Cosmic Perspective Chapter 18

What is the upper limit to the mass of a white dwarf?

1.4 solar masses

How does a 1.2 solar mass white dwarf compare to a 1.0 solar mass white dwarf?

it has a smaller radius

Which of the following is closest in size (radius) to a white dwarf?

Earth

What kind of pressure supports a white dwarf?

electron degeneracy pressure

What is the ultimate fate of an isolated white dwarf?

it will cool down and become a cold black dwarf

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?

the white dwarf will explode completely as a white dwarf supernova

How is an X-ray burst (in an X-ray binary system) similar to a nova?

both involve explosions on the surface of stellar corpse

Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?

the spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white dwarf supernova does not

Which of the following is closest in size (radius) to a neutron star?

a city

From an observational standpoint, what is a pulsar?

an object that emits flashes of light several times per second or more, with near perfect regularity

From a theoretical standpoint, what is a pulsar?

a rapidly rotating neutron star

What do we mean by the singularity of a black hole?

it is the center of a black hole, a place of infinite density where the known laws of physics cannot describe the conditions

Which of these objects has the smallest radius? a 1.2M(sun) white dwarf, a 0.6M(sun) white dwarf, or Jupiter

a 1.2M(sun) white dwarf

Which of these objects has the largest radius? a 1.2M(sun) white dwarf, a 1.5M(sun) neutron star, or a 3.0M(sun) black hole

3.0M(sun) black hole

If we see a nova, we know that we are observing

a white dwarf in a binary system.

What would happen if the Sun suddenly became a black hole without changing its mass?

Earth would remain in the same orbit

What neutrons tar must have had its angular momentum changed by a binary companion?

a pulsar that pulses 600x per second

Viewed from a distance, how would a flashing red light appear as it fell into a black hole?

its flashes would shift to the infrared part of the spectrum

Which of these black holes exerts the weakest tidal force on an object near its event horizon? a 10M(sun) black hole, a 100M(sun) black hole, or a 10^6M(sun) black hole

a 10^6 M(sun) black hole

Where do gamma ray bursts tend to come from?

extremely distant galaxies

Why do more massive white dwarfs have smaller radii?

Greater mass means greater gravitational pressure, squeezing the matter tighter due to electron degeneracy pressure.

What causes a nova?

Accreted hydrogen on the white dwarf's surface reaches fusion temperature and ignites explosively—but leaves the white dwarf intact.

How were neutron stars discovered?

Jocelyn Bell detected regular radio pulses in 1967, later identified as rotating neutron stars—pulsars.

Why do pulsars appear to “pulse”?

Because their magnetic poles (where radiation beams emerge) are misaligned with the rotation axis, sweeping like a lighthouse.

What happens if a companion's orbital direction is opposite to a neutron star’s rotation?

Conservation of angular momentum causes the neutron star’s rotation to slow down.

How are X-ray bursts from neutron stars different from novae on white dwarfs?

They result from helium fusion, not hydrogen, and emit X-rays rather than visible light.

What role does friction play in an accretion disk?

It transfers angular momentum outward, causing infall and heating the disk.

What defines the event horizon of a black hole?

It's the boundary where the escape velocity equals the speed of light—nothing can escape.

What is the Schwarzschild radius formula?

Rs​= 2GM/c² ≈ 3 km × (M / M☉)

Why are tidal forces weaker near a supermassive black hole?

The gradient in gravity is smaller due to its large radius, making tidal effects gentler.

What happens to time near the event horizon?

Time slows down due to gravitational time dilation.

What happens when neutron stars merge?

They release gravitational waves, possibly form a black hole, and create heavy elements like gold and platinum.

What confirmed black hole mergers?

LIGO detected gravitational waves from merging 30-solar-mass black holes in 2015.

What are gamma-ray bursts (GRBs)?

Extremely powerful explosions observed in distant galaxies—some linked to neutron star mergers or massive stellar collapse.

How does the size of a 1 M☉ white dwarf compare to Earth?

Approximately the same size as Earth (radius ≈ 6,000 km)

What happens to the size of a white dwarf as its mass increases?

It decreases in size—more mass compresses the core further due to stronger gravity.

What triggers a nova?

Accreted hydrogen reaches fusion temperature (~10 million K) and ignites explosively on the white dwarf's surface.

What is the typical mass and size of a neutron star?

About 1.4–2.0 M☉ and ~10–20 km in radius

How fast can pulsars rotate?

Up to 1,000 times per second

What is the escape velocity from the Schwarzschild radius?

Equal to the speed of light (c)

What is the neutron star mass limit?

About 2–3 M☉; above this, neutron degeneracy pressure fails and the star collapses into a black hole