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Stellar Evolution
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Life Cycle of stars
Stars
Mass determines a star’s
temperature, luminosity,
and diameter.
 Hydrostatic equilibrium is
when the inward pressure
of gravity is equal to the
outward pressure from
fusion and radiation.
 If this equilibrium does not
exist the star will expand
or collapse.

Fusion
Hydrogen molecules fuse
together to form helium in
the core of a young star.
 Some older, bigger stars can
either fuse helium to form
other elements or no fusion
happens at all.

Star Formation-The
beginning of all stars
All stars form in the same
manner.
 The star begins as a cloud of
interstellar gas and dust
called a nebula.
 The nebula collapses on
itself as a result of its own
gravity.
 The cloud begins to rotate
around the center and when
the center gets hot it is
called a protostar.

Fusion Begins-A true star
is born
The heat of the protostar
increases until it is hot
enough to start fusion in the
center.
 Once fusion begins it is now
stable and a true star.
 This is called the “Main
Sequence” stage! Star
spends most of it’s life here!

Average size star-Red
Giant
The rest of the life cycle
depends on the MASS of the
star.
 Only the core of a star is hot
enough to fuse hydrogen
into helium, when the
hydrogen is gone the star
begins to expand.
 This expansion turns the
star into a red giant.

When the star is a red giant
it begins to lose gas from its
outer layers.
 The star gets so large its
gravity isn’t strong enough
to hold some of the gases
together.
 The core however heats up,
so hot that helium now can
fuse to carbon.
 When the core has used up
all of the helium it is now
entirely carbon.

Back to nebula
The stars mass will never
get high enough to fuse
carbon, so no more energy is
produced.
 The outer layers of gas
expand and are driven off.
 This gas is called a planetary
nebula.
 Only the core is left which is
a white hot ball of carbon
called a white dwarf.

White Dwarfs

White Dwarfs are about the
same size as Earth and are
considered dead stars.
Massive Stars
For stars bigger than the sun
a slightly different path is
taken.
 They form about the same
way, only hydrogen is used
up faster, because they are
so bright.
 These massive stars become
red giants many times, each
time it uses up a new layer
of gases by fusing different
elements together.

Massive Stars
The star expands to a larger
size and becomes a
supergiant.
 As the star expands, each
time it loses some gases in
the outer shell and gets
smaller in mass.
 Eventually the star becomes
a white dwarf. Instead of
being made of only carbon it
can be made of many
different elements.

Supernovae
Some stars do not lose
enough mass to become a
white dwarf.
 These stars are too massive
to be stable and meet a
violent end.
 Once reactions in the core
have made IRON, no more
energy is produced and the
core collapses in on itself.
 Protons and electrons merge
to form neutrons and
become a neutron star!

Supernovae
Neutron stars are incredibly
dense, 3 times the mass of
our sun but only 10 km in
radius!
 The neutron star forms so
fast that the gases around
the star begin to collapse
 When the gases reach the
dense core they explode
outward into a supernova.
 Supernovae spread the
heavier elements around the
universe.

Supermassive Stars (Black
Holes)
Some stars skip the neutron
star stage because they are
supermassive.
 The gravity is so strong that
not even light can escape.

H-R Diagrams
The properties of mass,
luminosity, temperature and
diameter are closely related.
 A Hertzsprung –Russell
diagram puts all the stars on
a graph based on luminosity
and surface temperature.
 90% of all stars fall along a
broad strip called the “main
sequence” which runs
diagonally from top left
(hot, bright) to bottom right
(cool, dim)
