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The end stages of stars
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No homework due next week
Review
The Hertzsprung-Russell diagram
Review
A star’s spectral type simply depends on its surface
temperature
Review
Stars spend most of their life on the main sequence of the
Hertzsprung-Russell diagram, where they are fusing
hydrogen into helium in their cores.
Review
The fusion rate depends strongly on the mass of the star;
the most massive stars (>100 Msun) burn their fuel very
rapidly, so they are very hot and don’t live long.
Review
Low-mass stars (down to ~0.1 Msun) burn their fuel very
slowly, so they are cool and live very long.
Review
After the core of the star has been converted into pure helium the
fusion stops (although it will still occur in a shell around the
core). The star leaves the main sequence, expanding to become a
red giant or a supergiant.
Review
What happens next depends on the mass of the star:
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If the star is massive enough, then the core will be
hot and dense enough to star fusing the helium into
carbon. But it stops there.
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If the star is even more massive (a supergiant), then
the carbon core will start to fuse into heavier
elements.
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And even more massive stars will fuse even heavier
elements in their cores. The most massive stars will
eventually build up an iron core, which can no longer
fuse.
Q: Which is brighter?
A. K-type main sequence star
B. K-type giant
C. they are about the same
Q: Which is brighter?
A. K-type main sequence star
B. K-type giant
C. they are about the same
Q: Which is larger?
A. K-type main sequence star
B. K-type giant
C. they are about the same
Q: Which is larger?
A. K-type main sequence star
B. K-type giant
C. they are about the same
Q: Which is more luminous?
A. A-type supergiant
B. K-type supergiant
C. they are about the same
Q: Which is more luminous?
A. A-type supergiant
B. K-type supergiant
C. they are about the same
Q: Which is larger?
A. A-type supergiant
B. K-type supergiant
C. they are about the same
Q: Which is larger?
A. A-type supergiant
B. K-type supergiant
C. they are about the same
Review
Red giants, and supergiants:
Homework problem 13.47
The distance from Earth of the red supergiant Betelgeuse
is approximately 643 light-years. If it were to explode as a
supernova, it would be one of the brightest stars in the
sky. Right now, the brightest star other than the Sun is
Sirius, with a luminosity of 26LSun and a distance of 8.6
light-years.
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How much brighter in our sky than Sirius would the
Betelgeuse supernova be if it reached a maximum
luminosity of 8.0×109 LSun?
luminosity
apparent brightness =
4𝛑×distance2
How stars die — when M<8Msun
So what happens when all fusion finally stops in a red
giant?
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The core (which is now made of either helium or
carbon) contracts and heats.
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Through strong stellar winds and other processes,
the diffuse out layers of the star are ejected creating a
planetary nebula.
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The dense stellar core remains, and is now called a
white dwarf.
How stars die — when M<8Msun
white dwarf
Hot ionized gas
ejected from the star
How stars die — when M<8Msun
(Note that planetary nebulae
have nothing to do with planets)
How stars die — when M<8Msun
How stars die — when M<8Msun
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The white dwarf starts out very hot, but there is no
energy source so it cools off over time
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The gas also starts out hot, and initially is kept hot by
the white dwarf, but it will also cool off over time. So
the lifetime of a PN is relatively short, just a few 10s of
thousands of years.
How stars die — when M>8Msun
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The central core (which, in the most massive stars, is
made of iron) undergoes a sudden gravitational
collapse, reducing in size until all the electrons in the
atoms are smashed down into the nulcei.
➡
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All of the protons in those nuclei become neutrons.
The outer layers of the star explode outward (although
how exactly this happens is not completely known).
This is a supernova. The dense core remains as a
neutron star.
How stars die — when M>8Msun
neutron star
Hot ionized gas
ejected from by the supernova
The crab nebula: the supernova was observed in 1054
How stars die — when M>8Msun
How stars die — when M>8Msun
This is the remnant of the supernova observed
by Tycho Brahe in 1572
How stars die — when M>8Msun
This is the remnant of the supernova recorded by
Chinese astronomers in 386 A.D.
How stars die — when M>8Msun
Supernovae are incredibly bright, perhaps 1010Lsun.
Stellar remnants
What is left of the dense stellar core after the planetary
nebula or the supernova explosion depends on the mass
of the core. It can be a
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white dwarf
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neutron star
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black hole
Stellar remnants — white dwarfs
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A white dwarf is incredibly dense, more than 2000
pounds per cubic centimeter.
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Consists of either helium or hydrogen, and is support
by electron degeneracy pressure.
Stellar remnants — white dwarfs
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A white dwarf is incredibly dense, more than 2000
pounds per cubic centimeter.
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Consists of either helium or hydrogen, and is support
by electron degeneracy pressure.
Ordinarily, electrons can
have a choice of
occupying different
energy states in their
atoms
Stellar remnants — white dwarfs
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A white dwarf is incredibly dense, more than 2000
pounds per cubic centimeter.
