Transcript Death of the Stars

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Stellar Evolution
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Mass Dependence
Mass – Luminosity relationship:
More massive stars are brighter.
To be brighter, they spend more energy.
Massive stars live shorter.
Luminosity = Mass4 (roughly)
Lifetime = Mass / Luminosity
Lifetime = 1 / Mass3 (roughly)
Example: A star 5 times more massive than the Sun 
(5)4 = 625 times brighter than the Sun
But it will live for (1/5)3 = 125 times shorter 
1010/125 = 80,000,000 years
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Stages of Life
Gas Cloud 
Main Sequence 
Red Giant 
Planetary Nebula
or Supernova 
Remnant
How long each star will last in each stage and what kind of
remnant will form depends on the initial mass and
composition of the star.
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Giant Molecular Cloud
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Protostar
Over a long period of
time, the gases in the
giant molecular cloud
begin to condense.
There are no nuclear
reactions in these
regions, but they are hot
enough to glow in the
infrared.
When fusion starts at the core, it stops the gas from collapsing further.
But gas from the gas cloud still continues to fall: Most of the energy of a
protostar comes from this gravitational energy.
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Protostar Clusters
Young stars are social:
Inside the molecular clouds, usually more than one star is born.
They are also born at around the same time.
Giant cloud is blown
away or evaporated
by a nearby,
powerful star and we
can see the young
stars forming.
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T-Tauri stage
When the young star is
“ripe”, it will produce
strong winds and blow
away the surrounding
cocoon gas and dust.
After the gas is blown
away, the star becomes
visible for the first
time.
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Lifetrack of the Sun
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Main Sequence Stage
Because of the Hydrostatic Equilibrium, all stars become stable.
They spend 90% of their life on the main sequence buring hydrogen in
their cores.
They start their lives in
clusters, however with the
gravitational influences from
other stars they wander
away from their clusters.
Pleiades cluster in the Taurus
constellation 
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Different Tracks
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Subgiants and Red Giants
Finally, all the hydrogen in the core is converted to helium, and the
temperature is not high enough to fuse helium (to convert helium into
carbon).
Hydrodynamic Equilibrium is destroyed, the core shrinks because of
gravity.
The layers outside of the core fall onto the core and heat it up to a
temperature where helium fusion can start.
The luminosity increases
immensely.
The heat generated in
the core expands the star
to become a subgiant and
then a red giant.
Large area, not enough
heat red surface
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Life of a Red Giant
This process takes about 1
billion years for a star like our
Sun.
The radius increases 100-fold.
Temperature of the core increases.
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Helium Flash
But the thermal pressure is not
high enough to counteract gravity.
-> new source needed
-> electron degeneracy pressure
-> but this does not depend on the
temperature
-> heats up the core without
expanding it
-> helium fusion rate increases
enormously
-> HELIUM FLASH
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Supergiants
Our Sun will also go thru the Red Giant stage. During this stage the
Sun will swell to be as large as Venus’ orbit, and maybe even engulf
the Earth (in a few billion years).
However, as we have seen, the Sun is an ordinary star, there are more
massive stars than the Sun, and when they go through the Red Giant
stage, they will not be just giants, but Supergiants:
Betelgeuse is one of these
supergiants, and not the biggest
one of them.
Giants have strong winds. They
lose material with these winds.
Their surfaces are colder than
their main sequence equivalents.
But their cores are much denser
and warmer.
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Smaller Stars
Once helium is converted into
carbon, the star is not massive
enough to go through the same
cycle again.
Weak grip on its outer layers ->
matter flows away from the
surface
(Planetary) Nebula 
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Red Giant again (Massive Stars)
At the next stage, the collapse of the outer layers will heat the core up
even further, and this time carbon atoms will fuse to form Oxygen, Neon,
Magnesium, etc.
With each kind of fuel exhausted, the core will collapse and than the
gravitational energy will be converted into heat once again, and the
temperature of the core will increase to fuse even heavier elements.
