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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.
3. Eventually, the
temperature inside a
protostar becomes hot enough
for nuclear fusion reactions to
begin. 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 allows a sun-like star to now fuse helium into carbon.
This will counteract gravity and the outer layers of the star
will be 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. 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 white
dwarf made of
carbon.
After a white dwarf cools
enough to no longer
emit light or heat, it will
become a black dwarf.
Scientists, however, have
discovered that the time
required for a white
dwarf to cool enough to
become a black dwarf is
most likely longer than
the age of the Universe.
Therefore, scientists
believe that no black
dwarfs exist, yet.
Massive Stars
After the red giant phase, massive stars contract
again allowing the core to become hot enough to
fuse heavier and heavier elements until they reach
iron. When this occurs the star doesn’t have
enough energy to further fuse iron so gravity
quickly crushes the star, causing the protons and
electrons to combine and become neutrons.
At this moment, the entire outer portion of the star
is blown off in a massive explosion called a
supernova. This explosion creates elements that
are heavier than iron and enriches the universe.
Some massive stars will then have what is called a
neutron star remaining. A neutron star has a mass of
1.5 to 3 times the Sun’s mass, but a radius of only
10km!
Pulsars
• Most neutron stars will form pulsars which
are neutron stars that rotate rapidly and
emit a beam of electromagnetic
radiation.
A pulsar’s
magnetic poles
(where the beam
of radiation is
emitted) and it’s
rotational axis are
not exactly the
same. This results in
a “pulsing” look to
the star from our
perspective here
on Earth.
Atoms as Big as Mountains
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 black hole because its
gravity is so immense that
nothing, not even
light, can
escape it.
Travel Inside a Black Hole!
Life
Cycle of
Stars
life cycle of a star video