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Consists of either helium or hydrogen, and is support
by electron degeneracy pressure.
But if the density is high
enough, you would expect
that the electrons will all get
pushed down to the lowest
energy state. But this can’t
happen!
Stellar remnants — white dwarfs
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A white dwarf is incredibly dense, more than 2000
pounds per cubic centimeter.
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Consists of either helium or hydrogen, and is support
by electron degeneracy pressure.
There is a limit to how many
electrons can fit in each
energy level (for instance, the
ground state can only have
two electrons in it). This gives
rise to electron degeneracy
pressure.
Stellar remnants — neutron stars
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But if the core is more massive than 1.4Msun, then the
gravitational force is strong enough to overcome the
degeneracy pressure, and the electrons are smashed
down into the nuclei.
Stellar remnants — neutron stars
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But if the core is more massive than 1.4Msun, then the
gravitational force is strong enough to overcome the
degeneracy pressure, and the electrons are smashed
down into the nuclei.
Subrahmanyan Chandrasekhar
(1910-1995)
Stellar remnants — neutron stars
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But if the core is more massive than 1.4Msun, then the
gravitational force is strong enough to overcome the
degeneracy pressure, and the electrons are smashed
down into the nuclei.
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What is left is a neutron star — basically a ball of pure
neutrons — which is supported by neutron degeneracy
pressure. A neutron star is about 108 times denser than
a white dwarf (think of something with the mass of the
Sun compressed into a sphere just a few kilometers
across).
Stellar remnants — neutron stars
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Neutron stars rotate very rapidly due to the
conservation of angular momentum (the current
record-holder is 716 times per second). They can also
emit pulses of radio waves every time the rotate.
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They were discovered in 1967 by Jocelyn Bell and
Antony Hewish
Stellar remnants — black holes
If the original star is more massive than about 25Msun,
then the neutron degeneracy pressure isn’t strong enough
to support the neutron star from further gravitational
collapse. What is left is a black hole.
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This is an object so dense that — if you’re close enough
— it’s impossible to escape. Not even light can escape.
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This point of no return is called the event horizon. It
has radius of 10km for a 1Msun black hole.
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What goes on inside of the event horizon? Since we
can’t see past the event horizon, we have no idea… and
we may never know!
Stellar remnants — black holes
event horizon
??
Stellar remnants — black holes
Black holes are perhaps the most mysterious objects in the
universe.
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According to Einstein’s theory of gravity, they are
literal “holes” in spacetime. Once an object falls in, it
is cut off from the rest of the Universe forever.
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If you do have the misfortune to fall into a black hole,
you won’t even live to see what goes on inside the
event horizon because you’ll be ripped apart by tidal
forces before you even get there.
Stellar remnants — black holes
Black holes
What would happen if you fell into a black hole, other
than the fact that you’d get ripped apart?
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Time would slow down for you relative to outside
observers. By the time you reached the event horizon,
your time would actually stop for those outside
observers — you would appear frozen.
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Light gets redshifted as it leaves a gravitational field. So
outside observers would also see you become infinitely
redshifted.
Black holes
Yes, they really do exist
Black holes
But it is now generally believed that Einstein’s theory of
gravity doesn’t fully describe what happens in a black
hole. If we could somehow combine Einstein’s theory
with quantum physics, then we might realize that they’re
much more than a big, voracious ball of nothing.
Black holes
But it is now generally believed that Einstein’s theory of
gravity doesn’t fully describe what happens in a black
hole. If we could somehow combine Einstein’s theory
with quantum physics, then we might realize that they’re
much more than a big, voracious ball of nothing.
For instance, Stephen Hawking
used quantum arguments to
show that black holes actually
emit light with a perfect thermal
spectrum. A 1Msun black hole
has a temperature of 6x10-8
Kelvin
Q: If the Sun was suddenly squeezed small enough to
become a black hole, (it won't happen, but just
suppose…!)
A. Earth would get sucked in
B. Earth would continue in orbit pretty much as
before
C. Earth would get very cold
D. B and C
E. None of the above
Q: If the Sun was suddenly squeezed small enough to
become a black hole, (it won't happen, but just
suppose…!)
A. Earth would get sucked in
B. Earth would continue in orbit pretty much as
before
C. Earth would get very cold
D. B and C
E. None of the above
Chemical evolution of the universe
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Shortly after the big bang, the universe consisted
almost entirely of hydrogen and helium, with just a tiny
amount of lithium. Heavier elements are created by
nuclear fusion within the cores of stars.
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Planetary nebulae and supernovae are responsible for
expelling these elements out into the universe, which
can then be recycled into subsequent generations of
stars and planets.
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By the time the Sun was formed, about 2% of the matter
in the Milky Way was converted into heavier elements
— the stuff that we are made of.
Chemical evolution of the universe
We are made of starstuff — Carl Sagan