Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.
answer
The radius of a white dwarf is determined by a balance between the inward force of gravity and the outward push of "electron degeneracy pressure."
A(n) "nova" occurs when hydrogen fusion ignites on the surface of a white dwarf in a binary system.
A(n) "massive star supernova" occurs when fusion creates iron in the core of a star.
A white dwarf in a close binary system will explode as a supernova if it gains enough mass to exceed the "white dwarf limit (1.4 solar masses)".
A(n) "accretion disk" consists of hot, swirling gas captured by a white dwarf (or neutron star or black hole) from a binary companion star.
A(n) "white dwarf supernova" can occur only in a binary system, and all such events are thought to have the same luminosity.
question
Match the items below with the correct type of supernova.
answer
WD: Can only occur in a binary system. Spectra always lack strong hydrogen lines. Can occur in a very old star cluster. Star explodes completely, leaving no compact object behind. Has a brighter peak luminosity.
MSS: Black hole or neutron star left behind. Can only occur in a galaxy with ongoing star formation.
question
Each item below describes an observation of a hypothetical supernova. Classify each observation as either "Not surprising" if it fits in with our current understanding of supernovae, or "Surprising" if the observation would cause us to rethink our understanding of supernovae.
answer
Not surprising: A white dwarf supernova in a galaxy of only old stars. Two massive star supernovae occur in the same young star cluster. A massive star supernova leaves behind no detectable compact object. A massive star in a binary system explodes.
Surprising: An isolated star like our Sun explodes as a white dwarf supernova. A young (5 million years) star explodes as a white dwarf supernova.
question
Listed following are several astronomical objects. Rank these objects based on their diameter, from largest to smallest. (Note that the neutron star and black hole in this example have the same mass to make your comparison easier, but we generally expect black holes to have greater masses than neutron stars.)
answer
main-sequence star of spectral type A, Jupiter, a one-solar-mass white dwarf, the Moon, a two-solar-mass neutron star, the event horizon of a two-solar-mass black hole
question
Listed following are several astronomical objects. Rank these objects based on their mass, from largest to smallest. (Be sure to notice that the main-sequence star here has a different spectral type from the one in Part A.)
answer
a typical black hole (formed in a supernova), a typical neutron star, a one-solar-mass white dwarf, main-sequence star of spectral type M, Jupiter, the Moon
question
Listed following are several astronomical objects. Rank these objects based on their density, from highest to lowest.
answer
the singularity of a black hole, a typical neutron star, a one-solar-mass white dwarf, a main-sequence star
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From the viewpoint of an observer in the orbiting rocket, what happens to time on the other rocket as it falls toward the event horizon of the black hole?
answer
Time runs increasingly slower as the rocket approaches the black hole.
question
As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket __________.
answer
slows down as it approaches the event horizon, and never actually crosses the event horizon
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From Part B, you know that from afar we'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?
answer
Its light will become so redshifted that it will be undetectable.
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If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________.
answer
at a constant, normal rate as you approach the event horizon
question
If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would __________.
answer
accelerate as you fall and cross the event horizon completely unhindered
question
Listed following are distinguishing characteristics of different end states of stars. Match these to the appropriate consequence of stellar death.
answer
WD: in a binary system, it can explode as a supernova. has mass no greater than 1.4 MSun. typically about the size (diameter) of Earth. supported by electron degeneracy pressure.
NS: sometimes appears as a pulsar. usually has a very strong magnetic field.
BH: viewed from afar, time stops at its event horizon. size defined by its Schwarzschild radius.
question
The following items describe observational characteristics that could indicate that an object is either a white dwarf or a neutron star. Match each characteristic to the correct object.
answer
WD: may be surrounded by a planetary nebula. emits most strongly in visible and ultraviolet. may be in a binary system that undergoes nova explosions.
NS: may be in a binary system that undergoes X-ray bursts. can have a mass of 1.5 solar masses. may be surrounded by a supernova remnant. may repeatedly dim and brighten more than once per second.
question
The following items describe observational characteristics that may indicate that an object is either a neutron star or a black hole. Match each characteristic to the correct object; if the characteristic could apply to both types of object, choose the bin labeled "Both neutron stars and black holes."
answer
NS: may emit rapid pulses of radio waves. may be in a binary system that undergoes X-ray bursts.
BH: is detectable only if it is accreting gas from other objects. can have a mass of 10 solar masses.
Both: may be located in an X-ray binary. may be surrounded by a supernova remnant.
question
The Chandra X-Ray Observatory has detected X rays from a star system that contains a main-sequence star of spectral type B6. The X-ray emission is strong and fairly steady, and no sudden bursts have been observed. Which of the following statements are reasonable conclusions about this system?
