Chapter 13 The Bizarre Stellar Graveyard

13.1 White Dwarfs Our Goals for Learning What is a white dwarf? What can happen to a white dwarf in a close binary system?

What is a white dwarf?

White dwarfs are the cores of dead stars exposed after they shed their outer layers in planetary nebulae. Which type of stars turn into white dwarfs at the end of their lives? What are the white dwarfs made of? Sirius A, the brightest star on our sky, and Sirius B, its binary companion, which is a white dwarf star. X ray image (which of the two stars is more luminous in X ray?).

What supports them from collapsing under the pressure of gravity? White dwarfs cool off and grow dimmer with time.

What supports them from collapsing under the pressure of gravity? Electron degeneracy pressure. Because of the laws of Quantum Mechanics, electrons can be compressed only up to a some point. Compressing them, one makes them move faster and faster. White dwarfs cool off and grow dimmer with time.

A white dwarf of Suns mass is about the same size as Earth. And the earth is of the size of a typical Sun Spot – matter in white dwarfs is very dense. One teaspoon of this matter would weigh several tons on earth.

White dwarfs shrink when you add mass to them because their gravity gets stronger.

Shrinkage of White Dwarfs Quantum mechanics says that electrons in the same place cannot be in the same state Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space In order to avoid being in the same state some of the electrons need to move faster Is there a limit to how much you can shrink a white dwarf?

The White Dwarf Limit Einstein’s theory of relativity says that nothing can move faster than light When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can no longer support it Chandrasekhar found (at age 20!) that this happens when a white dwarf’s mass reaches 1.4 M sun. 1.4 M sun is the maximal mass of white dwarf! S. Chandrasekhar

What can happen to a white dwarf in a close binary system?

In a close binary system, gas from a companion star can spill toward a white dwarf, forming a swirling accretion disk around it.

Hydrogen that accretes onto a white dwarf star builds up in a shell on the surface. When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova.

Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space

Nova or Supernova? Supernovae are MUCH MUCH more luminous!!! (about 10 million times) Nova: H to He fusion of a layer, white dwarf left intact

What have we learned? What is a white dwarf? A white dwarf is the core left over from a low-mass star, supported against the crush of gravity by electron degeneracy pressure. What can happen to a white dwarf in a close binary system? A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk. As hydrogen builds up on the white dwarf’s surface, it may ignite with nuclear fusion to make a nova.

13.2 Neutron Stars Our Goals for Learning What is a neutron star? How were neutron stars discovered? What can happen to a neutron star in a close binary system?

What is a neutron star?

A neutron star is the ball of neutrons left behind by a massive-star supernova What supports a neutron star against gravity?

Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. Degeneracy pressure of neutrons!

A neutron star is about the same size as a small city (10 km in size). Neutron stars are essentially giant atomic nuclei made almost entirely of neutrons and held together by gravity.

How were neutron stars discovered?

The first neutron star was discovered by Bell Burnell in Using a radio telescope she noticed very regular pulses of radio emission coming from a single part of the sky.

Pulsars are neutron stars that give off very regular pulses of radiation.

Pulsar at center of Crab Nebula pulses 30 times per second

Pulsars are rotating neutron stars that act like lighthouses Beams of radiation coming from poles look like pulses as they sweep by Earth

A pulsar’s rotation is not aligned with magnetic poles. Neutron stars rotate fast (why?). They also have strong magnetic fields. The collapse of an iron core bunches the magnetic field lines running through the core far more tightly, which intensifies the magnetic fields. What is the main cause of magnetic field? What could it be in a neutron star?

Why Pulsars must be Neutron Stars Circumference of NS = 2π (radius) ~ 60 km Spin Rate of Fast Pulsars ~ 1000 cycles per second Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS Anything else would be torn to pieces!

Pulsars spin fast because core’s spin speeds up as it collapses into neutron star Conservation of angular momentum

Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No

Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No

What happens to a neutron star in a close binary system?

Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary

Accreting matter adds angular momentum to a neutron star, increasing its spin Episodes of fusion on the surface lead to X-ray bursts

Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? A.The neutron star’s rotation would speed up. B.The neutron star’s rotation would slow down. C.Nothing, the directions would cancel each other out.

Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? A.The neutron star’s rotation would speed up. B.The neutron star’s rotation would slow down. C.Nothing, the directions would cancel each other out.

Thought Question If you dropped a mountain onto the surface of a neutron star, what would happen? A.It would heat up (conservation of energy) B.It would flatten out (gravitational compression) C.It would bounce off. D.It would turn into pure neutrons and emit neutrinos.

Thought Question If you dropped a mountain onto the surface of a neutron star, what would happen? A.It would heat up (conservation of energy) B.It would flatten out (gravitational compression) C.It would bounce off. D.It would turn into pure neutrons and emit neutrinos.

What have we learned? What is a neutron star? A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova. How were neutron stars discovered? Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the existence of neutron stars.

What have we learned? What can happen to a neutron star in a close binary system? Neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot accretion disks. The hot gas emits strongly in X rays, so we see these systems as X-ray binaries. In some of these systems, frequent bursts of helium fusion ignite on the neutron star’s surface, emitting X-ray bursts.

