Transcript Birth, Lives, and Death of Stars

Chapter 22:
The Death of Stars
What happens to old stars?
How does death differ for small
and large stars?
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Stage 8: Planetary Nebula or
Supernova
The outer layers are ejected as the core shrinks to its most
compact state.
A large amount of mass is lost at this stage as the outer layers
are returned to the interstellar medium.
For the common low-mass stars (I.e with masses of 0.08 to 5
times the mass of the Sun during their main sequence stage),
the increased number of photons flowing outward from the
star's hot, compressed core will push on the carbon and silicon
grains that have formed in the star's cool outer layers to eject
the outer layers and form a planetary nebula.
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Stellar Nucleosynthesis
H, He, some Li, Be, B produced during the Big
Bang.
Other elements produced in stars through
nuclear fusion.
When the outer layers of a star are thrown back
into space, the new, heavy elements can later
form stars and planets.
Source for the stuff our Earth is made of.
All of the atoms on the Earth except hydrogen
and most of the helium are recycled star
material -- they were not created in the big bang.
They were created in stars.
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Stellar Nucleosynthesis (Cont’d)
Atoms from helium to iron are made in Star cores.
Low mass stars can only synthesize helium.
Stars similar to our Sun can synthesize He, C, O.
Massive stars (M* > 5 solar masses) can synthesize
He, C, O, Ne. Mg, Si, S, Ar, Ca, Ti, Cr, Fe.
Elements heavier than iron are made in supernova
explosions from the combination of the abundant
neutrons with heavy nuclei.
Synthesized elements are dispersed into interstellar medium
by the supernova explosion.
Elements later incorporated into giant molecular clouds.
Eventually become part of stars and planets.
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Degenerate matter
When atoms become super-compressed,
particles bump right up against each other to
produce a kind of gas, called a degenerate
gas.
Normal gas exerts higher pressure when it is
heated and expands, but the pressure in a
degenerate gas does not depend on the
temperature.
The laws of quantum mechanics must be
used for gases of ultra-high densities.
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Degenerate Gas
Only certain energies are permitted in a closely confined space.
The particles are arranged in energy levels like rungs of an energy
ladder. In ordinary gas, most of the energy levels are unfilled and the
particles are free to move about. But in a degenerate gas, all of the lower
energy levels are filled.
Only two particles can share the same energy level in a given
volume at one time.
For white dwarfs the degenerate particles are the electrons. For neutron
stars the degenerate particles are neutrons.
How close particles can be spaced depends inversely on their
masses.
Electrons are spaced further apart in a degenerate electron gas than the
neutrons in a degenerate neutron gas because electrons are much less
massive than neutrons.
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Consequences (1)
Degenerate gases strongly resist compression.
Degenerate particles (electrons or neutrons) locked into place
because all of the lower energy shells are filled up.
The only way they can move is to absorb enough energy to
get to the upper energy shells.
This is hard to do!
Compressing a degenerate gas requires a change in the
motions of the degenerate particle. But that requires A LOT of
energy.
Degenerate particles have no ``elbow room'' and their jostling
against each other strongly resists compression. The
degenerate gas is like hardened steel!
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Consequences (2)
The pressure in a degenerate gas depends only on
the speed of the degenerate particles NOT the
temperature of the gas.
But to change the speed of degenerate particles
requires A LOT of energy because they are locked
into place against each other.
Adding heat only causes the non-degenerate
particles to move faster, but the degenerate ones
supplying the pressure are unaffected.
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Consequences (3)
Increasing the mass of the stellar core
increases the compression of the core.
The degenerate particles are forced closer
together, but not much closer together
because there is no room left.
A more massive stellar core remnant will be
smaller than a lighter core remnant.
This is the opposite behavior of regular
materials: usually adding mass to something
makes it bigger!
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White Dwarfs
Form as the outer layers of a
low-mass red giant star puff
out to make a planetary nebula.
Since the lower mass stars make the white
dwarfs, this type of remnant is the most
common endpoint for stellar evolution.
