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Stellar Remnants
Stellar
Evolution
• The end of a star’s life as with
its birth depends on its mass.
• Solar mass stars generally will
evolve smoothly to a white
dwarf.
• Larger masses result in several
endings:
– Supernova, Black hole, Neutron
star
White Dwarfs
• Hot compact stars
• Mass of the Sun, size of the
Earth
• Core of carbon and oxygen
• Very compact, no fuel supply
• Increasing mass causes them
to shrink.
Degenerate Matter
• White dwarfs are
very dense objects.
• They are made of
degenerated matter.
• It is so dense that a
beach ball size lump
would weigh as much
as an ocean liner. ( 1
ton per cm3)
The End of a High-Mass Star
A high-mass star can continue to fuse elements in
its core right up to iron (after which the fusion
reaction is energetically unfavored).
As heavier elements are
fused, the reactions go
faster and the stage is
over more quickly.
A 20-solar-mass star
will burn carbon for
about 10,000 years, but
its iron core lasts less
than a day.
The End of a High-Mass Star
The neutrinos escape; the neutrons are compressed
together until the whole star has the density of an
atomic nucleus, about 1015 kg/m3.
The collapse is still going on; it compresses the
neutrons further until they recoil in an enormous
explosion as a supernova.
Supernovae
A supernova is a one-time event – once it
happens, there is little or nothing left of the
progenitor star.
There are two different types of supernovae,
both equally common:
Type I, which is a carbon-detonation
supernova;
Type II, which is the death of a high-mass
star just described.
Supernovae
Carbon-detonation supernova: white dwarf
that has accumulated too much mass from
binary companion
If the white dwarf’s mass exceeds 1.4 solar
masses, electron degeneracy can no longer
keep the core from collapsing.
Carbon fusion begins throughout the star
almost simultaneously, resulting in a carbon
explosion.
The Formation of the Elements
The elements that can be formed through
successive alpha-particle fusion are more
abundant than those created by other fusion
reactions:
The Formation of the Elements
The last nucleus in the alpha-particle chain is
nickel-56, which is unstable and quickly decays
to cobalt-56 and then to iron-56.
Iron-56 is the most stable nucleus, so it
neither fuses nor decays.
However, within the cores of the most massive
stars, neutron capture can create heavier
elements, all the way up to bismuth-209.
The heaviest elements are made during the
first few seconds of a supernova explosion.
The Cycle of Stellar Evolution
Star formation is
cyclical: stars form,
evolve, and die.
In dying, they send
heavy elements into
the interstellar
medium.
These elements then
become parts of new
stars.
And so it goes.
Chandrasekhar
Limit
• Extra mass increases the gravity and
compresses it.
• The Chandrasekhar limit implies that a
star greater than 1.4 solar mass
cannot evolve smoothly to become a
white dwarf.
• Too much mass causes white dwarf to
collapse.
Binary
System
• One star maybe a white dwarf
• Mass can be transferred between
them.
• An accretion disk spirals toward the
compact.
• Some binary systems produce a
nova explosion if the gas layer
reaches ignition temperature of
hydrogen.
• Novae may repeat the process if
it doesn’t accumulate too much
mass.
• If the mass is over the
Chandrasekhar –Limit, it will
explode.
Neutron Star
• In very massive stars, the
core is iron.
• When the iron core
collapses, the result is a
supernova explosion.
• The collapsing core is
squeezed past degenerate
matter.
• A neutron star is formed.
Supernovae
• There are two types of
supernovae.
• Type I luminosity declines rapidly
at first, then slowly as time
passes.
• Type II maintains brightness for
up to 100 days and then declines
in luminosity.
• A supernova leaves behind a
nebula.
• This is called a supernova
remnant.
Neutron Stars
Neutron stars,
although they have 1–3
solar masses, are so
dense that they are
very small. This image
shows a 1-solar-mass
neutron star, about 10
km in diameter,
compared to
Manhattan.
Pulsars
• 1967 Jocelyn Bell noticed odd
radio signal.
• Discovered the first pulsar, a
pulsating star.
• These objects seem to be
associated with super nova
remnants. (SNR)
• It is believed that a pulsar is a
rapidly rotating neutron star.
• Instead of pulsing, a pulsar is a rapidly
spinning neutron star.
• From the period of the pulse, it had to be
extremely dense but associated with a
supernova explosion.
• Spin of a neutron star and its magnetic
field generates powerful electric fields.
• Emission created by accelerating charges
called synchrotron radiation (low energy
radiation at radio wavelengths.)
Pulsars
But why would a neutron star flash on and off?
This figure illustrates the lighthouse effect
responsible:
Strong jets of matter are
emitted at the magnetic
poles, as that is where
they can escape. If the
rotation axis is not the
same as the magnetic
axis, the two beams will
sweep out circular paths.
If the Earth lies in one of
those paths, we will see
the star blinking on and
off.
Black Holes
The mass of a neutron star cannot exceed
about 3 solar masses. If a core remnant is
more massive than that, nothing will stop its
collapse, and it will become smaller and
smaller and denser and denser.
Eventually the gravitational force is so intense
that even light cannot escape. The remnant
has become a black hole.
Black Hole
• Core remnant mass that is
greater than 3 solar masses.
• A Black hole would result
from a star’s core complete
collapse.
• Degenerate neutrons con
hold up core against its own
gravity.
Event Horizon
• Is the boundary of a
black hole.
• The distance at which the
escape velocity equals the
speed of light for the
size of the black hole.
• Space is so curved that
any light emitted is bent
back to the point mass.
• The size of the event
horizon is called the
Scharzschild radius.
Black Holes
The radius at which the escape speed from
the black hole equals the speed of light is
called the Schwarzschild radius.
The Earth’s Schwarzschild radius is about a
centimeter; the Sun’s is about 3 km.
Once the black hole has collapsed, the
Schwarzschild radius takes on another
meaning – it is the event horizon. Nothing
within the event horizon can escape the
black hole.
Distorted
Space-Time
• As you approach a black hole,
time slows down.
• This happens because the
gravity field has distorted
space-time.
• Einstein’s theory of
relativity helps explain this.
Detecting
Black Holes
• Event horizon is only several miles
across.
• Black hole and visible star will orbit
around the center of mass between
them.
• Guestamate the mass of visible star,
then using Kepler’s 3rd Law, calculate
the mass of the other object.
• If mass is too large for neutron star
or white dwarf, most likely a black
hole.
• Black holes advertise their presence
with X-rays.
• Gas material
will form an
accretion disk.
• Particles rub
against each
other.
• This causes
heat that is
hot enough to
emit X-Rays.
• Gravitational Waves:
– If black hole orbits a
companion, it motion generates
a gravitational wave.
– Hawking Radiation:
– Black star emits blackbody
radiation.
– Using Wien’s Law you can
calculate the temperature of
the black hole. (6 x 10-8K)