ASTR 342: Chapter 11 Neutron Stars and Black Holes

At what mass does a star's core become a black hole when the star dies? Which stars become black holes?

About 75% of the mass of the star is ejected into space in the supernova. The fate of the left-over core depends on its mass. If the left-over core is about 1.4 to 5 times the mass of our Sun, it will collapse into a neutron star. If the core is larger, it will collapse into a black hole. To turn into a neutron star, a star must start with about 7 to 20 times the mass of the Sun before the supernova. Only stars with more than 20 times the mass of the Sun will become black holes.

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

Because over time, pulsars and neutron stars slow down. The fastest ones are therefore younger.

What evidence can you cite that pulsars are neutron stars?

Only neutron stars are small enough to produces such short pulse lengths.

True or False: Medium mass stars end up as neutron stars

True

What is a singularity?

a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate.

Consider the following objects: black hole, neutron star, white dwarf.(a) Rank the objects in order of increasing mass. (b) Rank the objects in order of increasing size. (c) Rank the objects in order of decreasing density. (d) Rank the objects in order of increasing rotation rate.

(a) white dwarf>neutron star>black hole (b) black hole>neutron star>white dwarf (c) black hole>neutron star>white dwarf (d) white dwarf>neutron star>black hole

How does the lighthouse model explain pulsars?

1) A pulsar does not pulse but rather emits beams of radiation that sweep around the sky as the neutron star rotates. If the beams do not sweep over Earth, the pulses will not be detectable by Earth's radio telescopes.2) The mechanism that produces the beams involves extremely high energies and is not fully understood.3) Modern space telescopes observing from above Earth's atmosphere can image details around young neutron stars and even located isolated neutron stars whose beams of electromagnetic radiation do not sweep over Earth.

What is the Chandrasekhar mass limit? What does it have to do with neutron stars and black holes?

1.4 times the mass of our sun, Beyond this limit, stars at the end of their lives either explode into a supernova or explode and then collapse into a neutron star or even a black hole.

Suppose you weigh 160 pounds on Earth. About how much would you weigh on a neutron star?

30 trillion pounds

Which model most accurately describes the pulsar you see in the simulation?

A lighthouse with a spinning beam

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

A normal star, even a small white dwarf, is much too big to pulse that fast. Nor could a star with a hot spot on its surface spin fast enough to produce the pulses. Even a small white dwarf would fly apart if it spun 30 times a second.

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

After a massive star has exhausts its fuel source of lighter elements (hydrogen, helium), it becomes much hotter. As a result large jets of plasma are ejected very high speeds. Eventually the star runs out of fuel to counterbalance the effects of gravity, so it collapses in upon itself (this happens very fast). Then the subatomic particles repel each other forcing the stars outer layers to erupt into an amazing large supernova. The bursts from x-rays and gamma rays are released during this event.

How can we observe black holes if light can't escape from them?

Black holes can not be observed directly. We can only observe the accretion disk of infalling matter (gas and dust) surrounding the black hole. Usually x-rays.

What changes would occur if the sun were replaced with a one-solar-mass black hole?

Earth's orbit would not change and the planets would disappear from view.

True or False: High mass stars end up as white dwarfs.

False

True or False: Low mass stars end up as black holes

False

What is a GRB? What causes them?

Gamma Ray Bursts are short bursts of gamma radiation we detect with gamma ray telescopes in orbit. The source of these GRB's is thought to be from supernova explosions of distant stars.

How can a black hole emit X-rays?

It's not the black hole itself that emits x-rays. It's the accretion disk or other infalling matter that emits xrays, superheated by friction as it falls in.

What is a neutron star? What are some of its features and properties? What size stars become neutron stars?

Medium size stars become neutron stars when they die. A neutron star is a city-sized ball of neutrons with the mass of several suns. A neutron star is a very strange object. It is a trillion times denser than lead. Its gravity is billions of times stronger than Earth's. Some neutron stars have beams of energy streaming from them as they spin, just light a giant light-house in space. Neutron stars are usually observed in X-rays because they are so hot. Typical size: 10 km (6 miles) diameter Mass: 1.4 - 3 Msun # Neutrons: 1057 Density: r ~ 1014 g/cm3 Gravity: 190 Billion g's (1 g = Earth gravity) Magnetic Field: Trillion times Earth's Surface Temperature: ~ Million degrees

How are neutron stars and white dwarfs similar? How do they differ?