If the remaining mass of the core is less than
1.4 solar masses, the pressure from the
degenerate electrons (called electron
degeneracy pressure) is enough to prevent
further collapse.
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White Dwarfs Density
Because the core has about the mass of the
Sun compressed to something the size of the
Earth, the density is tremendous: around 106
times denser than water (one sugarcube
volume's worth of white dwarf gas has a mass >
1 car)!
A higher mass core is compressed to a smaller
radius so the densities are even higher.
Despite the huge densities and the ``stiff''
electrons, the neutrons and protons have room
to move around freely---they are not
degenerate.
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Radius of a White Dwarf
Adding more
mass causes
the radius to
decrease!
At about 1.4
solar
masses, the
size
becomes
zero!
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White Dwarf Cooling
White dwarfs shine simply from the release of the heat
left over from when the star was still producing energy
from nuclear reactions.
There are no more nuclear reactions occurring so the
white dwarf cools off from an initial temperature of
about 100,000 K.
The white dwarf loses heat quickly at first cooling off to
20,000 K in only about 100 million years, but then the
cooling rate slows down: it takes about another 800
million years to cool down to 10,000 K and another 4
to 5 billion years to cool down to the Sun's
temperature of 5,800 K.
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From Giant to White Dwarf
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White Dwarf Cooling
(2)
Their rate of cooling and the distribution of their current
temperatures can be used to determine the age of our galaxy or
old star clusters that have white dwarfs in them.
However, their small size makes them extremely difficult to detect.
The HST can detect these small dead stars in nearby old star
clusters called globular clusters.
Analysis of the white dwarfs provides an independent way of
measuring the ages of the globular clusters and provide a
verification of their very old ages derived from main sequence
fitting.
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Death of Massive Stars
Rare high-mass stars (masses of 5 - 50 times the
Sun's mass in main sequence stage) end their life in a
different way.
When a massive star's iron core implodes, the protons
and electrons fuse together to form neutrons and
neutrinos.
The core, once the size of the Earth, becomes a very
stiff neutron star about the size of a small town in less
than a second.
The in falling outer layers hit the core and heat up to
billions of degrees from the impact.
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Death of Massive Stars Supernova
Enough of the huge number of neutrinos produced
when the core collapses interact with the gas in outer
layers, helping to heat it up.
During the supernova outburst, elements heavier than
iron are produced as free neutrons produced in the
explosion rapidly combine with heavy nuclei to
produce heavier and very rare nuclei like gold,
platinum, uranium among others.
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Supernova Explosion
The superheated gas is blasted into space carrying a
lot of the heavy elements produced in the stellar
nucleosynthesis process.
This explosion is a supernova.
Expanding gas crashes into the surrounding
interstellar gas at thousands of kilometers/second,
the shock wave heats up the interstellar gas to very
temperatures and it glows.
Strong emission lines of neutral oxygen and ionized
sulfur distinguish their spectra from planetary nebulae
and H II regions.
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Supernova Explosion (cont’d)
Also, the ratio of the strengths of the individual doublyionized oxygen is that expected from shock-wave
heating.
Planetary nebulae and H II regions are lit up by the
action of ultraviolet light on the gas, while supernova
glow from shock-wave heating.
Gas from supernova explosions also has strong radio
emission with a non-thermal continuous spectrum that
is produced by electrons spiraling around magnetic
field lines.
Gas from recent explosions (within a few thousand
years ago) are visible with X-ray telescopes as well.
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Crab Nebula
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A famous
supernova
remnant is the
Crab Nebula.
Chinese
astronomers
recorded the
explosion on
July 4, 1054
Anasazi
Indians painted
a picture of it.
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Vela Supernova
Occurred long before the Crab
Nebula
Much more spread out.
Parts have run into regions
of the interstellar medium
of different densities.
For that reason and because
of turbulence in expanding
supernova gas, the remnant
seen today is wispy strands
of glowing gas.
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Supernova Output
Neutrinos formed when the neutron core is
created fly away from the stiff core, carrying
most of the energy from the core collapse away
with them.
