What is a Star? (GCSE Physics/ Astronomy)

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What is a star?
Look up in the night sky and you'll see lots of stars. But what is a star?
In a scientific sense, a star is ball of hydrogen and helium with enough
mass that it can sustain nuclear fusion at its core. Our Sun is a star, of
course, but they can come in different sizes and colours.
Lets learn what a star is:
75% of the matter in the Universe is hydrogen and 23% is helium, these
are the amounts left over from the Big Bang. These elements exist in
large stable clouds of cold molecular gas. At some point a gravitational
disturbance, like a explosion will cause a cloud of gas to collapse,
beginning the process of star formation.
As the gas collects together, it heats up. Conservation of momentum
from the movement of all the particles in the cloud causes the whole
cloud to begin spinning. Most of the mass collects in the centre, but the
rapid rotation of the cloud causes it to flatten out into a protoplanetary
disk. It's out of this disk that planets will eventually form.
The protostar at the heart of the cloud heats up from the gravitational
collapse of all the hydrogen and helium, and over the course of about
100,000 years, it gets hotter and hotter becoming a Tauri star. Finally
after about 100 million years of collapse, temperatures and pressures at
its core become sufficient that nuclear fusion can ignite. From this point
on, the object is a star.
Nuclear fusion is what defines a star, but they can vary in mass. And the
different amounts of mass give a star its properties. The least massive
star possible is about 75 times the mass of Jupiter. In other words, if
you could find 74 more Jupiter's and mash them together, you'd get a
star. The most massive star possible is still an issue of scientific
disagreement, but it's thought to be about 150 times the mass of the
Sun. More than that, and the star just can't hold itself together.
The least massive stars are red dwarf stars, and will consume small
amounts over tremendous periods of time. Astronomers have calculated
that there are red dwarf stars that could live 10 trillion years. They put out
a fraction of the energy released by the Sun. The largest supergiant
stars, on the other hand, have very short lives. A star like Eta Carinae,
with 150 times the mass of the Sun is emitting more than 1 million times

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Sun. It has probably only lasted a few million
years and will soon detonate as a powerful supernova destroying itself
completely.
Most stars are in the main sequence phase of their lives, where they're
doing hydrogen fusion in their cores. Once this hydrogen runs out, and
only helium is left in the core, the stars have to burn something else. The
largest stars can continue fusing heavier and heavier elements until they
can't sustain fusion any more.…read more

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