Transcript Black Holes and Neutron Stars

Black Holes and
Neutron Stars
Dead Stars
Copyright – A. Hobart
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Goals
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What are neutron stars and pulsars?
What are black holes?
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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Quark Stars
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Neutrons (and protons) are made of quarks.
Gravity could crush neutrons into free quarks.
Called strange matter (a type of quark).
Astronomers think they may have seen a quark star.
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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
Earth
Jupiter
Sun
Mass
Radius
cm 
 1M

R EH  3km  
M

300 x Earth
3 mSun 
6 x 1024 kg
300,000 x
Earth
3 km
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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: g-ray
bursts:
• Most violent explosions in the universe.
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