Transcript The Vampire Stars - d_smith.lhseducators.com

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White Dwarfs, Novae, and
Type 1a Supernovae:
The Vampire Stars
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White Dwarfs
• ~Twice the size of the earth.
• Typical surface temperatures of
20,000 to 100,000 Kelvin.
• 200,000 times the density of the
earth.
• Made of C and O, possibly crystalline.
Earth
White Dwarf
Relative size of sun’s core;
contains ½ the mass of the sun.
Credit: SOHO, NOAANews, NASA
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• Contains about ½ the mass of the
original star: from 0.4 to 1.4 solar
masses.
• This upper limit (1.4 solar masses) is
the Chandrasekhar Limit.
(If the core of the star is heavier
than 1.4 solar masses, it will turn
into a neutron star instead of a white
dwarf.)
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• Nuclear fusion has completely shut
down – the star shines only from
residual heat.
• If there’s no nuclear fusion to
provide outward pressure…why
doesn’t the white dwarf instead
collapse further into a neutron star
or black hole?
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• At the enormous density of a white
dwarf, the empty space between
atoms is squeezed out.
• Electrons of atoms repel electrons of
other atoms, providing an outward
pressure which the star’s gravity isn’t
strong enough to overcome.
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• Degenerate electron pressure:
Atoms are so close together that
their atomic orbitals merge.
Electrons simply flow around all the
atoms (electrons are degenerate.)
Sometimes white dwarfs
explode!
• Since at least half of the stars occur
in binary systems, we ought to find
many white dwarfs in binary systems
with other stars.
• If a white dwarf is in a close binary
system with a red giant or
supergiant, its gravity will pull
hydrogen gas from the larger star.
The gas from the red giant spirals into the
white dwarf – forming an accretion disk.
Ready to go BOOM!
• When enough hydrogen from the red
giant accumulates on the surface of
the white dwarf, the high
temperatures cause the H to fuse
into He.
• The star briefly flares hundreds of
times brighter than normal.
A nova in Hercules.
www.if.ufrgs.br/oei/stars/ novas/novas.htm
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• Although the explosion is violent,
most of the energy and mass comes
from the “stolen” hydrogen gas. The
star itself isn’t destroyed.
• The star can go through the process
dozens, even hundreds, of times.
Nova light curve
http://zebu.uoregon.edu/~js/ast122/images/nova_light_curve.gif
http://www.seed.slb.com/en/watch/
cosmos/images/nova.jpg
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Sometimes it goes too far…
• A Nova happens when the white
dwarf is far below the Chandrasekhar
Limit of 1.4 Msun.
• What happens if the white dwarf is
just below or right at the
Chandrasekhar limit?
Type 1 Supernova
• As hydrogen from the red giant piles
onto the surface of the white dwarf,
it may cause the total mass of the
white dwarf to surpass the
Chandrasekhar Limit.
• The electron repulsion can no longer
support the star, so the entire star
collapses and explodes.
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• Since a Type 1 Supernova always
occurs at the same mass, (1.4 solar
masses), these supernovas always
explode with the same brightness.
• This makes them perfect “standard
candles” or distance markers.
http://antwrp.gsfc.nasa.gov/apod/ap061224.html
Supernova 1994, a Type 1 supernova
in a distant galaxy
SN2006X in Galaxy M100 (near Coma Berenices)
Credit: FORS Team, 8.2-meter VLT, ESO
Before & After SN 2005cs in M51 (Whirlpool Galaxy) in constellation Canes Venatici.
Credit: http://www.nasaimages.org, R. Jay GaBany
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Type II or “Core Collapse”
Supernovas
Large stars begin like any other
star
• Stars that eventually become Type II
supernovae begin with 3 to >100
times the mass of the sun.
• A nebula collapses to become a
protostar; nuclear fusion ignites becomes a main sequence star
(where it lives most of its life); begins
to use up its fuel.
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• As the H fuel in the core is used up,
the core shrinks and heats up, while
the outer layers swell and cool – the
star becomes a red giant.
• As the core shrinks & heats it begins
fusing He (the ash from the previous
reaction) into C and O.
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• The layer right next to the core begins
fusing H into He – so now 2 fusion
reactions are occurring.
• A small star would stop here, the core
not being hot enough to do anything
with the C and O.
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He fusing into
C and O.
(Triple-alpha
Process)
No fusion in outer layers.
H fusing into He.
(Proton-Proton Chain)
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• A large star’s core can collapse and
heat further. The core begins to fuse
C and O into Neon (Ne), Magnesium
(Mg), and Silicon (Si).
• The next layer out fuses He into C
and O. The layer outside that fuses H
into He. The star starts to resemble
the layers of an onion.
Onion
layers
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• The production of Ne, Mg, and Si
continues for only a few hundred years.
