Stellar Evolution
Pressure vs. Gravity
Pressure vs. Gravity
• Two determining forces that govern a
star’s existence
Gravity pushes inwardly
Pressure pushes outwardly
Pressure
• Stars support themselves against
gravitational collapse by generating
thermonuclear energy
• Most stars are composed mainly of
Hydrogen and thus fuse together to form
Helium.
Proton-Proton Chain
• p + p  d + e+ + νe
• p + d  3He + γ
• 3He + 3He  4He + p + p
• These reactions take about 5 billion years
to go into effect.
Low Mass Star Evolution
• Our sun is a example of low mass star that
uses Hydrogen as fuel
• Lower mass stars have less energy
available to fuse heavier elements
together
• Slower burning process; longer lives
Gravity
• When star runs out of fuel gravity wins out
• Contraction produces heat
• Helium core then becomes hot enough to
begin burning Helium for fuel to produce
Carbon
• To radiate the energy produced by the
Helium burning, the star expands into a
Red Giant.
White Dwarf
• Outer Hydrogen envelope, i.e. planetary
nebula eventually drifts off
• Hot remnant core is a white dwarf
• No more support from burning fuel.
• Thermal motion of the ions will become
less important and eventually degenerate
electron pressure opposes gravitational
collapse.
Importance of White Dwarfs
• More accurate estimate of the age of the
universe
• At these extremes of temperature and
pressure we expect to observe deviations
from existing theories of matter
High Mass Star Evolution
• Approx. 8 solar masses or greater
• More massive stars have higher core
temperatures so, burn beyond C.
• H He C Ne O Si Fe
• Cannot burn beyond Fe because it is the most
tightly bound state. Burning Fe would no longer
release energy, but require energy absorption.
• These stars have short violent lives.
Nucleosynthesis
Massive Star Collapse
• Burn Hydrogen up through Carbon, Neon,
Oxygen & Silicon
• Iron Core Formation & burning shells
• Catastrophic collapse of Iron Core.
• Happens very quickly
End of Road
• Two possible outcomes for massive stars.
Neutron Star
Black Hole
Summary
• Stellar evolution depends on gravity and
pressure.
• Less Massive stars have less energy so
end up as white dwarfs. These stars live
very long
• More massive stars have more energy so
end up as neutron stars or black holes.
They have very short lives