Download Death - Wayne State University Physics and Astronomy

The Death of Stars
12 April 2005
AST 2010: Chapter 22
1
Stellar Questions
• What happens to old stars?
• How does death differ for small and
large stars?
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2
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 (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
• Helium and heavier elements produced in
stars through nuclear fusion
• When the outer layers of a star are thrown
back into space, the processed material can
be incorporated into gas clouds that will 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 created in
stars
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Stellar Nucleosynthesis (Cont’d)
• Atoms heavier than He up to the iron atoms 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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Consequences
• 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
• They 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
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White Dwarf’s Cooling (1)
• 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 Dwarfs 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
• Because it is above the atmosphere, the HST can
detect these small dead stars in nearby old star
clusters called globular clusters
• Analysis of the white dwarfs may provide 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 (1)
• 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 (2)
• Planetary nebulae and H II regions are
lit up by the action of ultraviolet light
on the gas, while supernovae glow from
shock-wave heating
• Gas from supernova explosions also has
strong radio emission with a nonthermal 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
21
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
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• Supernova
occurred in satellite
galaxy of the Milky
Way at beginning
of 1987
• Called SN1987a
• Kamiokande
neutrino detector
saw a burst of
neutrinos
• Confirmation of
supernova models
• Left image shows
star before it went
supernova
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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 (1)
• 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)
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Novae (2)
• They are now known to be caused by old,
dead stars
• The spectra of a nova shows blue-shifted
absorption lines showing that a hot dense
gas is expanding 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 lowerdensity surface of the expanding cloud
• After a few days the gas has expanded and
thinned out enough to just produce blueshifted emission lines
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Novae (3)
• After the 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
• 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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Novae (4)
• 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 I supernova to
distinguish them from the supernova (called
a type II supernova) that occurs when a
massive star's iron core implodes to form a
neutron star or black hole
• Type I supernovae are several times brighter
than type II supernovae
• Tycho’s supernova was a type I
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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 × 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 (1)
• 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”)
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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
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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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