Neutron stars and white dwarfs are similar because they can have about the same mass but a white dwarf would be a different size. If a Neutron star containing a little over 1 solar mass compressed to a radius of about 10 km, a comparable white dwarf with the same mass would be about the size of Earth.A interior of a white dwarf is mostly carbon and oxygen nuclei immersed in a whirling storm of degenerate electrons.-A neutron star is just a core of neutrons left behind. If the collapsing core is more massive than the Chandrasekhar limit of 1.4 solar masses, then it cannot reach stability as a white dwarf because the weight is too great to be supported by degenerate electrons. (227)

If neutron stars are hot, why aren't they very luminous?

Neutron stars are hot but have very small surface area so their luminosity is very low. Their peak wavelength is in the x-rays.

Why is there an upper limit to the mass of neutron stars?

Neutron stars have an upper limit of about 3 M☉ because if they are any larger than that, then gravity overwhelms the strength of the neutrons and it collapses into a black hole.

Discuss the possible causes of gamma-ray bursts

Soft Gamma Ray Repeaters (SGRs) are Galactic events probably caused by neutron starquakes: shifts in the magnetic fields around highly magnetic neutron stars (magnetars). Higher energy Gamma Ray Bursts are extra-galactic events cause by one of two mechanisms: When two neutron stars are in a binary system they could loose orbital energy to gravitational waves, spiral in, and eventually collide. Hypernova: the largest stars may have too much gravity to produce a normal Type II Supernova. Instead when they collapse they could beam gamma rays out the poles and then get all collapsed into a black hole. If we're in line with the beam then we see a gamma ray burst.

If a black hole is so black, how can it emit electromagnetic radiation?

The electromagnetic radiation comes from the accretion disk, not the black hole

Neutron stars can be described in simple terms as a great big ball of solid neutrons. Where did all these neutrons come from, and why are there no other types of material inside it?

The neutrons came from the star's nuclei, whereas all the protons and electrons combined to form neutrons during the supernova explosion.

What evidence is there that black holes really exist?

The observation of binary X-ray sources with mass greater than 3 solar masses.

What does the short length of pulsar pulses tell you?

The short pulses and the discovery of the pulsar in the Crab Nebula were strong evidence that pulsars are neutron stars.

What is gravitational time dilation?

Time slows down in strong gravitational fields near massive objects.

True or False: All pulsars are believed to be neutron stars, but not all neutron stars are pulsars.

True, pulsars are a sub-set of neutron stars

What evidence can you cite that black holes really exist?

We can see binary systems where one object is unseen but highly massive: too massive for a neutron star. That leaves only one choice: a black hole.

Why do you expect neutron stars to spin rapidly?

When neutron stars form, they collapse to a very small radius, which means they will spin fast because any small rotation they had before the collapse gets greatly magnified due to the conservation of angular momentum.

Why do you expect neutron stars to have a powerful magnetic field?

When neutron stars form, they collapse to a very small radius. They have strong magnetic fields because the original magnetic field gets concentrated in this very small star.

Where did all the neutrons in a neutron star come from? Why no protons or electrons?

When stars four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons

Electromagnetic waves of what waveband are emitted by the accretion disk around a black hole?

X-Rays

What is a pulsar?

a rapidly pulsing radio source, that spins very fast. It is a form of a neutron star.

What is the lower limit for the mass of neutron stars?

about 1.4 solar masses

What prevents neutron stars from contracting to a smaller size?

degenerate neutrons

How can a neutron star in a binary system generate X-rays?

mass flows into a hot accretion disk around the star

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

not all of the sun's mass is inside its schwarzchild radius.

What is an event horizon? What is the escape velocity at the event horizon?

the boundary defining the region of space around a black hole from which nothing (not even light) can escape, the escape velocity for an object within the event horizon exceeds the speed of light, escape velocity is : The speed required for a projectile to escape from a body's gravitational field.

Why do we expect that neutron stars spin rapidly?

the law of conservation of angular momentum