Transcript Stellar Evolution II

Stellar Evolution II
The Upper End of the Main Sequence:
How massive can a star get?
Larger clouds of gas (GMCs) tend to fragment into smaller ones
before collapsing to form stars – very massive stars are rare
• Stars with masses above 50 MSUN are unstable – nuclear reactions
in their core produce energy at such a fast rate that they blow off
their outer layers, losing mass.
•
Eta Carinae
• Mass: 100-150MSUN
• Giant eruption in 1843
Made it 2nd brightest star
in sky for a short time
•Created two giant lobes
of hot gas expanding away
from the star
Evolution of Stars on the Main Sequence
• Core starts with same fraction of
hydrogen as whole star
• Fusion changes H  He
• For each reaction, the star loses 4
H and gains only one He, so
pressure decreases and gravity
squeezes the core more tightly
• Core gradually shrinks and gets
hotter, increasing the pressure to
compensate
• Energy generation rate gradually
increases, so star gets more
luminous and the surface gets
hotter. Increased pressure of
radiation increases the radius.
H > He
Evolution of Stars on the Main Sequence
Evolution of Stars on the Main Sequence
• Lifetime of stars on the main sequence:
• More massive stars burn their fuel faster, so will use it up
quicker > have smaller lifetimes
• Lifetime T = 1/M2.5
• O5 - 1,000,000 yr
• A0 - 440,000,000 yr
• G0 - 8,000,000,000 yr
• K0 - 17,000,000,000 yr
• M0 - 56,000,000,000 yr
• (age of universe: 13,600,000,000 yr)
Main Sequence Stars:
• Energy generated by fusing H to He in their core
• Luminosity and surface temperature increase as mass increases
Post-MS Evolution of Low Mass Stars
(M < 8MSUN)
•
What happens when the core runs out of Hydrogen?
H
H > He
Post-MS Evolution of Low Mass Stars
(M < 8MSUN)
• Core stops producing energy
– gravity causes it to contract
and heat up
He
H > He
• Layer surrounding the core
also contracts and heats up
enough to start fusing H to He
• Outer parts of star expand
because star is H – burning layer
is producing more energy than is
required to balance gravity
• Surface gets cooler because of
increased area > star becomes red
giant
Post-MS Evolution of Low Mass Stars
(M < 8MSUN)
Post-MS Evolution of Low Mass Stars
(M < 8MSUN)
Degenerate Gases
Normal Gas:
• Compress normal gases > particles move faster >
increased pressure and temperature
• Heat a normal gas > pressure increases
Degenerate Gases
Degenerate Gases:
• If gas is dense enough, particles have no where to move – if you
compress the gas, the particles cannot move faster; they simply ‘wiggle’
more energetically
• Compress degenerate gas >
temperature increases but
pressure remains the same
• Heat a degenerate gas >
pressure stays the same
Helium Flash
• Matter in core is fully ionized – all electrons are free of their atoms
• Most pressure in the core is from the electrons
• As the core of Helium ash shrinks, it becomes degenerate – its
temperature will increase but its pressure will remain the same
• Density now around 1000,000 grams/cubic cm (about 1000 tonnes/cc)
• As the temperature of the core passes 100,000,000 K, it can start He
fusion into Carbon
Helium Flash
• He
ignites > produces energy > temperature increases, but pressure
stays the same – the core cannot respond to the increased temperature
by expanding
• Increases temperature > increased He fusion rate > increased energy
production > increased temperature > increased He fusion rate >
increased energy production > increased temperature > increased He
fusion rate > increased energy production > increased temperature > ....
• Explosion! – the Helium
Flash
Helium Flash
• For a few minutes the core generates 100,000,000,000,000 times
more energy per second than the sun – 100 times more energy per
second than all the stars in the Milky Way combined
• Does not destroy the star – energy is absorbed in outer layers. No
outward sign of explosion
• Helium flash only occurs in stars between 0.5 and 3 MSUN
• After a few minutes, the core becomes so hot that the gas becomes
normal again, and pressure increases
Helium Burning
• He
fused to C, O in core
• H still fusing to He in
shell around core
He >
C, O
H > He
Helium Burning
• Extra energy from He
Fusion causes core to
Expand
• This forces H burning
shell to expand.
He > C, O
H > He
Helium Burning
• Expansion cools H
burning shell, which then
absorbs heat from the
envelope, causing it to
shrink a little and get
hotter
He > C, O
H > He