Some energy goes into driving the gas
envelope outward.
The rest of the energy goes into making the
supernova as bright as 1011 Suns
as bright as an entire galaxy!
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SN 1987a
after
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Supernova
occurred in
satellite galaxy of
the Milky Way at
beginning of 1987
Called SN1987a.
Kamiokande
detector (Japan)
saw a burst of
neutrinos.
Confirmation of
supernova models.
before
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HST Images of SN1987a
The material from the
explosion is expanding
outward at over 9.5
million km/hr
preferentially into two
lobes that are roughly
aligned with the bright
central ring.
Central bright ring and
two outer rings are
from material ejected
by the star before its
death.
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Supernova Rate in the Universe
Supernovae are very rare
about one every hundred years in any given galaxy
because the stars that produce them are rare.
But… there are billions of galaxies in the universe,
simple probability says that there should be a few
supernovae happening somewhere in the universe during a
year and that is what is seen!
Because supernovae are so luminous and the energy
is concentrated in a small area, they stand out and
can be seen from hundreds of millions of light years
away.
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Stage 9: Core Remnant
Core mass < 1.4 solar masses,
Star core shrinks down to a white dwarf the size of the
Earth.
Core 1.4 < mass <3 solar masses,
Neutrons bump up against each other to form a degenerate
gas.
Forms a neutron star about the size of small city.
Neutrons prevent further collapse of the core.
Core > 3 solar masses : Complete collapse
As it collapses, it may momentarily create a neutron star and
the resulting supernova rebound explosion.
Gravity finally wins. Nothing holds it up.
Becomes a black hole
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Novae and Supernovae Type I
An isolated white dwarf has a boring future: it simply
cools off, dimming to invisibility.
White dwarfs in binary systems where the companion
is still a main sequence or red giant star can have
more interesting futures.
If the white dwarf is close enough to its red giant or
main sequence companion, gas expelled by the star
can fall onto the white dwarf.
The hydrogen-rich gas from the star's outer layers
builds up on the white dwarf's surface and gets
compressed and hot by the white dwarf's gravity.
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Novae
Eventually the hydrogen gas gets dense and hot
enough for nuclear reactions to start. The reactions
occur at an explosive rate.
The hydrogen gas is blasted outward to form an
expanding shell of hot gas.
The hot gas shell produces a lot of light suddenly.
From the Earth, it looks like a new star has appeared
in our sky.
Early astronomers called them novae (``new'' in
Latin).
They are now known to be caused by old, dead stars.
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Novae
The spectra of novae show blue-shifted absorption
lines from hot dense gas expelled towards us at a few
thousands of kilometers per second.
The continuum is from the hot dense gas and the
absorption lines are from the lower-density surface of
the expanding cloud.
After a few days the gas has expanded and thinned
out enough to just produce blue-shifted emission lines.
After a nova burst, gas from the regular star begins to
build up again on the white dwarf's surface.
A binary system can have repeating nova bursts.
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Type Ia Supernovae
If enough mass accumulates on the white dwarf
to push it over the 1.4 solar mass limit, the
degenerate electrons will not be able to stop
gravity from collapsing the dead core.
The collapse is sudden and heats the carbon
and oxygen nuclei left from the dead star's red
giant phase to temperatures great enough for
nuclear fusion.
The carbon and oxygen quickly fuse to form silicon
nuclei.
The silicon nuclei fuse to create nickel nuclei.
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Type Ia Supernovae
A huge amount of energy is released very quickly with
such power that the white dwarf blows itself apart.
This explosion is called a Type Ia supernova to
distinguish it from the supernovae (called type II
supernovae) that occur when a massive star's iron
core implodes to form a neutron star or black hole.
Type Ia supernovae are several times brighter than
Type II supernovae.
Tycho’s supernova was a type Ia.
Type Ia supernovae are used as “standard candles”.
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Tycho’s Supernova and Companion
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Neutron Stars
If the core mass is between 1.4 and 3 solar masses,
the compression from the star's gravity will be so great
the protons fuse with the electrons to form neutrons.
