Download Stellar Evolution

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. 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 elements into iron. When this occurs the
star’s mass is so immense, it can no longer
support it causing a sudden, violent, collapse.
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
1.5-3 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 pulsars
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.
http://www.youtube.com/watch?
v=VgGXaIU0lTU&list=TLo9gX
g6qecwQ
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.