Mastering Astronomy: Neutron Stars, Black Holes

From Part B, you know that from afar you'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why? Even though you won't see it cross the event horizon, it does cross it, and that means you can no longer see it. Its light will become so redshifted that it will be undetectable. The black hole's blackness will drown out the light of the rocket. Tidal forces will squeeze the in-falling rocket to an undetectably thin line.

Its light will become so redshifted that it will be undetectable. (As the video shows, viewed from afar the light of the in-falling rocket becomes increasingly redshifted. As it approaches the event horizon, the redshift approaches infinity, meaning all its light is stretched to such enormous wavelengths that no detector could see it, even in principle.)

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. Obtain high-resolution images of the compact object, so that you can determine whether it emits any light. Study the X-ray emission to determine the temperature of the gas in the accretion disk.

Measure Doppler shifts in the spectrum of the main-sequence star so that you can determine the mass of the compact object. (The Doppler shifts will allow you to determine the speed (or at least the component of the speed that is in your line of sight) of the main-sequence star as it orbits the compact object. You can then use this speed along with the known mass of the main-sequence star to determine the compact object's mass (or a lower limit on its mass). If this mass is greater than the neutron star limit of about 3 solar masses, then the object must be a black hole.)

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."

Neutron Star: •may emit rapid pulses of radio waves •may be in a binary system that undergoes X-ray bursts. Black Hole: •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. (Be sure you understand why each of these observations goes with the indicated object(s). For example, radio pulses and X-ray bursts cannot come from black holes, because both are caused by events that happen on the surface of a compact object. Note also that the fact that two important items go in the "Both" bin indicates that it can be more difficult to distinguish between neutron stars and black holes than between white dwarfs and neutron stars.)

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? The main-sequence star orbits either a white dwarf or a neutron star. The main-sequence star orbits either a neutron star or a black hole. The main-sequence star must orbit a white dwarf. The main-sequence star is emitting X rays. Gas from the main-sequence star makes an accretion disk around another object. The main-sequence star must orbit a black hole. Some time in the next few decades, this system will undergo a nova explosion. The main-sequence star must orbit a neutron star.

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. (The system is an X-ray binary, which means the companion to the main-sequence star can be either a neutron star or a black hole. The X rays come from the hot accretion disk consisting of gas that the compact object's gravity is pulling away from the main-sequence star.)

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? Time runs increasingly faster as the rocket approaches the black hole. Time runs increasingly slower as the rocket approaches the black hole. Time is always the same on both rockets.

Time runs increasingly slower as the rocket approaches the black hole. (In this video, your view is similar to the view from the orbiting rocket. Notice that the clock on the falling rocket ticks more slowly as it approaches the black hole, indicating that time is slowing down.)

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.

White Dwarf: •may be surrounded by a planetary nebula •emits most strongly in visible and ultraviolet •may be in a binary system that undergoes nova explosions Neutron Star: •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 (Be sure you understand why each of these observations goes with the indicated object. For example, white dwarfs may be surrounded by planetary nebulae because they are the remains of low-mass stars, while neutron stars form only in supernovae.)

Listed following are distinguishing characteristics of different end states of stars. Match these to the appropriate consequence of stellar death.

White Dwarf: •supported by electron degeneracy pressure •typically about the size (diameter) of Earth •has a mass no greater than 1.4 MSun •in a binary system, it can explode as a supernova Neutron Star: •sometimes appears as a pulsar •usually has a very strong magnetic field Black Hole: •size defined by its Schwarzschild radius •viewed from afar, time stops at its event horizon

If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would __________. slow down and come to a stop at the event horizon slow down and cross the event horizon at low speed accelerate as you fall and cross the event horizon completely unhindered

accelerate as you fall and cross the event horizon completely unhindered (From your point of view, time runs normally in your spaceship and gravity must accelerate you as you fall toward the black hole. There is no physical barrier at the event horizon, so you cross it unhindered.)

If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________. increasingly faster as you approach the event horizon at a constant, normal rate as you approach the event horizon increasingly slower as you approach the event horizon

at a constant, normal rate as you approach the event horizon (All motion is relative, and you will always consider your own clock to be running at a normal rate.)

As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket __________. slows down as it approaches the event horizon and never actually crosses the event horizon slows down near the event horizon so that it crosses the event horizon at a low speed moves at constant speed as it approaches and crosses the event horizon accelerates as it falls and crosses the event horizon at high speed

slows down as it approaches the event horizon and never actually crosses the event horizon (Viewed from afar, time comes to a stop at the event horizon of a black hole, so falling objects never appear to cross the event horizon.)