Transcript The Stellar Cycle

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The Birth, Life and Death of Stars
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How can we learn about the lives of stars when
little changes except on timescales much longer
than all of human history?
Suppose you had never seen a tree before, and you
were given one minute in a forest to determine the
life cycle of trees. Could you piece together the
story without ever seeing a tree grow?
This is about the equivalent of a human lifetime to
the lifetime of the Sun.
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Stellar
“Forest”
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The Stellar Cycle
New (dirty) molecular
clouds are left
behind by the
supernova debris.
Cool molecular clouds
gravitationally collapse
to form clusters of stars
Molecular
cloud
Stars generate
helium, carbon
and iron through
stellar nucleosynthesis
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The hottest, most
massive stars in the
cluster supernova –
heavier elements are
formed in the explosion.
Star Birth
• Cold gas clouds contract
and form groups of stars.
• When O and B stars begin
to shine, surrounding gas
is ionized
• The stars in a cluster are
all about the same age.
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Cloud Collapses to Form Stars
Radiation from
protostars arises
from the conversion
of gravitational
energy to heat.
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Pre-Main Sequence Contraction
• Protostars
contract until
core reaches
HHe fusion
temperature.
• Low mass
protostars
contract more
slowly.
• Nature makes
more low-mass
stars than highmass stars.
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Anatomy of a Main Sequence Star
Hydrogen
burning core
shell
Hydrogen
fuel
Helium
“ash”
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Up the red giant branch
As hydrogen in the core is being used up, it starts to contract,
raising temperature in the surrounding. Eventually, hydrogen
will burn only in a shell. There is less gravity from above to
balance this pressure. The Sun will then swell to enormous
size and luminosity, and its surface temperature will drop,  a
red giant.
Sun in ~5 Gyr
Sun today
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Helium fusion at the center of a giant
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While the exterior layers expand, the helium
core continues to contract, while growing in
mass, and eventually becomes hot enough (100
million Kelvin) for helium to begin to fuse into
carbon
Carbon ash is deposited in core and eventually a
helium-burning shell develops. This shell is
itself surrounded by a shell of hydrogen
undergoing nuclear fusion.
For a star with M< 1 Msun, the carbon core
never gets hot enough to ignite nuclear fusion.
In very massive stars, elements can be fused
into Fe.
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The Sun will expand and cool again, becoming a red giant.
Earth will be engulfed and vaporized within the Sun. The
Sun’s core will consist mostly of carbon.
•Red Giants create most of the Carbon in the universe
(from which organic molecules—and life—are made)
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H, He, C burning
Since fusing atomic nuclei repel each other
because of their electric charge, the order of
easiest to hardest to fuse must be
(1)
H, He, C
(2)
C, He, H
(3)
H, C, He
(4)
He, C, H
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Carbon-triple alpha process
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The Sun’s Path
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Planetary Nebula Formation
• When the Red Giant
exhausts its He fuel
– the C core collapses  white
dwarf
– No fusion going on inside …
this is a dead star.
• He & H burning shells
overcome gravity
– the outer envelope of the
star is blown outward  a
planetary nebula
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What holds the white dwarf from collapsing?
• As matter compresses, it
becomes denser.
• Eventually, the electrons
are forced to be too close
together. A quantum
mechanical law called the
Pauli Exclusion Principle
restricts electrons from
being in the same state (i.e.,
keeps them from being too
close together).
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Indistinguishable particles
are not allowed to stay in
the same quantum state.
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What holds the white dwarf from collapsing?
• The resulting outward pressure which keeps the
electrons apart is called electron degeneracy
pressure – this is what balances the weight.
• Only if more energy drives the electrons into
higher energy states, can the density increase.
• Adding mass can drive electrons to higher
energies so star shrinks.
• At 1.4 solar masses—the Chandrasekhar Limit—
a star with no other support will collapse, which
will rapidly heat carbon to fusion temperature.
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WD has a size slightly less than that of the earth. It is so
dense, one teaspoon weights 15 tons! WD from an isolated
star will simply
cool, temperature
dropping until it is no
1
teaspoon
=
1
elephant
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longer
a “black dwarf”.
Sun’s life
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What is a planetary nebula?
(1) A large swarm of planets surrounding a star.
(2) A disk of gas and dust around a young star.
(3) Glowing gas in Earth’s upper atmosphere.
(4) Ionized gas around a white dwarf star.
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The lead-up to disaster
• In massive stars (M > 8 Msun),
elements can be fused into
Fe.
• Iron cores do not immediately
collapse due to electron
degeneracy pressure.
• If the density continues to
rise, eventually the electrons
are forced to combine with
the protons – resulting in
neutrons.
• Now the electron degeneracy
pressure disappears.
• What comes next … is core
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collapse.
Supernova! Type II (Core-Collapse)
• The core implodes, but no fuel there, so it collapses until
neutron degeneracy pressure kicks in.
• Core “bounces” when it hits neutron limit; huge neutrino
release; unspent fuel outside core fuses…
• Outer parts of star are blasted outward.
• A tiny “neutron star” or a black hole remains at the
center.
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Supernova 1987a before/after
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Production of Heavy Elements
(There is evidence that the universe began
with nothing but hydrogen and helium.)
• To make elements heavier than iron extra
energy must be provided.
• Supernova temperatures drive nuclei into each
other at such high speeds that heavy elements
can be made.
• Gold, Silver, etc., -- any element heavier than
iron, were all made during a supernova.
We were all once fuel for a stellar furnace.
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Parts of us were formed in a supernova!
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Life of a 15 solar mass star
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Stellar Evolution in a Nutshell
0.5 MSun < M < 8 MSun
M > 8 MSun
Mcore < 3MSun
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Mass controls the
evolution
of a star!
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The H-R diagram
1. Which of these star is the
hottest?
2. What are Sun-like stars (0.5
Msun < M < 8 Msun) in common?
3. What about red dwarfs
(0.08 Msun < M < 0.5 Msun) ?
4. Where do stars spend most
of their time?
5. Which is the faintest? the
sun, an O star, a white
dwarf, or a red giant?
O
Stars with M < 0.08 Msun  Brown
dwarf (fusion never starts)
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Answers: 1. OU6_StarLife
star, 2. end as a WD, 3. no RG phase, lifetime
longer than the age of the Universe, 4. MS, 5. WD
The evolution of 10,000 stars
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If we came back in 10 billion years,
the Sun will have a remaining mass
about half of its current mass.
Where did the other half go?
• It was lost in a supernova explosion
• It flows outward in a planetary nebula
• It is converted into energy by nuclear fusion
• The core of the Sun gravitationally collapses,
absorbing the mass
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A star cluster containing _____
would be MOST likely to be a few
billion years old.
(1) luminous red stars
(2) hot ionized gas
(3) infrared sources inside dark clouds
(4) luminous blue stars
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