Check all that apply.
-The main-sequence star must orbit a white dwarf.
-The main-sequence star orbits either a white dwarf or a neutron star.
-The main-sequence star must orbit a black hole.
-The main-sequence star is emitting X rays.
-The main-sequence star orbits either a neutron star or a black hole.
-The main-sequence star must orbit a neutron star.
-Gas from the main-sequence star makes an accretion disk around another object.
-Some time in the next few decades, this system will undergo a nova explosion.
answer
-The main-sequence star orbits either a neutron star or a black hole.
-Gas from the main-sequence star makes an accretion disk around another object.
question
What is the key observation needed to determine whether the compact object in Part C is a neutron star or a black hole?
-Measure Doppler shifts in the spectrum of the main-sequence star so that you can determine the mass of the compact object.
-Study the X-ray emission to determine the temperature of the gas in the accretion disk.
-Obtain high-resolution images of the compact object, so that you can determine whether it emits any light.
answer
-Measure Doppler shifts in the spectrum of the main-sequence star so that you can determine the mass of the compact object.
question
Listed below are some characteristics of flat, spherical, and saddle-shaped geometries. Match each of these characteristics to the corresponding geometry.
answer
Flat: sum of angles of a triangle is always 180. circumference of a circle is exactly 2Οr.
Spherical: sum of angles of a triangle is greater than 180. straightest possible path is a segment of a great circle. lines that are initially parallel eventually converge.
Saddle-shaped: sum of angles of a triangle is less than 180. lines that are initially parallel eventually diverge.
question
The figures below show several different astronomical objects. Rank the objects based on the amount that spacetime is curved (relative to flat spacetime) very near the surface (or event horizon) of each of the objects, from least to greatest.
answer
red giant, main-sequence star, white dwarf, neutron star, black hole
question
The figures below show the same astronomical objects as shown in Part A. Rank the objects based on the amount that each would deflect the path of a photon of light from a very distant galaxy (as viewed from Earth), assuming the photon passes very near the surface (or event horizon) of each object, from smallest to largest.
answer
red giant, main-sequence star, white dwarf, neutron star, black hole
question
The figures below show several different astronomical objects. Rank the objects based on the strength of the gravitational force that would be felt by a spacecraft traveling at a distance of 10 AU from the center of each of the objects, from weakest to strongest. If the gravitational force is equal for two (or more) cases, show this equality by dragging one figure on top of the other(s).
answer
all the same
question
The figures below show several different astronomical objects. Rank the objects based on the amount that spacetime is curved (relative to flat spacetime) at a distance of 10 AU from the center of each of the objects, from least to greatest. If two (or more) cases are equal, show this equality by dragging one figure on top of the other(s).
answer
all the same
question
The figures below show several different astronomical objects. Rank the objects based on the amount that spacetime is curved (relative to flat spacetime) very near the surface (or event horizon) of the objects, from least to greatest.
answer
red giant, the Sun, white dwarf, black hole
question
The figures below show several different astronomical objects. Rank the objects based on the acceleration a spaceship would have as it passed very near the surface (or event horizon) of each object, from smallest to largest.
answer
red giant, the Sun, white dwarf, black hole
question
Click on "Show Orbits" and choose an elliptical orbit. Where does an object on an elliptical orbit experience the greatest acceleration?
answer
where spacetime has the most curvature
question
Imagine that the Sun could be turned into a black hole without changing its mass. How would Earth's orbit change?
answer
Earth's orbit would not change.
question
Imagine that the Sun gained mass without changing its radius. How would the structure of spacetime change at the distance of Earth's orbit?
answer
Spacetime would become more curved at Earth's orbit.
question
The data in the figure show how the orbital period of the two neutron stars changes with time. Based on these data, what can we conclude?
answer
shorter and the two neutron stars are moving closer together
question
Einstein's general theory of relativity predicts that two orbiting neutron stars should radiate gravitational waves. How does the fact that the orbital period is getting shorter support this prediction?
answer
Gravitational waves must carry some energy away from the system, and this loss of energy must cause the orbits to get smaller over time.
question
Notice how closely the actual data points match the theoretical prediction (red curve). Which of the following statements are supported by this close match of prediction and data?
answer
-We can continue to test the predictions of general relativity by continuing to measure the orbital period of this system in the future.
-These data provide strong evidence for the existence of gravitational waves as predicted by the general theory of relativity.
