the likely reason that we cannot find any examples of the first generation stars? a. the first generation stars are too faint to be visible now b. the first generation stars formed such a long time ago that the light from them has not yet had time to reach us c. the first generation stars were all very massive and exploded as supernova d. the first generation stars formed with only H and He and therefore have no spectral features
c
What is the smallest mass a newborn star can have? a. 0.10 mass of the sun b. 0.20 the mass of the sun c. 0.08 mass of the sun d. 0.01 the mass of the sun
c
What new method have astronomers utilized to find merging black holes and neutron stars at very great distances from the Earth? a. x-ray bursts b. gamma-ray bursts c. gravitation waves d. infrared variability
c
What prevents the pressure from increasing as a cloud contracts due to its gravity? a. as the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup b. the pressure is transferred from the center of the cloud to its outer edges where it can dissipate c. thermal energy is converted to radiative energy by molecules like CO and released as photons
c
What prevents the pressure from increasing as a cloud contracts due to its gravity? a. as the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup b. the pressure is transferred from the center of the cloud to its outer edges where it can dissipate c. thermal energy is converted to radiative energy by molecules like CO and released as photons d. excess pressure is released in jets of material from young stars
c
What slows down the contraction of a star-forming cloud when it makes a protostar? a) production of fusion energy b) magnetic fields c) trapping of thermal energy inside the protostar
c
What would happen if the Sun suddenly became a black hole without changing its mass? a) The black hole would quickly suck in Earth. b) Earth would gradually spiral into the black hole. c) Earth would remain in the same orbit.
c
Where do gamma-ray bursts come from? a) neutron stars in our galaxy b) binary systems that also emit X-ray bursts c) extremely distant galaxies
c
Which event marks the beginning of a supernova? a. the onset of helium burning after a helium flash in a star with mass comparable to that of the sun b. the sudden outpouring of X rays from a newly formed accretion disk c. the sudden collapse of an iron core into a compact ball of neutrons d. the beginning of neon burning in an y massive star
c
Which kinds of star are most common in a newly formed star cluster? a) O stars b) G stars c) M stars
c
Which of the following statements about novae is not true? A) A star system that undergoes a nova may have another nova sometime in the future. B) A nova involves fusion taking place on the surface of a white dwarf. C) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now. D) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova. E) The word nova means "new star" and originally referred to stars that suddenly appeared in the sky, then disappeared again after a few weeks or months.
c
Which of these black holes exerts the weakest tidal force on an object near its event horizon? a) a 10 Msun black hole b) a 100 Msun black hole c) a 10^6 Msun black hole
c
Which of these objects has the largest radius? a) a 1.2 MSun white dwarf b) a 1.5 Msun neutron star c) a 3.0 Msun black hole
c
Why are Cepheid variables so important for measuring distances in astronomy? a. they all have the same luminosity b. they all have the same period c. their luminosity can be inferred from their period d. they are close enough to have a detectable parallax
c
You discover a binary star system in which one member is a 15Msun main-sequence star and the other star is a 10Msun giant. Why would you be surprised, at least at first? a. it doesn't make sense to find a giant in a binary star system c. the two stars in a binary system should both be at the same point in stellar evolution c. the two stars should be the same age, so the more massive one should have become a giant first
c
an H-R diagram for a globular cluster will show a horizontal branch - 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? a. they have inert (non-burning) carbon cores b. their sole source of energy is hydrogen shell burning c. they generate energy through both hydrogen fusion and helium fusion d. in a particular star cluster, all horizontal branch stars have the same spectral type
c
1) What is the upper limit to the mass of a white dwarf? A) There is no upper limit. B) There is an upper limit, but we do not yet know what it is. C) 2 solar masses D) 1.4 solar masses E) 1 solar mass
d
After a massive star supernova event, what is left behind? a. always a white dwarf b. always a neutron star c. either a white dwarf or a neutron star d. either a neutron star or a black hole
d
After a supernova event, what is left behind? a. always a white dwarf b. either a white dwarf or a neutron star c. always a black hole d. either a neutron star or a black hole
d
In order to predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star? a. its luminosity b. its overall abundance of elements heavier than helium c. how much oxygen it now has in its core d. its mass
d
What is a helium flash? a. the ignition of helium shell burning in a high-mass star with a carbon core b. a sudden brightening of a low-mass star, detectable from Earth by observing spectral lines of helium c. it is another name for the helium fusion reaction d. the sudden onset of helium fusion in the core of a low-mass star
d
What kind of star is most likely to become a white-dwarf supernova? a. an O star b. a star like our Sun c. a binary M star d. a white dwarf star with a red giant binary companion e. a pulsar
d
When does a protostar become a true star? a. when the star is 1 million years old b. when the central temperature reaches 1 million Kelvin c. when the thermal energy becomes trapped in the center d. when nuclear fusion begins in the core
d
Which of the following sequences correctly describes the stages of life for a low-mass star? a. red giant, protostar, main-sequence, white dwarf b. white dwarf, main-sequence, red giant, protostar c. protostar, red giant, main-sequence, whtie dwarf d. protostar, main-sequence, red giant, white dwarf
d
Which of the following statements about black holes is not true? A) If you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits (or reflects) becomes more and more redshifted. B) If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls nearer to the black hole. C) A black hole is truly a hole in spacetime, through which we could leave the observable universe. D) If the Sun magically disappeared and was replaced by a black hole of the same mass, Earth would soon be sucked into the black hole. E) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.
d
the vast majority of stars in a newly formed star cluster are a. very high mass, type O and B stars b. red giants c. about the same mass as our Sun d. less massive than our Sun
d