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Stars and the Universe

Stars  -  Life Cycle - Main Sequence.

What is a Main Sequence Star?

After nuclear fusion reactions have begun,
there are two opposing forces.
Gravity is the force causing the star to contract.
The heat released from fusion causes the star to expand.
These opposing forces balance each other
and the size of the star stays almost constant (the same).
The star has entered a stable phase (meaning it stays the same)
and it is now called a main sequence star.

The length of time that a star lasts as a main sequence star
depends on how big it is. Big stars
use up their hydrogen at a much faster rate than small ones.
The Sun has been a main sequence star for five billion years.
It is expected to last for another five billion years.
The Sun is about half way through its main sequence phase.

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