The core becomes a super-dense ball of neutrons.
Only the rare, massive stars will form these remnants
in a supernova explosion.
Neutrons can be packed much closer together than
electrons so even though a neutron star is more
massive than a white dwarf, it is only about the size of
a city.
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Neutron Stars
The neutrons are degenerate and
their pressure (called neutron
degeneracy pressure) prevents
further collapse.
Neutron stars are about 30 kilometers across, so
their densities are much larger than even the
incredible densities of white dwarfs: 2 x 1014 times
the density of water.
Recently, the Hubble Space Telescope was able to
image one of these very small objects.
Even though it is over 660,000 K, the neutron star is
close to the limit of HST's detectors because it is at
most 27 kilometers across.
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Pulsars
In the late 1960's astronomers discovered radio
sources that pulsated very regularly with periods of
just fractions of a second to a few seconds.
The periods are extremely regular---only the ultra-high
precision of atomic clocks can show a very slight
lengthening in the period.
At first, some thought they were picking up signals
from extra-terrestrial intelligent civilizations.
The discovery of several more pulsars discounted that
idea---they are a natural phenomenon called pulsars
(short for “pulsating star”).
Vela pulsar
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Pulsars (2)
Normal variable stars (stars near the end of their life in
stages 5 to 7) oscillate in brightness by changing their
size and temperature.
The density of the star determines the pulsation
period--denser stars pulsate more quickly than low
density variables.
However, normal stars and white dwarfs are not dense
enough to pulsate at rates of under one second.
Neutron stars would pulsate too quickly because of
their huge density, so pulsars must pulsate by a
different way than normal variable stars.
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Pulsars (3)
A rapidly rotating object with a
bright spot on it could produce
the quick flashes if the bright
spot was lined up with the Earth.
Normal stars and white dwarfs cannot rotate fast enough
because they do not have enough gravity to keep themselves
together; they would spin themselves apart.
Neutron stars are compact enough and strong enough to rotate
that fast. The pulsar at the center of the Crab Nebula rotates 30
times every second.
In the figure it is the left one of the two bright stars at the center
of the HST image
Crab pulsar
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Pulsars (4)
Another clue comes from the length of each pulse
itself.
Each pulse lasts about 1/1000th of a second (the time
between pulses is the period mentioned above).
An important principle in science is that an object
cannot change its brightness faster than it takes light
to cross its diameter.
Even if the object could magically brighten everywhere
simultaneously, it would take light from the far side of the
object longer to reach you than the near side.
Fastest known pulsar B1937
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Pulsars (5)
Observed change in brightness to be smeared out
over a time interval equal to the time it would take
the light from the far side of the object to travel to
the near side of the object.
If the object did not brighten everywhere
simultaneously, then a smaller object could produce
a pulse in the same interval. The brightness
fluctuation timescale gives the maximum size of an
object.
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Pulsar Size
The 1/1000th of second burst of energy means that the pulsars
are at most (300,000 kilometers/second) × (1/1000 second) =
300 kilometers across.
This is too small for normal stars or white dwarfs, but fine for
neutron stars.
When neutron stars form they will be spinning rapidly and have
very STRONG magnetic fields (109 to 1012 times the Sun's).
The magnetic field is the relic magnetic field from the star's
previous life stages.
The magnetic field is frozen into the star, so when the core
collapses, the magnetic field is compressed too.
The magnetic field becomes very concentrated and much
stronger than before.
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Summary
Init. Mass
(Msun)
< 0.01
Final Mass
(Msun)
< 0.01
0.01 to 0.08
0.01 to 0.08 Brown dwarf (H and He)
0.08 to 0.25
Final disposition
Planet
White dwarf, mostly He
0.25 to 10
< 1.4
White dwarf, mostly C & O
10 to 12
< 1.4
White dwarf, O, Ne, Mg
12 to 40
<3
Supernova  neutron star
> 40
>3
Supernova  black hole
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