ch 11 neutron stars and black holes

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.

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 did scientists predict the existence of black holes?

If the collapsing core of a supernova has a mass greater than 3 solar masses, then degenerate neutrons cannot stop the contraction, and it must contract to a very small size—perhaps to a singularity, an object of zero radius. Near such an object, gravity is so strong that not even light can escape, and the region is called a black hole. The outer boundary of a black hole is the event horizon; no event inside is detectable. The radius of the event horizon is the Schwarzschild radius, amounting to only a few kilometers for a black hole of a few solar masses. Once matter falls into a black hole, it loses all of its properties except for mass, electrical charge, and angular momentum. Rotating black holes are described by a model called the Kerr solution.If the collapsing core of a supernova has a mass greater than 3 solar masses, then degenerate neutrons cannot stop the contraction, and it must contract to a very small size—perhaps to a singularity, an object of zero radius. Near such an object, gravity is so strong that not even light can escape, and the region is called a black hole. The outer boundary of a black hole is the event horizon; no event inside is detectable. The radius of the event horizon is the Schwarzschild radius, amounting to only a few kilometers for a black hole of a few solar masses. Once matter falls into a black hole, it loses all of its properties except for mass, electrical charge, and angular momentum. Rotating black holes are described by a model called the Kerr solution.

What is the evidence that black holes really exist?

If you were to leap into a black hole, your friends who stayed behind would see two relativistic effects. They would see your clock slow relative to their own clock because of time dilation. Also, they would see your light redshifted to longer wavelengths. You would not notice these effects, but you would feel powerful tidal forces that would deform and heat your mass until you grew hot enough to emit X rays. Any X rays your mass emitted before crossing the event horizon could escape. To search for black holes, you must look for binary star systems in which mass flows into a compact object and emits X rays. If the mass of the compact object is greater than about 3 solar masses, then the object cannot be a neutron star and is presumably a black hole. A number of such objects have been located.

how are neutron stars and white dwarfs similar and different

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. The major difference is due to the way in which they are formed. 1. White dwarfs are formed from the collapse of low mass stars, less than about 10 time the mass of the Sun. This star loses most of its mass in a wind, leaving behind a core that is less than 1.44 solar mass. On the other hand, neutron stars are formed in the catastrophic collapse of the core of a massive star. Other differences follow: 2. A white dwarf is supported by electron degeneracy pressure, a neutron star by neutron degeneracy pressure (go look those terms up for a quick physics lesson). 3. A white dwarf has a larger radius --about 600 times 4. A neutron star has a stronger gravitational field -about 400,000 times 5. Finally, neutron stars have higher temperatures at birth, spin faster, and have stronger magnetic fields, among other things.

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. The exact value is not well known because neutron star material is impossible to create in a laboratory environment. We have to rely completely on untested theory.

What evidence can you cite that pulsars are neutron stars?

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

how do astronomers know neutron stars really exist?

Pulsars, rapidly pulsing radio sources, were discovered in 1967, and were eventually understood to be spinning neutron stars. The discovery of a pulsar in the supernova remnant called the Crab Nebula was a key link in the story. Pulsars are evidently spinning neutron stars that emit beams of radiation from their magnetic poles. As they spin, they sweep the beams around the sky; if the beams sweep over Earth, pulses can be detected. A spinning neutron star slows as it radiates its energy into space. Most of the energy emitted by a pulsar is carried away as a pulsar wind. Dozens of pulsars have been found in binary systems, which allows astronomers to estimate the masses of the neutron stars. Such masses are consistent with the predicted masses of neutron stars. In some binary systems, mass flows into a hot accretion disk around the neutron star and causes the emission of X rays. The fastest pulsars, the millisecond pulsars, appear to be old pulsars that have been spun up to high speed by mass flowing from binary companions. Planets have been found orbiting at least one neutron star. They may be the remains of a companion star that was mostly devoured by the neutron star.

How can a black hole emit X-rays?

The X-rays come from hot gas orbiting around the black hole in an accretion disk. As the gas orbits, magnetic stresses cause it to lose energy and angular momentum, thus spiralling slowly in towards the black hole. The orbital energy is transformed into thermal energy, heating up the gas to millions of degrees, so it then emits blackbody radiation in the X-ray band. Once the gas gets closer than a few times the horizon radius, it plunges into the black hole, so while some X-rays can still escape just before the horizon, most are emitted a fair bit outside.

why wouldn't an accretion disk orbiting a giant star get as hot as an accretion disk orbiting a compact object

The particles in an accretion disc around a small compact object are moving faster and have more energy.

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.

how does theory predict the existence of neutron stars

When a supernova explodes, the core collapses to very small size. Theory predicts that protons and electrons will combine to form a degenerate neutron gas. The collapsing core cannot support itself as a white dwarf if its mass is greater than 1.4 solar masses, the Chandrasekhar limit. If its mass lies between 1.4 solar masses and about 3 solar masses, it can halt its contraction and form a neutron star. A neutron star is supported by the pressure of the degenerate gas of neutrons. Theory predicts that a neutron star should be about 10 km in radius, spin very fast because it conserves angular momentum as it contracts, have a high temperature, and have a powerful magnetic field.

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

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

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

Discuss the possible causes of gamma-ray bursts

shifts in magnetic fields of magnetars or the merger of binary neutron star pairs or neutron/black hole pairs

What evidence is there that black holes really exist

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