This creation of heavier elements from lighter elements is called stellar
nucleosynthesis.
Stellar nucleosysnthesis continues until all of the core is converted to
Iron.
After Iron, there is no energy production possible, so the core collapses.
This time, no energy production can stop the collapse.
What happens??
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Big Stars – Small Stars
Both big and small stars live most of their lives on the Main Sequence.
They both go through the Red Giant Stages.
But for stars there exists a limit, called the Chandrasekhar limit.
Chandrasekhar limit: 1.4 Msun
Stars with M > 1.4 Msun are big stars, with M < 1.4 Msun are small stars.
At the stars under the Chandrasekhar limit, electron degeneracy
pressure is strong enough to stop the collapse of the star.
Electron degeneracy pressure: You cannot put two electrons into the
same place (state), they will resist.
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White dwarfs
The shrinkage stops at a radius equal to Earth’s radius (Sun compressed into a
volume as large as the Earth).  White Dwarf
Leftover heat has to be radiated thru a small surface, so white dwarfs are very
small, very hot and very bright.
The light pressure pushes the outer layers further away forming a planetary
nebula.
Planetary
nebula have
regular shapes
and better
defined colors
coming from
the emission
from hydrogen,
oxygen and
nitrogen.
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Remnants:
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Novae
Isolated white dwarfs are boring, they just fade to become brown dwarfs
first, and then black dwarfs.
However, if the white dwarf has a companion from which it can “steal” some
material, then things get interesting:
White dwarf
H + He stolen from
the companion
The surface of the white dwarf is normally not at the fusion temperature
of hydrogen. However the accumulated H + He increase the temperature
where H fusion starts at the surface of the white dwarf.
With a major explosion which can be seen from the Earth (novae = new
star in the sky) the H + He layer is blasted outwards, then the whole cycle
begins anew.
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Supernovae Type I
If a white dwarf with mass very close to 1.4 Msun steals some more
hydrogen from its companion, then the star goes over the
Chandrasekhar limit.
The electron degeneracy pressure cannot hold the star.
The core collapses very fast and the resulting explosion disrupts the
star (the star blows itself apart).
Nothing is left as a remnant of this supernova.
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Supernovae Type II
Sandeulet in the Large Magellanic Cloud: 40 Msun
Main sequence  9-10 million years (H burning)
Red supergiant  1 million years (He burning)
Blue supergiant  1000 years (C burning)
 few years (N burning)
 few days ( Fe)
Electron degeneracy pressure is not strong enough to hold up the
gravitation  the star collapses further
Electrons and protons fuse to form neutrons and neutrinos are
released.
From our Sun which is 0.000016 lightyears away we detect 1
neutrino every three days.
During this collapse, from Sandeulet, which is 170,000 ly away we
detected 11 neutrinos in 13 seconds.
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Neutron Degeneracy
The star will collapse until the neutron degeneracy pressure is reached.
The star is composed of only neutrons.
White dwarf  Earth diameter, Neutron star  about 30kms in diameter
It takes 0.2 seconds to collapse from the white dwarf to neutron star.
Heated core starts to expand immediately outwards.
The outer layers are falling inwards at relativistic speeds (comparable to
the speed of light)
When the expanding core collides with the outer layers we have a BIG
explosion.  Type II Supernova.
So much energy is released that all nuclear reactions can occur
simultaneously. This is how the heavier elements (than iron) are created.
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How big is the explosion?
Crab Nebula
In July 4th, 1054, the Chinese
astronomers reported that there is a
new Sun in the sky, visible during the
daytime as well.
The same event is recorded by every
tribe on the world.
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After the Explosion
Now the outer layers are gone, what happens to the star??
The answer depends on the initial mass:
Minitial < 3 Msun  Neutron star
Minitial > 3 Msun  Black Hole
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Neutron Stars
If the star has more mass than 1.4 Msun,
electron degeneracy pressure is not
strong enough against the gravity, so the
star collapses rapidly.
