Space: The Life Cycle of Stars

Stars undergo a series of evolutionary stages, dictated by nuclear reactions happening within the star.
The balance between gravitational forces, pulling everything towards the star’s centre, and nuclear forces, pushing everything outwards, controls the star’s life cycle.

All stars originate from a nebula, which is a giant cloud of dust and hydrogen gas in space.
The nebula begins to compress or collapse under its own gravitational pull, which increases pressure and temperature at its core.

Parts of the compressed nebula form a rotating protostar.
As the gravitational potential energy of the gas and dust decreases, it gets converted into heat, raising the temperature.

When the core temperature of the protostar reaches a sufficient level (around 15 million degrees Celsius), nuclear fusion of hydrogen begins, creating a main sequence star.
The star stays in the main sequence stage, fusing hydrogen into helium, for about 90% of its life cycle.

Once the hydrogen fuel in the core runs out, the star begins to contract under gravity.
This contractive phase results in an increase in pressure and temperature until helium begins to fuse into heavier elements, and the outer layers of the star expand, creating a red giant or red super giant.

For stars similar to or smaller than the Sun, the red giant eventually sheds its outer layers, leaving behind a hot core known as a white dwarf.
The white dwarf eventually cools down and dims into a black dwarf.
For stars much larger than the Sun, the red super giant undergoes a more dramatic end–a supernova explosion.
Depending on the initial mass of the star, a supernova may leave behind a dense core known as a neutron star, or if the star is massive enough, it forms a black hole.

The mass of a star determines its life cycle, lifespan, and end state - higher-mass stars have shorter lives and more dramatic ends.
Knowledge of a star’s mass can allow astrophysicists to predict its evolution and eventual fate.