• Eventually, when all the C and O is used
up, the core shrinks once more, heats to
over 600,000,000 K and starts fusing Mg +
Si into iron (Fe).
• Energy must be added to fuse iron into a
heavier element, so fusion stops with iron.
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• The production of iron from Mg & Si
happens very quickly, less than 1
day.
• When the iron core gets massive
enough it implodes! (The needed
mass is 1.4 x the mass of the sun –
the Chandrasekhar limit!)
Inverse Beta Decay
• The gravity is so great in the core,
that protons & electrons get squashed
together into neutrons:
p+ + eno
(inverse beta decay.)
• The core becomes a neutron star.
Neutron Stars
• Neutron stars are formed from stars
that were originally 3 – 8 times the
mass of the sun.
• They’re held up against gravity simply
by the neutrons being jammed in
tightly next to each other. This is
called “neutron degeneracy”.
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Black Holes
• What happens if the star was
originally more than 8 solar masses?
• Even the pressure of the neutrons is
overcome, and the neutron star
collapses into a black hole.
What about the rest of the layers?
• When the iron core collapses into a
neutron star or black hole (at nearly
the speed of light), the outer layers
follow it in.
• The outer layers “bounce” or rebound
off the immensely hot new neutron
star and a gigantic explosion occurs!
• The rebound is helped out by a blast
of gamma rays & neutrinos.
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Credit: aether.lbl.gov/www/tour/elements/stellar/rebound.gif
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How often do SN happen?
• On average, about every 100 years
for any given galaxy.
• Our own galaxy has had several
during recorded history:
“Recent” Supernovas
• July 4th, 1054, in
Taurus, 6500 light
years away. This
resulted in the
Crab Nebula. It
was recorded by
Anasazi Indians in
the American
southwest.
The Crab Nebula in Taurus
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• In 1572 Tycho
Brahe saw a
supernova in
Cassiopeia –
(16,000 light
years away)
• The Chinese also
saw and recorded
the appearance
of a “guest” star.
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From Astronomie Populaire, by
Camille Flammarion, 1884.
The
remnant
of
Cassiopeia A.
Credit: NASA/GSFC/U.Hwang et al.
This is
the
brightest
radioemitting
object in
the sky.
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Ancient SN’s
• When a supernova explodes, it
leaves behind a cloud or supernova
remnant.
• These remnants last for hundreds of
thousands of years. New (small)
stars can be formed from their gases
and dust.
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This is how
the whole
Cygnus Loop
SN remnant
looks –
remarkably
like the cloud
from an
ordinary
explosion.
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Future Supernovas
• At present, astronomers are waiting
for another star to go supernova in
our galaxy: Eta Carinae.
Credit: N. Smith, J. A. Morse (U. Colorado) et al., NASA
A Very Special Supernova
• The only close supernova that
astronomers have been able to study
in detail is SN 1987A in the Large
Magellanic Cloud, 170,000 light years
away.
• The SN happened in the Tarantula
Nebula.
March 23, 1987
The Large Magellanic Cloud
The Tarantula Nebula
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SN 1987A
• This supernova was different than
many – when it exploded, it was a
blue supergiant.
After the Explosion
• The brightness of SN 1987A has
been monitored for the past 22 years.
It didn’t follow the usual pattern
(suddenly bright, with a quick fall-off).
• Rather, it got suddenly bright, grew
brighter, then faded off gradually.
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• A couple of years after the supernova
faded, it suddenly brightened again.
• It wasn’t the supernova itself, but its
light reflecting off a cloud of dust
behind the SN. This reflected light is
a “light echo”.
http://apod.nasa.gov/apod/ap060125.html
Expanding light echos.
Credit: ESO (European Southern Obs.)
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• This supernova has been observed
extensively. Over the years, we’ve
seen shock waves from the explosion
slam into the clouds of gas that the
star gave off just before it exploded.
• The shock waves heat the gas,
producing rings.
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• The shock waves heat the shells of
gas hot enough to give off X-rays.
Credit: NASA/CXC/SAO/PSU/D.Burrows et al.
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Nucleosynthesis
• Supernovas can make elements up to
the mass of Fe (atomic number 26)
before they explode. However, there
are 80+ elements that are heavier
than iron.
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• During the explosion, there are a lot
of very fast, high energy neutrons
flying around. Sometimes, one of
these neutrons hits an iron nucleus:
56 Fe
26
+ no
57 Fe
26
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• The extra neutron inside the heavy
iron nucleus can split into a proton
and an electron:
57 Fe
26
57 Co
27
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• This process of adding a neutron,
then the neutron splitting into a proton
and electron can happen over and
over, producing elements heavier
than iron.
57 Co
27
+ no
58 Ni
28
etc.
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• The next time you look at your
‘significant other’ – remember they
truly are made of stars.
• So was your lunch today….that’s
what that funny taste was!