Electrons fuse with protons to form
neutrons and a huge number of neutrinos
are generated as a byproduct.
The star shrinks until all the electrons
and protons are converted to neutrons.
But there is a limit to the volume you can
squeeze the neutrons into  Neutron
Degeneracy Pressure.
Finally, the star becomes about 30kms in diameter, and about 1,000,000
degrees at the surfaces.
In the 1930s, the existence of neutron stars were proposed, however as
they were very small, they were only (it was) observed after the HST.
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Pulsars
1967 – graduate student  Bell  radio source with a short, periodic
signal
 jumped to a very logical conclusion and named it LGM.
LGM: Little Green Men
Later, other sources were found too  natural source
Renamed: Pulsating Stars, or pulsars.
Source is unknown: Normal stars change their brightness towards the
end of their lives (expanding and contracting), however the period of
these changes are never in the seconds’ range.
To pulsate with a high frequency, the star must be very dense and very
small, so the only likely known source are the
Neutron Stars.
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Strong Magnetic Fields
When the star becomes smaller,
it starts to rotate faster  gives
stronger magnetic field.
Faster rotation when the size gets
smaller is a result of conservation of
angular momentum.
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Lighthouse Model
Electrons and protons get caught in this field and strike the neutron star
with a high velocity, giving out a very strong radiation.
But this does not explain the high frequency of the pulsar.
Actually the pulsar does not “pulse”, however it radiates a sharp beam from
the magnetic poles.
Because the whole star rotates quickly, only from time to time the Earth
stands on the path of this sharp beam, hence we see the pulsar as pulsating.
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Remnants of Massive Stars
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… and then nothing is left…
If the star’s mass is larger than 3 Msun, nothing can stop the collapse and
the star shrinks…
Force of Gravitation: FG = GmM / r2
On Earth, if we have to leave the planet we have to shoot a rocket up, and
if the rocket can overcome this force it can escape from Earth.
If shoot the rocket up with a speed greater than 11km/s, it can leave the
Earth.
To leave the surface of the Sun, you need a speed of 620km/s.
When the Sun becomes a white dwarf, you will need 6400km/s.
If the Sun became a neutron star, you would need 94,300 km/s.
The speed of light is 300,000 km/s, so if we could squeeze even the Sun
to about 3 kms radius, even light would not be able to escape.
AND, much bigger stars are squeezed into almost nothingness… so
nothing can escape from the surface.
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Curvature of Spacetime
Gravitation is a manifestation of the curvature of spacetime.
Mass changes the curvature of spacetime around itself.
The more massive is the object the higher will be the curvature of space.
All objects follow the shortest path of the curvature of spacetime.
Earth does not revolve around the Sun because of gravitation, but because
it follows the shortest path around the Sun.
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Deflection of light
Light, just like all the other moving objects, follow the curvature of
spacetime.
In our daily life, we do not observe any such curvature of spacetime,
hence it is very difficult to believe such theoretical concepts.
However, astronomers observe this curvature by the deflection of light
around the Sun.
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Detecting Black Holes
The event horizon of black holes is too small to be optically observed
from the Earth. Instead we must rely on indirect methods of
observation.
First, if a star seems to be rotating around “nothing”, that nothing is
probably very small to be observed from the Earth, but very massive,
hence it can be a black hole.
Also, the black holes will steal material from their normal companions,
and this gas, while falling into the black hole, will emit X-rays.
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Hawking Radiation
Particle – antiparticle pairs are
created almost everywhere in the
universe.
However, they annihilate each other
very quickly, hence we do not observe
these event routinely.
But when this particle – antiparticle
pair creation occurs right along the
boundary of the event horizon, one of
the pair might be caught by the black
hole whereas other one escapes and
can be observed by outside sources.