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Stellar Evolution
‘The life-cycle of stars’
Star Energy
• Nuclear Fusion – a nuclear reaction in which to
atoms are fused together…
• New elements are created and energy is
released.
• This process is responsible for creating ALL
elements found in the universe…
in other words, we are all made from star dust.
Star Energy (con’t)
• Hydrogen fusion
H + H  He +
Energy
• Helium fusion
He + He  Be +
More Energy
• As the mass of elements increases,
energy production increases
1. Nebula
• A cloud of gas and
dust
• Gravity causes the
cloud to collapse and
condense
• Temperatures begin
to increase = Glows
• Fusion begins at
VERY high temps.
(Some of the extra gas and dust may form planets)
Main Sequence Stars
• Core reaches a temp
of ~15 million K.
• Hydrogen begins to
fuse into Helium in
the core.
• ~90% of stars lifetime
is spent in the main
sequence stage.
•Classified based on temperature and luminosity
Giant Stars
• Core decreases in size
as
• All (or most) H is
consumed
• He fusion is occurring –
producing more energy
• Diameter increases x10
• Surface temp decreases
as star expands
Super Giants
• Form from massive stars
• A chain of reactions take
place in the core producing
He, C, O, Ne, Mg, Si, S, Ar,
Ca, Ti, Cr, Fe
• Highest temperature = Blue
Super Giants
• Usually explode in a
tremendous event called a
supernova
Nova/Super Nova
• The core of a giant star
produces too much
energy in it’s core and
causes the outside of
the star to expand.
• The outer layer of
gasses are blown out
to space (nova, or
supernova), leaving
behind a small, hot
core…
White Dwarf
• Stars decrease in size ~ same diameter as
earth, mass stays the same
• Mass of sun, size of Earth
• DENSITY INCREASES TREMENDOUSLY
D = 1,000,000 g/cm3
1 sugar cube > 1 car
• Solid, but still hot, so it is glowing
• Luminosity decreases
Neutron stars
• Form from the remains of extremely
massive stars after a supernova.
• Very small ~ 30 km across
• D = 2 x 1014
• 1 sugar cube = mass of humanity
The fate of our Sun