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Black Holes Black Holes and Neutron and Neutron

StarsStars

Dead Stars

Copyright – A. Hobart

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GoalsGoals

• What are neutron stars and pulsars?• What are black holes?• What happens near a black hole?• How do we see black holes?• What happens when neutron stars and

black holes are in binaries?

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Supernova Remnant

• Recall: In the death of a high-mass star, the core is converted to neutrons and collapses catastrophically.

• The collapse and rebound creates a supernova.

• But what happens to the neutrons already at the very center of the core?

• The central core is left behind as a small, dense, sphere of neutrons a neutron star.

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Neutron Stars

• A giant ball of neutrons.

• Mass : at least 1.4 x mass of the Sun.

• Diameter: 20 km!• Density: 1018 kg/m3

– A thimble weighs as much as a mountain• Day: 1 – 0.001 seconds!

• Magnetic fields as strong as the Sun, but in the space of a city.

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Pulsars• Interstellar

Lighthouses.• See periodic bursts

of radiation.• Perfect clocks.• While every pulsar

is a neutron star, the opposite isn’t true.

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Crab Nebula Pulsar

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Pulsar Motion

• Pulsars born in the center of supernovae explosions.

• Non-symmetric explosions lead to huge “kick.”

• Large velocity pulsars.

• v = 800 – 1000 km/s!Guitar Nebula – copyright J.M. Cordes

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Neutron Degeneracy

• Neutron stars are held up by neutron degeneracy pressure.– Recall electron degeneracy pressure for white

dwarfs.

– For white dwarfs, maximum mass of 1.4 Msun

• For neutron stars, maximum mass ~3Msun

• What happens if a high-mass star is SO big that its central core is bigger than this?

• What happens when gravity is stronger than even neutron degeneracy pressure?

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Density• Density = mass per volume• From Red Giant cores to White Dwarfs to

Neutron Stars, density has been increasing.• As density increases, the force of gravity on

the surface increases.• The greater the force, the higher the

escape velocity:– How fast you need to go in order to escape the

surface.

• How dense can something get?• How strong can the force of gravity be?• What if the escape velocity is faster than

light?

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Black Hole

• When a high-mass star’s core is greater than ~3 x Msun, then, when it collapses, the density becomes so high not even light can escape!

• The star collapses to form a Black Hole.

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The Event Horizon• Event Horizon = black hole “surface”

Object Mass Radius

Earth 6 x 1024 kg 1 cm

Jupiter 300 x Earth 3 m

Sun 300,000 x Earth

3 km

SunEH M

Mkm3R

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Curved Space• Einstein related

gravity forces to space curvature.

• Black holes deeply warp space.

• Everything falls in, nothing can climb out.

• How does this work?

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General Relativity2 Main Postulates:1. The speed of light is always c.

Thou shalt not add your speed to the speed of light!

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General Relativity

2. Accelerating reference frames are indistinguishable from a gravitational force.

The Star Tours ride at Disneyland is awesome!

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Gravity Bends Light

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Gravitational Lenses

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c

distancetime

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Gravity makes time slow

S

F

S

F

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Black Holes

• Light is bent by the gravity of a black hole.

• The event horizon is the boundary inside which light is bent into the black hole.

• Approaching the event horizon time slows down relative to distant observers.

• Time stops at the event horizon.

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Seeing Holes

• Can’t see black hole itself, but can see matter falling into a hole.

• Gravitational forces stretch and rip matter: heats up.

• Very hot objects emit in X-rays (interior of Sun)

• Cygnus X-1.http://www.owlnet.rice.edu/~spac250/steve/ident.html

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What would be a good source of material for a hungry black hole?

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Binaries• Gravitational tides pull matter off big low

density objects towards small high density objects.

Cygnus X-1

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Nova and Supernova• Similar situation with white dwarf-gas

giant pairs.• White dwarf accretes matter from giant.• If enough material falls fast enough it

will ignite and fuse on the w.d surface:• Nova!• If enough mass falls onto white dwarf

that Mwd > 1.4 x Msun:• White dwarf collapses to a neutron star.• Supernova! (now there are two types)

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Binary Pulsars

• Neutron stars can also be in binaries.

• General Relativity says they will eventually spiral into one another.

• Result: -ray bursts:

• Most violent explosions in the universe.