-If gravitational waves really are responsible for the decay in this system, we should see similar decay in other systems with two neutron stars.
question
Suppose the binary neutron star system did not emit gravitational waves. Which of the following shows how the data would look in that case?
answer
Cumulative change in orbital period should be 0. Straight red horizontal line across the graph.
question
Match the terms in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each term only once.
answer
The "event horizon" is the boundary of a black hole beyond which a spacecraft could not even in principle transmit a radio signal to observers outside the black hole.
A star that happens to be hidden behind the Sun might become viewable during a solar eclipse due to the bending of light known as "gravitational lensing".
According to relativity, at the center of a(n) "black hole" spacetime becomes infinitely curved.
Two neutron stars orbiting each other at close distance will gradually spiral in toward each other because they lose orbital energy to "gravitational waves".
If a(n) "wormhole" can really exist, it might be able to connect two widely separated locations in the universe.
"Gravitational time dilation" is the slowing of time that you observe when you look at an object with a strong gravitational field.
Spectral lines of hydrogen on a white dwarf appear at longer wavelengths than the same lines observed in the spectrum of the Sun, illustrating what we call "gravitational redshift".
question
Which of the following statements about degeneracy pressure is not true?
answer
Degeneracy pressure can arise only from interactions among electrons.
question
The more massive a white dwarf, the _________.
answer
smaller its radius
question
Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
answer
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.
question
The white dwarf that remains when our Sun dies will be mostly made of ______.
answer
carbon
question
Which statement about accretion disks is not true?
answer
The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
question
According to present understanding, a nova is caused by _________.
answer
hydrogen fusion on the surface of a white dwarf
question
Which of the following is not true about differences between novae and supernovae?
answer
Supernovae eject gas into space but novae do not.
question
Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
answer
No, because it is not orbited by another star.
question
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?
answer
The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
question
Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star?
answer
Pulsars are easy to identify by their almost perfectly steady periods of pulsation.
question
Which statement about pulsars is not thought to be true?
answer
Pulsars can form only in close binary systems.
question
How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
answer
The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
question
Which statement concerning black hole masses and Schwarzschild radii is not true?
answer
In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
question
Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
answer
Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.
question
Which of statement below about black holes is not true?
answer
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.
question
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?
answer
intense X-ray bursts
question
Which of the following observatories is most likely to discover a black hole in a binary system?
answer
the Chandra X-Ray Observatory
question
Which of the following statements about gamma ray bursts is not true?
answer
The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
question
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 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because ________.
answer
the red giant will probably supernova within the next million years
question
Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) 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 radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least to start the process of killing him first)?
answer
X rays from the accretion disk
question
Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) 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 billion years ago. Which of the following statements should you expect to find as you continue to read this book?
answer
They evolved on a different planet in a different star system, and moved to their current location.
question
A white dwarf is _________.
answer
what most stars become when they die
question
A typical white dwarf is _________.
answer
as massive as the Sun but only about as large in size as Earth
question
If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh _________.
answer
as much as a truck
question
The maximum mass of a white dwarf is _________.
answer
about 1.4 times the mass of our Sun
question
What is an accretion disk?
answer
a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
question
According to our modern understanding, what is a nova?
answer
an explosion on the surface of a white dwarf in a close binary system
question
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?
answer
The white dwarf will explode completely as a white dwarf supernova.
question
A neutron star is _________.
answer
the remains of a star that died in a massive star supernova (if no black hole was created)
question
A typical neutron star is more massive than our Sun and about the size (radius) of _________.
answer
a small asteroid (10 km in diameter)
question
If you had something the size of a sugar cube that was made of neutron star matter, it would weigh _________.
answer
about as much as a large mountain
question
Pulsars are thought to be _________.
answer
rapidly rotating neutron stars
question
How is an X-ray burst (in an X-ray binary system) similar to a nova?
answer
Both involve explosions on the surface of stellar corpse.
question
What is the basic definition of a black hole?
answer
an object with gravity so strong that not even light can escape
question
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _________.
answer
3 solar masses
question
What do we mean by the event horizon of a black hole?
answer
It is the point beyond which neither light nor anything else can escape.
question
Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit?
answer
Nothing - Earth's orbit would remain the same.
question
What do we mean by the singularity of a black hole?
answer
It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
question
What makes us think that the star system Cygnus X-1 contains a black hole?
answer
It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
question
The Schwarzschild radius of a black hole depends on ________.
answer
only the mass of the black hole
question
Scientists have detected thousands of gamma ray bursts. The evidence suggests that most or all of these bursts _________.
answer
have occurred in distant galaxies
question
Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
answer
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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