Transcript Stellar Remnants White Dwarfs, Neutron Stars & Black Holes

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
pulsating 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
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Earth we see a brief pulse.
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 10 Solar
masses the remnant of the star left behind after a supernova
explosion is too large (about 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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