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Stars form when giant gas clouds start to clump together and collapse
in upon themselves. This clumping is often caused by energy waves
from nearby exploding supernovae. As they collapse, the molecules
that form the clouds compress in upon each other to form helium
and hydrogen. Once gravity pushes them together strongly enough,
nuclear fission will begin to occur in the core of the star. This
process will continue throughout the life of the star, until all
of the fuel is used up.
Stars with a large amount of mass will
eventually become so unstable that they will explode in what is
called a supernova; these stars eventually collapse upon themselves
to form black holes. They are so much lager than our sun that we cannot imagine being on a planet in orbit of them. Check out the largest stars known at Space.com. Medium sized stars, like our Sun, grow larger
as they shift from hydrogen building fission to helium building
fussion and change color from yellow to red. They become Red Giants.
After red giants expand all of their fuel, they contract to become
white dwarfs, and then evaporate over billions of years into planetary
nebula. Small and medium sized stars don’t have enough mass to
supernova; they shrink down to become white dwarfs and then cool
off, becoming black dwarfs.
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