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Star Life Cycle
Stellar Evolution: Life of the
 The most massive stars have Star
the shortest lives. Stars that
are 25 to 50 times that of the
sun live for only a few
million years. Stars like our
Sun live for about 10 billion
years. Stars less massive than
the Sun have even longer life
spans
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Stars are like humans they
are born, live and die
The Birth of Stars
Stars are formed in
1. Nebulae, interstellar
clouds of dust and gas
(mostly hydrogen).
These stellar nurseries are
abundant in the arms of spiral
galaxies.
In these stellar nurseries,
dense parts of these clouds
undergo gravitational collapse
and compress to form a
rotating gas globule.
It begins to spin as it shrinks
NEBULA PICTURES
Protostar stage: The nebula
flattens and the center condenses:
Temps start to increase due to friction
(more collision of molecules).
When temps reach 10 million K,
NUCLEAR FUSION begins
Nuclear Fusion generates the energy
for a star.
When fusion begins, it is officially
considered a STAR. (yeah!)
Fusion – combining a lightweight
nuclei into a heavier nuclei
Protostar
Pictures
Main-Sequence Stage
The protostar is now a
stable main sequence
star which will remain
in this state for about 10
billion years. After that,
the hydrogen fuel is
depleted and the star
begins to die.
Stars live out the
majority of their lives in
a phase termed as the
Main Sequence.
Longest stage of a star
Main Sequence Star
RED GIANT STAGE
When a star has burned between 10% and 20% of its hydrogen,
its core will to run out of fuel.
At this stage, the star is entering the end of its life.
The diameter of the star can increase by a factor of 200, while its
cooling is translated into a reddening of its radiation : the star is
becoming what is called a red giant.
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Star has run out of
Hydrogen atoms in the core
to undergo fusion.
Our sun has used only about
5% of its Hydrogen
Star expands about 10x
bigger and cools.
Its luminosity increases,
temp decreases.*****
Helium fuses to form
Carbon, and the core
shrinks.
It begins losing outer layers
BETELGEUSE –RED GIANT
F.Y.I.
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Only a few million years
old, Betelgeuse is already
dying. Astronomers
predict that it's doomed to
explode as a soon, within
1,000 years or so, an event
that will be spectacular for
Earth's future inhabitants.
(Conceivably, it's already
happened as Betelgeuse is
640 light-years away!)
•Star size comparison
Burnout and Death
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Fate of a Star depends on
its mass
All stars, regardless of
size, run out of fuel and
collapse due to gravity
A star will become
either a black dwarf,
neutron star, or
black hole,
depending on how
massive it was. .
Death of low-mass stars
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Never evolve into red giants
Remain stable main-sequence stars until
hydrogen is gone
Collapse into white dwarfs
**Death of medium-mass stars**
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Sun-like Stars
(Mass under 1.5 times the mass of the Sun)
Red
Giant --> Planetary Nebula -->White Dwarf
--> Black Dwarf
**Death of massive stars**
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Huge Stars
(Mass between 1.5 to 3 times the mass of the Sun)
Red SuperGiant --> Supernova --> Neutron Star
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Giant Stars
(Mass over 3 times the mass of the Sun)
SuperGiant --> Supernova --> Black Hole
Red
PLANETARY NEBULA
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This is left when a giant loses its outer layers
of gas.
Leaves only the core.
The core will become a white dwarf
Planetary nebula pictures
WHITE DWARF
Only hot, dense core is left of this star.
 It will shine for billions of years before
cooling.
 Stable star with no nuclear fuel,radiates left
over fuel for billions of years
 Could become a black dwarf, but the
universe is not old enough to form these
White Dwarf Pictures
SUPER NOVA
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Super Giant eventually
loses its outer layers in an
explosion leaving only
the core – this is the
super nova.
Eventually this core collapses (in an instant). As
the iron atoms are crushed together in this
gravitational collapse, the core temperature rises
to about 100 billion degrees.
The repulsive electrical forces
between the atoms' nuclei overcomes
the gravitational forces, causing a
massive, bright, short-lived explosion
called a supernova. During the
explosion, shock waves, blow away the
star's outer layers.
NEUTRON STAR
After a Super Nova explosion, the core may
condense into a small core of neutrons.
 It is so dense – 1 spoonful of Earth would weigh
100 million tons.
 Rotates very rapidly.
If the star's remaining mass is between 1 1/2 to 3 times the
mass of the Sun, it will collapse into a small, dense
neutron star (about ten miles in diameter, about 1.4 times
the mass of the Sun, with an extraordinarily strong magnetic
field, and rapid spin).
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Neutron Star Pictures
BLACK HOLE
This is the 3rd predicted result to a star.
 This is the most dense core of a star that
can be left.
 Gravity is so strong, light cannot escape.
 Makes it look like a dark hole in space.
If the star's remaining mass is greater than three times
the mass of the Sun, the star contracts tremendously
and becomes a black hole
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Black Hole Pictures
Anatomy of a black hole
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The density of a star is pre-determined based
on its weight (the amount of dust and gas)
it begins with.
Smaller stars become white dwarfs.
Very large stars become neutron stars or
black holes.
•Life Cycle of a Star Video