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Stellar Evolution
From Nebula
Neutron Star
Basic Structure
The more massive the star
the hotter it is, the hotter it
is the brighter it burns
• Mass is the most important
• Determines brightness,
temperature & diameter (volume)
Pressure Balance
Force of gravity
pulling in =
Pressure from
fusion pushing
Otherwise it
would expand &
Sun’s Energy
Remember how the
sun produces
Sun’s Energy Cont’d
• Not all stars fuse H into He
• Helium can be fused to Carbon,
Carbon to nitrogen and more…
• All to produce enough energy to fight
Stellar Evolution
Stars change throughout their life
Changes in what they fuse as
they run out of fuel causes them
to adjust to differing pressure
The Beginnings
• All stars start
out the same
• Begin as
nebulas: diffuse
cloud of dust
and gas
Fusion Begins
• When enough pressure
builds and enough heat
is produced H is fused
to helium
• Has enough force
pushing out to balance
gravity pulling in
• Remains this way for
up to ten billion years
Average Size Stars
• When a star begins fusing Hydrogen
into Helium it enters the Main
• The amount of H is limited so it runs
out (~10bil yrs)
• Outer regions of the core push out
the surface
Becomes a Red Giant
Average Size Stars
• During Red giant stage
Fuses He into Carbon
Outer layers expand
He fusion last a fraction as long as H fusion
Star then shrinks back to normal
Becomes stable for a while
Average Size Star
Outer layers are given off and star
becomes a planetary nebula
Average Size Stars
White Dwarf
• Star shrinks to that of Earth’s size
• Becomes stable
• No fusion is going on
Massive Stars
• Begin their life in the same as
medium sized stars
• Go through life cycle much
Think of them as a huge SUV’s,
very low gas mileage
Massive Stars
• Produce enough
heat to fuse
• From many
layers w/fusion
of different
Death of Massive Stars
• May also become white dwarfs
• After Iron, no more fuel to fuse
• Collapses violently & 2 things
can happen
1. Electrons & protons form solid ball of
neutrons (Neutron Star)
2. Outer gases rebound off a neutron star
shooting outward as a Supernovae
Neutron Star
Black Holes
• Happens to stars that begin 20x the
mass of the Sun
• Core of neutrons not strong enough
to fight gravity
• Continues to collapse until it’s so
massive that gravity won’t even let
light escape
Black Holes