Life Cycle of Stars
The Birth of a Star
In space, there exists huge clouds of gas and dust. These clouds
consist of hydrogen and helium, and are the birthplaces of new stars. Gravity
causes these clouds to shrink and become warmer. The body starts to collapse
under its own gravity, and the temperature inside rises. After the temperature
reaches several thousand degrees, the hydrogen molecules are ionized (electrons
are stripped from them), and they become single
protons. The contraction of the gas and the rise in temperature continue until
the temperature of the star reaches about 10,000,000 degrees Celsius (18,000,000
degrees Fahrenheit). At this point, nuclear fusion occurs in a process called
proton-proton
reaction. Briefly, proton-proton reaction
is when four protons join together and two are converted into neutrons; an 4He
nucleus is formed. During this process, some matter is lost and converted
to energy as dictated by Einstein's
equation. At this point,
the star stops collapsing because the outward force of heat balances the
gravity.
The Hydrogen Burning Stage
The proton-proton reaction occurs during a period called the
hydrogen-burning state, and its length depends
on the star's weight. In heavy stars, the great amount of weight puts a
large amount of pressure on the core, raising the temperature and speeding
up the fusion process. These heavy stars are very bright, but only live for a short
amount of time. After the energy from this deuteron-hydrogen fusion process ends, the
star begins to contract again, and the temperature and pressure subsequently increase.
Nuclear fusion occurs between the hydrogen and lithium & other light metals in the
star, but this process soon ends. Contraction starts again, and the extreme high
temperature and pressure cause the hydrogen to transform into helium through the
carbon-nitrogen-oxygen cycle. When all the hydrogen has been used up, the star is
at its largest size, and it is called a red giant. Different things can happen to the
star now.
Planetary Nebulas
One scenario is that the star will continue to make energy by using hydrogen and helium outside
of the core; its surface will rise and fall and the star will become a
variable star. After it gets out of control, the layers of gas will pull
away, forming a shell of gas known as a planetary nebula.
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White dwarf
The other scenario is that the star will continue to shine through the fusion of
helium nuclei, in the triple alpha
process.
The
star is now a white dwarf, and further contraction is prevented by the repulsion of
electrons in the core.
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Supernova
Very heavy stars will continue to fuse heavy
elements
in order to produce more energy. However, once iron is formed, it cannot be fused to make
more energy since it has such a high binding energy and
is therefore very stable. The core will collapse under gravity and huge amounts of gas on the surface of the star will explode out. This star is
now called a supernova.
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Neutron Star
After a supernova explosion, the iron core of the star may be
extremely heavy, and the force of gravity may be extremely large. It then
becomes a neutron star, where the repulsion between neutrons stops the
contraction caused by gravity. Neutron stars consist of matter that is
100 million times denser than white dwarf matter.
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Pulsars
A neutron star may spin rapidly after a supernova explosion, and it
may emit two beams of radio waves, light, and X-rays. These beams radiate
in a circle because the star is spinning, and it appears that the star is
pulsing on and off. Thus, it is given the name Pulsar.
Black Holes
Neutron-neutron repulsion can only counteract the force of gravity
if the core of the dead star weighs less than three times the weight of the
sun. In an extremely heavy core, no force can stop the matter from being
squeezed into a smaller and smaller space. Nothing can escape these black
holes; not even light.
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