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Class 17: Stellar evolution,
Part I
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Evolution of stars of various masses
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Red giants.
Planetary nebulae.
White dwarfs.
Supernovae.
Neutron stars.
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HR diagram gives clues to stellar evolution.
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Main Sequence (MS):
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Consists of stars living out the “normal” part of their
lives…
Stars on MS produce energy via steady hydrogen
burning (i.e., converting hydrogen into helium).
Stars of different mass lie at different points on the
main sequence.
 Mass-luminosity relation: L  M4.
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Eventually, the hydrogen “fuel” runs out and
the stars begin to die… they then leave the
main sequence…
Life of a 0.05 M “star”
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“Star” forms from rotating collapsing
gas cloud (recall formation of solar
system in class 2).
Core heats up to few million K.
Trace deuterium burns to form helium.
That’s it…
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Temperature never gets high enough to
initiate hydrogen burning.
So never really becomes a proper star.
Object becomes a “brown dwarf”.
This is the case up to about 0.08 M.
Evolution of the Sun
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Same beginning… cloud collapses.
This time, core is hot enough to initiate
hydrogen burning (p-p chain).
Steady hydrogen burning for 10 billion
years (5 billion years more to go…).
Then run out of hydrogen in core.
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Nuclear reactions slow then stop.
Core gradually collapses; outer parts of Sun
puff up tremendously – becomes red giant.
He burning starts (forming carbon).
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Once core helium is exhausted, the red giant
blows off its outer layers into space.
Produces a “planetary nebula”.
Only the core of the star is left – becomes a
white dwarf. Cools forever like a dying ember.
Evolution of a 10 M star
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Star forms as before.
H-burning much faster (“CNO cycle”)
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Only lasts a few million years.
When H is exhausted, core contracts, gets
hot enough for helium-burning (makes
carbon).
When He exhausted, core contracts and
gets hot enough for carbon burning.
And so on… until the core is turned into iron
(the most stable element).
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Get shell or “onion”
structure.
No more energy
available when core
becomes iron.
Catastrophic core
collapse…
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Core turns into
neutron star
Rest of star ejected in
a supernova explosion.
Core-collapse (type-II) supernovae
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Very powerful explosion
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1044 J released as radiation (VERY bright!).
100 more released in a neutrino pulse.
SN1987A
(LMC)
Cas-A remnant
Neutron stars
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The remnant of a SN explosion…
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Typical mass of 1.5 M but radius only 10 km!
Made of densely packed neutrons (1018 kg/m3)
– a teaspoonful would weigh a million tons!
Extreme properties…
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Very strong gravity on surface.
Very strong magnetic fields on surface.
Can spin very quickly (hundreds of times per
second)… gives rise to pulsars.