Stars start out as a Nebula –

A large diffuse cloud of gas mostly Hydrogen gas

As the gases collect together, they start to have more mass than the gas around them, so their gravity attracts more gas making more mass and so on.

This process is called Accretion

As mass increases, pressure increases until there is so much pressure in the middle of the ball that the hydrogen fuses into Helium.

This is a Thermo-nuclear fusion reaction, and the birth of the star.

Even though the star is exploding like a million nuclear bombs per second, it

doesn’t break apart. Why?

The outward force of the nuclear fusion is balanced by the inward pull of gravity, so it stays in equilibrium.

The star will stay in “Main sequence” until enough of the H is fused into He. At this point,

fusion slows down and gravity wins.

Three things can happen now…depending on the star’s mass.

If a star is small, it will expand to many times its original size as it burns the last of its fuel, becoming a Red Giant,

then it will collapse in on itself, forming a White Dwarf.

Medium sized stars will do the same, but will collapse as a neutron star – much bigger and denser

than a white dwarf.

When large stars collapse, they can have enough mass to reignite the He into heavier elements like Carbon etc. and they can enter a second or third life cycle burning their new fuel and creating any of the other heavier elements.

When these stars explode they form a super-nova and expel all their elements into space to become the seeds for other objects like stars or planets.

The remains become a Black Hole.