Transcript S1E4 Extreme Stars

The Birth, Life and Death of Stars
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.
Stellar
“Forest”
Imagine an enormous cloud of gas and dust
many light-years across. Gravity, as it always
does, tries to pull the materials together. A
few grains of dust collect a few more, then a
few more, then more still. Eventually, enough
gas and dust has been collected into a giant
ball that, at the center of the ball, the
temperature (from all the gas and dust
bumping into each other under the great
pressure of the surrounding material) reaches
15 million degrees or so.
A wondrous event occurs....
nuclear fusion begins and
the ball of gas and dust
starts to glow. A new star
has begun its life in our
Universe.
So what is this magical thing called
“nuclear fusion” and why does it start
happening inside the ball of gas and
dust? It happens like this.....
As the contraction of the gas
and dust progresses and the
temperature reaches 15
million degrees or so, the
pressure at the center of the
ball becomes enormous. The
electrons are stripped off of
their parent atoms, creating a
plasma.
Eventually, they approach each other so fast
that they overcome the electrical repulsion
that exists between their protons. The nuclei
crash into each other so hard that they stick
together, or fuse. In doing so, they give off a
great deal of energy.
This energy from fusion pours out from the
core, setting up an outward pressure in the gas
around it that balances the inward pull of
gravity. When the released energy reaches the
outer layers of the ball of gas and dust, it
moves off into space in the form of
electromagnetic radiation. The ball, now a
star, begins to shine.
Watching stars being born
The Bubble Nebula
Here you can see the old dust
and gas being blown away by
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heat of the new star.
The Stellar Cycle
Cool molecular clouds
gravitationally collapse
to form clusters of stars
New (dirty) molecular
clouds are left
behind by the
supernova debris.
Molecular
cloud
Stars generate
helium, carbon
and iron through
stellar nucleosynthesis
The hottest, most
massive stars in the
cluster supernova –
heavier elements are
formed in the explosion.
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Anatomy of a Main Sequence Star
Hydrogen
fuel
Helium
“ash”
Hydrogen
burning core
shell
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Throughout their lives, stars fight the inward
pull of the force of gravity. It is only the
outward pressure created by the nuclear
reactions pushing away from the star's core that
keeps the star “intact”. But these nuclear
reactions require fuel, in particular hydrogen.
Eventually the supply of hydrogen runs out and
the star begins its demise.
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.
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)
Death of a Star
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When all fuel runs out, the core collapses
Outer regions of star explode outwards: Supernova
SN shine more brightly than a galaxy for a few hours/days
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
Carbon-triple alpha process
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).
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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On-line Lessons: The Birth and
Death
of
Stars
The end of a Sun-like star
• The outer parts of the
star (that formed the Red
Giant) then drift off into
space and cool down
making a Planetary
Nebula.
•
Planetary nebulae have nothing to do with
planets, of course, they just look a bit
like them in small telescopes!
• Here you can see a
planetary nebula called
M57 with its White Dwarf
in the middle.
Image from the Liverpool
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”.
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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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.
What is left after a Supernova?
• Because the star was so big, the collapse does
not stop even with a White Dwarf, but an
even more dense object called a Neutron
Star is made.
• The density of a Neutron star is about 1x1018
kg/m3 (that is 1,000,000,000,000,000,000!)
• Sometimes the collapse cannot stop at all and
a Black Hole is made, from which not even
light can escape!
• The debris of the explosion is blown away and
forms a glowing cloud called a Supernova
Remnant.
The Crab Supernova
Remnant
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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We are made of stardust!
May 2006April 2004
Belinda Wilkes
Stellar Evolution in a Nutshell
0.5 MSun < M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mcore > 3MSun
Mass controls the
evolution of a star!
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
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?
1. It was lost in a supernova explosion
2. It flows outward in a planetary nebula
3. It is converted into energy by nuclear fusion
4. The core of the Sun gravitationally collapses,
absorbing the mass
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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?
1. It was lost in a supernova explosion
2. It flows outward in a planetary nebula
3. It is converted into energy by nuclear fusion
4. 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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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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White Dwarf and
Planetary
Nebula
Collapsing
cloud
A new
star
Sun-like
stars
Supernova
Remnant and
Neutron Star
Red
Giant
Massive
stars
Birth and Death of Stars - Summary