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Notes #2 Stellar
Evolution
• The star called the sun is changing and will burn out over a lifespan of approximately 10 billion years.
• Other than the hydrogen and helium formed at the time of the Big Bang, nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases electromagnetic energy. Heavier elements are produced when certain massive stars achieve a supernova state and explode.
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1. Stars are classified
based on their temperature
and luminosity using the H-
R diagram.
Main Sequence Stars have a relationship between temperature and luminosity
White dwarfs have a high temperature and a low luminosity
Giants and Supergiants have a high luminosity and a low temperature
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2. A star begins as a nebula, concentrates
to form a protostar, and then finally
becomes a main sequence star. Nebula: large cloud of mostly H gas
Rotates and contracts due to gravity
Protostar: a concentrated region within a nebula
Main sequence star a star like our sun
Creates energy from the fusion of H to form He (hydrogen fusion)
Maintains a stable size and luminosity
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Main Sequence Star
The force of gravity
would cause the star
to contract, but it is
balanced by the
expansion of the hot
gas
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3. A main sequence star can
eventually become a red giant or
supergiant when fusion changes. Red giant: becomes larger and red (low
temperature) Was a small main sequence star
Change occurs when hydrogen fusion ends and helium fusion (combination of helium to form carbon and oxygen) begins
Supergiant Was a large main sequence star
Helium fusion occurs followed by more phases of fusion create even heavier elements (up to iron on the periodic table)
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Why does a red giant become
so big?
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4. A red giant will eventually become
a nova, white dwarf, and planetary
nebula when fusion ends.
Planetary Nebula: After fusion ends in a
red giant the gases will drift away
White dwarf: Left over core of a red giant
Small, high temperature star
All of the heat is left over from when fusion
happened
Nova: Explosion of a white dwarf that is small
the star will return to being a white dwarf
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Planetary Nebula
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5. A supergiant will eventually explode in a
supernova and become a neutron star or
a black hole when fusion ends. Supernova: Explosion of a supergiant that completely
destroys the supergiant During the explosion fusion creates heavier elements
Neutron star: Left over core of a supergiant after a supernova explode; only give off radio waves
Black hole A massive star explodes inward
The increase in gravity will not allow visible light nor matter to escape
X-rays are created as matter falls into the black hole
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Supernova Explosion
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Black Hole