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Stellar Evolution (Star Life-Cycle)
Basic Structure
Mass governs a star’s
temperature, luminosity,
and diameter. In fact,
astronomers have
discovered that
the mass and the
composition (makeup) of
a star determine
nearly all its
other properties.
The balance between
gravity squeezing
inward and pressure
from nuclear fusion
and radiation pushing
outward, called
hydrostatic equilibrium,
must hold for a star to be stable.
Otherwise, the star would expand
or contract.
Fusion
Fusion reactions involving
elements other than hydrogen
can occur. Once a star’s
core has been converted
into helium, the helium
may fuse to form carbon
if the temperature is high
enough. At even higher temperatures, carbon can
react with helium to form oxygen, then neon,
then magnesium, then silicon, and finally, iron.
Life Cycle
1. All stars form in much the same
manner as the Sun did. The
formation of a star begins with a
cloud of interstellar gas and dust
called a nebula.
2. The nebula collapses on itself as a result of its own
gravity. As the cloud contracts, its rotation forces it
into a disk shape
with a hot condensed
object at the center
called a protostar.
Eventually, the
temperature inside a
protostar becomes hot enough
for nuclear fusion reactions to
begin. 3. Once this reaction
begins, the star becomes a main
sequence star (stable) because it then
has sufficient internal
heat to produce the pressure
needed to balance
gravity.
What happens during a star’s life cycle depends on
its mass.
For example, as a star like the Sun converts hydrogen
into helium in its core, it gradually becomes more
luminous because both the core density and
temperature rise slowly and increase the reaction rate.
It takes about 10 billion years for a star with the mass
of the Sun to convert all of the hydrogen in its core
into helium.
After millions to billions of years (depending on their initial
masses), stars run out of their main fuel - hydrogen. Once the
supply of hydrogen in the core is gone, nuclear processes
occurring there cease.
Without the outward pressure generated from these reactions
to counteract the force of gravity, the outer layers of the star
begin to collapse inward.
Just as during formation, when the material contracts, the
temperature and pressure increase. This newly generated heat
temporarily counteracts the force of gravity, and the outer
layers of the star are now pushed outward.
The star expands to larger than it ever was during its lifetime a few to about a hundred times bigger.
4. The star has become a red
giant. Afterwards, when the
helium in the core is all used up,
the star is left with a core made
of carbon.
Sun Sized Stars
A star of the Sun’s mass never
becomes hot enough for carbon to
react, so the star’s energy
production ends at this point. The
outer layers expand once again and
are driven off entirely. 5a. This
shell of gas is called a planetary
nebula. It has nothing to do with
planets, despite its name.
In the center of a
planetary nebula,
the core of the star
becomes exposed
as a small, hot
object about the
size of Earth. The
star is then a 6a.
white dwarf made
of carbon.
Massive Stars
After the red giant phase, massive stars contract
again allowing the core to become iron. When
this occurs no further energy-producing reactions
can occur, and the core of the star violently
collapses in on itself.
As the gravity continues to shrink the star, the
entire outer portion of the star is blown off in a
massive explosion called a 5b. supernova. This
explosion creates elements that are heavier than
iron and enriches the universe.
6b. This can become a neutron star. A neutron star
has a mass of 1.5-3 times the Sun’s mass, but a radius
of only 10km!
Some stars are too massive even to form
neutron stars. The mass of the star is so
great that the core of the star simply
continues to collapse, compacting matter
into a smaller and smaller volume. The
small, but extremely dense, object that
remains is called a 7. black hole
because its gravity is so immense
that nothing, not
even light, can
escape it.