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Stellar Evolution
Basic Structure of Stars
Mass and composition of stars determine
nearly all of the other properties of stars
 More massive a star is, the greater the
gravity is, and the hotter and denser the
star is inside
 Temperature inside stars determines rate
of nuclear reactions, which in turn affects
the energy output, or luminosity of the star

Fusion
Density and temperature increase toward
the center, where energy is generated by
nuclear fusion (hydrogen into helium)
 Stars not on main sequence either fuse
different elements in their core, or do not
undergo fusion at all
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Stellar Evolution and Life Cycle
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A star changes as it ages
because its internal composition
changes as nuclear fusion
reactions convert one element
into another
As the star’s core composition
changes, its density increases, its
temperature rises, and its
luminosity increases
When nuclear fuel runs out, star’s
internal structure and mechanism
for producing pressure must
change to counteract the force of
gravity
Star Formation

Begins with
cloud of
interstellar
gas and dust,
called a
nebula that
collapses in
on itself as a
result of its
own gravity
Star Formation
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As the nebula cloud
contracts, its
rotation forces it into
a disk shape with a
hot condensed
object at the center,
called a protostar
The condensed
object will become a
new star
Star Formation
Eventually, temperature inside protostar is
hot enough for nuclear fusion reactions to
occur
 Once fusion of hydrogen to helium occurs,
the star becomes stable because it has
sufficient internal heat to produce the
pressure needed to balance the pressure
of gravity
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Life Cycle of Average Stars (Our Sun)
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What happens during a star’s life cycle depends
on its mass
As a star like the Sun converts hydrogen to
helium in its core, it gradually becomes more
luminous because the core temperature and
density rise and increase the fusion reaction rate
Takes 10 billion years for average-sized star to
convert all the hydrogen in its core into helium
Life Cycle of Average Stars (Our Sun)
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Only innermost 10% of a star’s
mass can undergo nuclear
reactions because temperatures
outside the core never get hot
enough for nuclear reactions to
occur
When all the hydrogen in a star’s
core is gone, star has a helium
center and the outer layers have
mostly hydrogen gas
Some hydrogen will continue to
undergo reactions in the
outermost layer of the helium
core; energy produced in this
layer forces out layers of the star
to expand and cool
The star has now become a red
giant
Life Cycle of Average Stars (Our Sun)
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While the star is a red giant,
it loses gas from its outer
layers
Core becomes hot enough to
for helium to react and form
carbon
Star contracts to a more
smaller size where it is more
stable
Star never becomes hot
enough for carbon to react,
so star’s energy production
ceases at this point
Outer layers of gas expand
and are driven off
This outer layer of gas is
called a planetary nebula
Core of star becomes
exposed as a small, hot
object the size of Earth
called a white dwarf
White dwarf remains
Life Cycle of Massive Stars
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More massive star
begins its life
same way as
average sized
stars, but the
star’s lifetime is
much shorter
because the star
is so luminous
and uses its fuel
up quickly
Undergoes more
reaction phases
and produces
more elements in
its interior
Life Cycle of Massive Stars
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Star becomes a red
giant several times as it
expands following the
end of each fusion
reaction
As more shells are
formed, the star expands
to larger size and
becomes a supergiant
(EX: Betelgeuse)
Light Echo Illuminates Dust Around
Supergiant Star V838 Monocerotis (V838 Mon)
Size Comparison of Stars
Life Cycle of Massive Stars
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Some massive stars lose enough mass to
become white dwarfs
Stars that don’t lose that much mass come to a
more violent end
Once reactions in the core have produced iron,
no future reactions can occur and the core the
star quickly collapses on itself
Neutron star is formed while outer gas layers
are blown off in a massive explosion called a
supernova
Life Cycle of Massive Stars
Neutron Star
Supernova explosion
Life Cycle of Massive Stars
When they are formed
neutron stars rotate in
space. As they compress
and shrink, the rotation
occurs faster. Those
bodies that are still
spinning rapidly may emit
radiation that from Earth
appears to blink on and
off as the star spins, like
the beam of light from a
turning lighthouse. This
"pulsing" appearance
gives some neutron stars
the name pulsars.
The white dwarf in the AE Aquarii system
is the first star of its type known to give off
pulsar-like pulsations that are powered by its
rotation and particle acceleration.
Credit: Casey Reed
Life Cycle of Massive Stars
Some stars too massive to form neutron
stars. The core of these stars collapses
forever, compacting matter into smaller
and smaller volumes
 The small, extremely dense object that
remains is called a black hole
 Called a black hole because its gravity is
so great that nothing, not even light, can
escape
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Black Holes
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Estimated that black holes will
consume other stars at a rate of
about once every ten thousand
years in a typical galaxy
Journey to a Black Hole!
What happens if you fall into a
black hole?
Life Cycle of Stars – Summary