13.3 Black Holes: Gravity’s Ultimate Victory Our Goals for Learning What is a black hole? What would it be like to visit a black hole? Do black holes really exist?

What is a black hole?

A black hole is an object whose gravity is so powerful that not even light can escape it.

Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same

Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same Hint:

Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same Hint:

Escape Velocity Initial Kinetic Energy Final Gravitational Potential Energy = = (escape velocity) 2 G x (mass) 2 (radius)

Light would not be able to escape Earth’s surface if you could shrink it to < 1 cm

The “surface” of a black hole is the radius at which the escape velocity equals the speed of light. This spherical surface is known as the event horizon. The radius of the event horizon is known as the Schwarzschild radius.

A black hole’s mass (or any other mass according to Einstein!) warps space and time in vicinity of event horizon. We use the term hole, because nothing exits from it – it is really a hole in the observable universe.

No Escape Nothing can escape from within the event horizon because nothing can go faster than light. No escape means there is no more contact with something that falls in. It increases the hole mass, changes the spin or charge, but otherwise loses its identity.

Neutron Star Limit Quantum mechanics says that neutrons in the same place cannot be in the same state Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 M sun Some massive star supernovae can make black hole if enough mass falls onto core.

Beyond the neutron star limit, no known force can resist the crush of gravity. As far as we know, gravity crushes all the matter into a single point known as a singularity.

3 M Sun Black Hole The event horizon of a 3 M Sun black hole is also about as big as a small city Neutron star

Thought Question How does the radius of the event horizon change when you add mass to a black hole? A. Increases B. Decreases C. Stays the same

Thought Question How does the radius of the event horizon change when you add mass to a black hole? A. Increases B. Decreases C. Stays the same

What would it be like to visit a black hole?

If the Sun shrank into a black hole, its gravity would be different only near the event horizon Black holes don’t suck!

Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift

Time passes more slowly near the event horizon

Thought Question Is it easy or hard to fall into a black hole? A. Easy B. Hard

Thought Question Is it easy or hard to fall into a black hole? A. Easy B. Hard Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus

Thought Question Is it easy or hard to fall into a black hole? B. Hard Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus

Tidal forces near the event horizon of a 3 M Sun black hole would be lethal to humans Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

Do black holes really exist?

Black Hole Verification Need to measure mass  Use orbital properties of companion  Measure velocity and distance of orbiting gas It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 M Sun )

Some X-ray binaries contain compact objects of mass exceeding 3 M Sun which are likely to be black holes

One famous X-ray binary with a likely black hole is in the constellation Cygnus

What have we learned? What is a black hole? A black hole is a place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can ever escape, not even light.

What have we learned? What would it be like to visit a black hole? You could orbit a black hole just like any other object of the same mass. However, you’d see strange effects for an object falling toward the black hole: –Time would seem to run slowly for the object –Its light would be increasingly redshifted as it approached the black hole. –The object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it.

What have we learned? Do black holes really exist? No known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses, and theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea.

13.4 The Mystery of Gamma-Ray Bursts Our Goals for Learning What are gamma ray bursts? What causes gamma ray bursts?

The most energetic events in the Universe- there luminosity can briefly outshine the luminosity of thousands to million Galaxies! Come randomly from all directions in space -> outside of our Galaxy. They appear about once a day.

What causes gamma ray bursts?

Gamma ray bursts may signal the births of new black holes

At least some gamma ray bursts come from supernovae in very distant galaxies – black hole formation is likely cause.

What have we learned? What causes gamma ray bursts? Gamma-ray bursts occur in distant galaxies and are the most powerful bursts of energy we observe anywhere in the universe. No one knows their precise cause, although at least some appear to come from unusually powerful supernovae.

Review questions

Which event marks the beginning of a supernova? A. The beginning of neon burning in an extremely massive star B. The sudden initiation of the CNO cycle C. The onset of helium burning after a helium flash D. The sudden collapse of an iron core into a compact ball of neutrons

A. We'd see a cloud of gas expanding away from the position where Betelgeuse used to be. Over a period of a few weeks, this cloud would fill our entire sky. B. Betelgeuse would suddenly appear to grow larger in size, soon reaching the size of the full Moon. It would also be about as bright as the full Moon. C. Because the supernova destroys the star, Betelgeuse would suddenly disappear from view. D. Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime. Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?

A spinning neutron star has been observed at the center of a _________. A. red supergiant B. protostar C. planetary nebula D. supernova remnant

A. Brown dwarfs would outnumber all ordinary stars. B. Brown dwarfs would be responsible for most of the overall luminosity of our Milky Way Galaxy. C. Brown dwarfs would be extremely rare. D. Most of the brown dwarfs in the Milky Way Galaxy would be quite young in age. We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?

A. Black holes form when gravity overcomes degeneracy pressure. B. Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives. C. Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold. D. Degeneracy pressure can only be created by interactions among the electrons in an object. Which of the following statements about degeneracy pressure is NOT true?

A. Pulsars are very bright and therefore easy to find. B. We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars. C. Several pulsars are located within a dozen light-years of our solar system, making them useful for finding our solar system. D. Pulsars are easy to identify by their almost perfectly steady periods of pulsation. The Voyager spacecraft has a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why do you think the scientists chose pulsars rather than some other type of star?