The Life Cycles of Stars
I. Star Birth and Life
Imagine an enormous cloud of gas and dust many light-years
across. Gravity, as it always does, tries to pull the materials together.
A few grains of dust collect a few more, then a few more, then more still.
Eventually, enough gas and dust has been collected into a giant ball that,
at the center of the ball, the temperature (from all the gas and dust
bumping into each other under the great pressure of the surrounding
material) reaches 15 million degrees or so. A wondrous event occurs....
nuclear fusion begins and the ball of gas and dust starts to glow. A new
star has begun its life in our Universe.
So what is this magical thing called "nuclear fusion" and why
does it start happening inside the ball of gas and dust? It happens like
this..... As the contraction of the gas and dust progresses and the
temperature reaches 15 million degrees or so, the pressure at the center
of the ball becomes enormous. The electrons are stripped off of their
parent atoms, creating a plasma. The contraction continues and the nuclei
in the plasma start moving faster and faster. Eventually, they approach
each other so fast that they overcome the electrical repulsion that exists
between their protons. The nuclei crash into each other so hard that they
stick together, or fuse. In doing so, they give off a great deal of
energy. This energy from fusion pours out from the core, setting up an
outward pressure in the gas around it that balances the inward pull of
gravity. When the released energy reaches the outer layers of the ball of
gas and dust, it moves off into space in the form of electromagnetic
radiation. The ball, now a star, begins to shine.
New stars come in a variety of sizes and colors. They range from blue
to red, from less than half the size of our Sun to over 20 times the Sunís
size. It all depends on how much gas and dust is collected during the
star's formation. The color of the star depends on the surface temperature
of the star. And its temperature depends, again, on how much gas and dust
were accumulated during formation. The more mass a star starts out with,
the brighter and hotter it will be. For a star, everything depends on its
Throughout their lives, stars fight the inward pull of the force of
gravity. It is only the outward pressure created by the nuclear reactions
pushing away from the star's core that keeps the star "intact".
But these nuclear reactions require fuel, in particular hydrogen.
Eventually the supply of hydrogen runs out and the star begins its
II. Beginning of the End
After millions to billions of years, depending on their initial
masses, stars run out of their main fuel - hydrogen. Once the ready 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 toward the core. 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. The star
has become a red giant.
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