Download The Main Sequence

Stellar Evolution
Where do gold earrings come from?
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Goals
• Explain why stars evolve off the main sequence.
• What happens when they leave the main
sequence?
• How does mass affect what happens?
• How do stars die?
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The Main Sequence
• Stars characterized by
what holds them up.
• 90% held up by heat of
Hydrogen fusion
4H  He + Energy
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M.S. Lifetime
• More Massive 
Hotter
• Hotter  More
luminous
• More luminous 
shorter life
TLife
Mass ( M Sun )

 TSun
L( LSun )
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M13
M13 – Natalie Redfield ‘06
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Age of M13
12 billion years old
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The Main Sequence
• A star is a delicate
balance between
the force of
gravity pulling in,
and pressure
pushing out.
• Stars on the main
sequence fuse
hydrogen in their
core to produce
thermal pressure.
• Longest phase of a
star’s life.
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What then?
• When the hydrogen in the core is almost consumed
the balance between gravity thermal pressure
pushing out and gravity pushing in is disturbed.
• The structure and appearance of the star changes
dramatically.
• What happens then, depends on the star’s mass.
• Two cases:
– Low-mass (< 8 x mass of Sun)
– High-mass (> 8 x mass of Sun)
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Helium
Ash
• Heavier elements, sink to the “bottom.”
• After 10 billion years, core is “choked” with helium “ash”.
• H  He continues in shell around non-burning core.
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The Red Giant Branch
• Without fusion pressure in
core:
– Helium core collapses (no
counter to gravity)
– Density in core increases.
• 3He  C + Energy in core
• 4H  He + Energy in shell
• Extra energy results in extra
pressure. Star expands.
• The star gets bigger while
its outside gets cooler.
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The Onion Sun
• Red Giant Stars
• Layers of:
–
–
–
–
Non-fusing H
Fusing H
Fusing He
Non-fusing C “ash”
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…And the Solar System?
• A few million years from now:
– Sun becomes slightly brighter
– Ocean’s begin to evaporate
– “Hot House” Earth
• A few billion years from now:
–
–
–
–
Sun swells up
Engulfs the inner Solar System
Certain death for terrestrial planets
Possible “spring” on the Jovian ocean-moons!
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Red Supergiant
• What happens when the
Sun runs out of helium
in its core?
• Same as before.
• Core shrinks, surface
expands.
• Radius ~ 3 AU!
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Death
• Core is contracting and heating.
– Surface is cooling and expanding.
• Will it finally become hot enough in core for
Carbon to fuse?
• For the Sun: No.
• Gravity keeps contracting the core: 1000 kg/cm3!
• What stops it?
• Electron degeneracy pressure!
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Electron Degeneracy
Pressure from motion of atoms
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Electron Degeneracy
Pressure from electron shells
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NGC3242 – HST – Bruce Balick
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M57 – Ring Nebula
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M27 – Dumbbell Nebula – copyright VLT, ESO
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Cat’s Eye
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Eskimo Nebula
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Hourglass Nebula
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White Dwarf
• Mass of Sun
• Radius of Earth
• Hot as Sun’s core
• A million times denser than lead
• Slowly cool off
NGC2440 – HST – Bruce Balick
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High-Mass Stars
• Think back to the first
carbon core.
• How they get from
main sequence to the
carbon core stage is a
little different.
• Now however, there is
enough mass that it
becomes hot enough to
fuse carbon?
• Hot enough to eventually fuse lots of elements.
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The Iron Core
4H  He + Energy
3He  C + Energy
C + He  O + Energy
The ash of one reaction, becomes the fuel of
the next.
• Fusion takes place in the core as long as the
end result also yields energy.
• This energy causes pressure which counters
gravity.
• But Iron doesn’t fuse.
•
•
•
•
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Core-Collapse
•
•
•
•
•
•
Iron core – no outward pressure.
Gravity wins!
Star collapses rapidly!
Electron degeneracy can’t stop it.
Atomic structure can’t stop it.
Electrons and protons crushed together to
produce neutrons.
• Neutrons pushed together by force of gravity.
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Supernova
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Supernova
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Supernova
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Supernova
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Supernova
• The result of the catastrophic collapse is the
rebound and explosion of the core.
• From start of collapse to now: 1 second!
• Matter thrown back into the interstellar
medium.
• Matter rushing outwards, fuses with matter
rushing inwards.
• Every element after Fe is made in the instant
of a supernova!
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M1 – Crab Nebula – copyright VLT
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Veil Nebula – Lua Gregory (English ’05)
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NGC 4526 – 6 Million parsecs away
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Neutron Stars
• A giant ball of neutrons.
• Mass : at least 1.4 x mass of
the Sun.
• Diameter: 20 km!
• Density: 1018 kg/m3
– A thimble weighs as much
as a mountain
• Day: 1 – 0.001 seconds!
• Magnetic fields as strong as the Sun, but in
the space of a city.
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Pulsars
• Interstellar
Lighthouses.
• See periodic bursts
of radiation.
• Perfect clocks.
• While every pulsar is
a neutron star, the
opposite isn’t true.
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Crab Nebula Pulsar
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Neutron Degeneracy
• Neutron stars are held up by neutron degeneracy
pressure.
– Recall electron degeneracy pressure for white dwarfs.
– For white dwarfs, maximum mass of 1.4 Msun
• For neutron stars, maximum mass ~3Msun
• What happens if a high-mass star is SO big that
its central core is bigger than this?
• What happens when gravity is stronger than
even neutron degeneracy pressure?
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Density
• Density = mass per volume
• From Red Giant cores to White Dwarfs to Neutron
Stars, density has been increasing.
• As density increases, the force of gravity on the
surface increases.
• The greater the force, the higher the escape velocity:
– How fast you need to go in order to escape the
surface.
• How dense can something get?
• How strong can the force of gravity be?
• What if the escape velocity is faster than light?
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Singularity
• When a high-mass star’s
core is greater than ~3 x
Msun, then, when it
collapses, neutron
degeneracy pressure can’t
balance gravity.
• The star collapses to form
a singularity.
• No size at all.
• Density infinite.
• Escape velocity > c
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Black Hole Diagram
Singularity
Event Horizon
.
Schwarzschild Radius
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Seeing Holes
• Can’t see black hole
itself, but can see matter
falling into a hole.
• Gravitational forces
stretch and rip matter:
heats up.
• Very hot objects emit in
X-rays (interior of Sun)
• Cygnus X-1.
http://www.owlnet.rice.edu/~spac250/steve/ident.html
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Binaries
• Gravitational tides pull matter off big low density
objects towards small high density objects.
Cygnus X-1
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