Chapter 14: Neutron Stars and Black Holes

neutron star

A small, highly dense star composed almost entirely of tightly packed neutrons; radius about 10 km.

pulsar

A source of short, precisely timed radio bursts; thought to be a spinning neutron star.

lighthouse model

The explanation of a pulsar as a spinning neutron star sweeping beams of radio radiation around the sky.

pulsar wind

The flow of high-energy particles that carries most of the energy away from a spinning neutron star.

X-ray bursters

An object that produces occasional X-ray flares. Thought to be caused by mass transfer in a closed binary star system.

millisecond pulsars

A pulsar with a period of approximately a millisecond, a thousandth of a second.

singularity

The object of zero radius into which the matter in a black hole is thought to fall.

black hole

A mass that has collapsed to such a small volume that its gravity prevents the escape of all radiation; also, the volume of space from which radiation may not escape.

event horizon

The boundary of the region of a black hole from which no radiation may escape. No event that occurs within the event horizon is visible to a distant observer.

Schwarzschild radius

The radius of the event horizon around a black hole.

time dilation

The slowing of moving clocks or clocks in strong gravitational fields.

gravitational redshift

The lengthening of the wavelength of a photon its escape from a gravitational field.

gamma-ray bursts (GRBs)

A sudden burst of gammarays thought to be associated with neutron stars and black holes.

hypernova

The explosion produced as a very massive star collapses into a black hole; thought to be responsible for at least some gamma-ray bursts.

magnetars

A class of neutron stars that have exceedingly strong magnetic fields; thought to be responsible for soft gamma-ray repeaters.

How are neutron stars and white dwarfs similar?

• Both are compact objects.

• Both eventually cease to shine.

• Both are faint and difficult to detect.

• Both are produced by dying stars.

How are neutron stars and white dwarfs different?

White dwarfs are produced by the final gravitational contraction of the cores of low- to medium-mass stars. Neutron stars are produced from the collapsing core of a massive star as it undergoes a supernova explosion.

Why do neutron stars have an upper mass limit?

There is a maximum mass that can be supported against gravity by the outward pressure of degenerate neutrons.

Why do you expect neutron stars to spin more rapidly than white dwarfs?

As a massive star collapses it must rotate faster to conserve angular momentum. Neutron stars spin more rapidly because they are smaller in diameter than white dwarfs.

If neutron stars have hot surface temperatures, why aren't they very luminous?

Although neutron stars are hot, they are very small and have little surface area from which to radiate, so their luminosity is low.

Why do you expect neutron stars to have a more powerful magnetic field than a white dwarf?

When a star collapses into a neutron star, its magnetic field is squeezed into a small volume. Because neutron stars are smaller than white dwarfs, they have a more powerful magnetic field.

How did astronomers conclude that pulsars actually could not be pulsating stars?

Normal stars are much too large to pulse with a period as small as the period of a pulsar.

Why would astronomers naturally assume that the first discovered millisecond pulsar was relatively young?

When a pulsar first forms, it is spinning fast, and its rotation begins to slow as it radiates energy into space.

If the Sun has a Schwarzschild radius, why isn't it a black hole?

Not all of the Sun's mass is inside its Schwarzschild radius.

Can the Sun ever become a black hole? Why or why not?

No, the Sun's mass is too small.

In what sense is a black hole actually black?

It absorbs all light within the Schwarzschild radius and emits no light itself.

How can mass transfer into a compact object produce jets of high-speed gas?

Gas in an accretion disk is accelerated to high speeds and can interact with the compact object's magnetic field creating powerful jets of excited gas.

How can mass transfer into a compact object produce X-ray bursts?

Gas that flows from the accretion disk of a neutron star down to the surface of the neutron star accumulates in a dense layer that becomes degenerate until it ignites helium fusion to produce a burst of X-rays. The bursts repeat every time a large enough layer of degenerate fuel accumulates.

How can mass transfer into a compact object produce gamma-ray bursts?

Mass transferred to a neutron star as the result of a merger between it and another neutron star or a black hole can cause a violent explosion resulting in a gamma-ray burst.

The event horizon is which of the following?

• A sphere of radius RS around a black hole.

• The boundary that separates a black hole from the observable universe.

• The boundary beyond which events are undetectable by outside observers.

Which of the following is true of a billion-solar-mass black hole?

• It would fit inside the orbit of Mercury.

• It would have an RS of about 0.02 AU.

• It might be found in the core of a galaxy.

Not every object is a black hole, but every object has which of the following?

a Schwarzschild radius

A mass can be a black hole if which of the following is true?

It is inside its event horizon.

Stars with masses like those of the sun eventually become which of the following?

white dwarfs

Sun-like stars eject which of the following?

planetary nebulae

A white dwarf is supported by which of the following?

degenerate electrons

High-mass stars can eject which of the following?

supernovae remnants

A neutron star is supported by which of the following?

degenerate neutrons

The most massive stars are thought to explode as supernovae and leave behind which of the following?

black holes

The end state that an evolving star eventually reaches depends on which of its following attributes?

mass

Consider the following objects: black hole singularity, neutron star, white dwarf.

Rank the objects in order of increasing mass (from least to greatest). (Assume the objects are equal in diameter.)

white dwarf, neutron star, black hole singularity

Consider the following objects: black hole singularity, neutron star, white dwarf.

Rank the objects in order of increasing size (from smallest to largest). (Assume the objects are of equal mass.)

black hole singularity, neutron star, white dwarf

Consider the following objects: black hole singularity, neutron star, white dwarf.

Rank the objects in order of decreasing density (from greatest to least).

black hole singularity, neutron star, white dwarf