Stellar Remnants
White Dwarfs, Neutron Stars & Black Holes
Sirius & Sirius B a White Dwarf Star
• These objects normally emit
light only due to their very high
temperatures.
• Normally nuclear fusion has
completely stopped.
• These are very small, dense
objects.
• They exist in states of matter
not seen anywhere on Earth.
They do not behave like normal
solids, liquids or gases.
• They often have very strong
magnetic fields and very rapid
spin rates.
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White Dwarfs
• composed mainly of Carbon
& Oxygen
• formed from stars that are
no more than 8 Solar masses
• White Dwarfs can be no
more than 1.4 Solar masses
and have diameters about
the size of the Earth (1/100
the diameter of the Sun).
• If a White Dwarf is in a
binary system and close
enough to its companion
A White Dwarf pulling material
star it may draw material off
off of another star in a binary system this star. This material can
then build up on the surface
of the White Dwarf.
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White Dwarfs in Binary Systems
• This material pulled off the
companion star is mostly
Hydrogen.
• As it accumulates on the star
it may become hot enough for
nuclear fusion to occur.
• The Hydrogen begins to fuse
and the White Dwarf emits a
bright burst of light briefly.
• We see this on Earth as a
nova.
• This process can repeat as
new material accumulates.
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Another Kind of Supernova
• If too much material accumulates the White Dwarf may
collapse.
• Rapid fusion reactions of Carbon & Oxygen begin. Carbon &
Oxygen fuse into Silicon and Silicon into Nickel.
• The energy from this event may cause the entire White Dwarf
to explode leaving nothing behind.
• This is called a supernova but it is a different process from
that which occurs for massive stars.
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Stellar Remnants and the Chandrasekhar
• Stellar remnants
Limit
greater than 1.4
Solar masses
cannot form
White Dwarfs.
• Objects this
massive cannot
support their own
weight but
collapse to form
either Neutron
Stars or Black
Holes.
• This maximum
mass is called the
Chandrasekhar
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Limit.
Neutron Stars
Neutron Stars weigh more than
the Sun and are as large a city.
• Except for a thin crust of
iron atoms a neutron star is
composed entirely of
neutrons.
• The gravitational forces
inside a neutron star are too
strong for atoms to exist.
• Instead electrons get
crushed into the protons in
the atomic nucleus forming
neutrons.
• Neutron stars have very
intense magnetic fields and
very rapid rotation.
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Pulsars
• Neutron stars can
sometimes be directly
observed.
• Astronomers have
discovered rapidly
spinning stars emitting
strong, very regularly
timed bursts of radio
waves.
• These types of neutron
stars are called pulsars.
• Pulsar bursts are as regular
as some of the best clocks
on Earth.
As the beam from a pulsar sweeps past
Earth we see a brief pulse.
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The Discovery of Pulsars
• In 1967 in Cambridge England,
Jocelyn Bell, a graduate student in
astronomy, discovered very
regularly spaced bursts of radio
noise in data from the radio
telescope at Cambridge University.
• After eliminating any possible manmade sources she realized this
emission must be coming from
space.
• The regularity of these pulses at first
made her and her co-workers think
they had discovered alien life.
Jocelyn Bell Burnell in front • Later they realized these must be
of the radio telescope used to
due to rapidly spinning neutron
stars.
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discover pulsars.
Black Holes
• For Main Sequence stars of mass greater than about 20 Solar
masses the remnant of the star left behind after a supernova
explosion is too large (more than 3 Solar masses) to be a white
dwarf or even a neutron star.
• These remnants collapse to form Black Holes.
• No light can escape from a Black Hole which is why it’s black.
• We can only “see” Black Holes due to their effects on other
objects.
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Escape Velocity & Curved Space
All objects exert a gravitational
pull on all other objects in the Universe.
One way to picture gravity’s effect
is by imagining space as a rubber sheet.
Heavy objects bend this sheet more than
light objects. Black Holes are like tears
in this sheet.
• There is a minimum
velocity that an object
needs to escape the
gravitational pull of any
asteroid, planet, star, etc.
• This is the escape velocity
and depends on the mass
and radius of the object
• For the Earth the escape
velocity is about 11 km/sec.
• Since a Black Hole has so
much mass in so small a
space its escape velocity is
the speed of light 300,000
km/sec.
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Schwarzschild Radius:
The Radius of the Event Horizon
• The Event Horizon is the spherical region of
space surrounding the Black Hole from
which no light may escape
– once matter or light crosses the event horizon it
can never return
– tidal forces are extreme at the event horizon
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Two dimensional representation of
the Event Horizon
•Consider a 2-D universe (graph paper) instead of a 3-D universe.
•The massive Black Hole bends space (the graph paper).
•Light paths near the Black Hole are bent.
•Light paths that intersect the Event Horizon terminate at the Black
Hole
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Escape Velocity and Event Horizon
• Compare the escape velocities and event
horizons for the following:
–
–
–
–
A 200 pound person
The Sun
A 1.4 Msol white dwarf the size of the Earth
A 3 Msol neutron star the size of a city (10 km
radius)
– A 15 Msol Black Hole, nominally city-sized
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Escape Velocity and Event Horizon
col
col
col
col
1.:
2.:
3.:
4.:
label
escape velocity
light speed
escape velocity / light-speed
Human
Sun
WhiteDwarf
NeutronStar
BlackHole
0.010957189
61736768.
7.6575036e+08
2.8291341e+10
6.3261363e+10
3.0000000e+10
3.0000000e+10
3.0000000e+10
3.0000000e+10
3.0000000e+10
3.6523965e-13
0.0020578923
0.025525012
0.94304471
2.1087121
----------------------------------------------------------------col
col
col
col
1.:
2.:
3.:
4.:
label
schwarzschild radius
actual radius
schwarzschild radius/ actual radius
Human
Sun
WhiteDwarf
NeutronStar
BlackHole
1.3340000e-23
296444.44
415022.22
889333.33
4446666.7
100.00000
7.0000000e+10
6.3700000e+08
1000000.0
1000000.0
1.3340000e-25
4.2349206e-06
0.00065152624
0.88933333
4.4466667
•Look at the last column in each table
•Table I: the escape velocity for the neutron star is near light speed
•Table II: the event horizon radius for the BH is 4.44 times the radius of its matter
•The “event horizons” of the other objects are less than their actual sizes –14they
effectively have no event horizon.
Observing a Black Hole
• General approach to “observing” black
holes is an indirect approach – look for an
effect on an object that can be uniquely
attributed to an interaction with a black hole
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Observing a Black Hole
• A black hole in a close binary system
– An accretion disk may form around the black hole as it draws
in material from its companion
– Material swirling around at or near the speed of light at the
black hole’s event horizon will emit X-rays due to the extreme
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temperatures
Observing a Black Hole
• If the black hole is eclipsed by the companion, an x-ray
telescope will observe the periodic disappearance of the
x-ray signal
• From the periodicity of the X-rays and the known mass
of the companion, the mass of the invisible black hole
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can be found
Observing a Black Hole
• If this mass exceeds the maximum allowed for a
neutron star (Cygnus X-1 and A0620-00 are two
examples), a black hole is currently the only
known object that can have high mass and not be
visible (and yet its companion is)
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