Life Cycle of Stars

Basics of Stars

Stars are giant spheres of hot gas that emit light and heat.
The Sun is a star, located at the centre of our solar system.
Stars are mainly made of hydrogen and helium - the two lightest elements.

Stars begin their lives in nebulae, which are large clouds of gas and dust.
Dense regions within nebulae start to collapse under their own gravitational pull, forming a protostar.
As the protostar continues to collapse, the pressure and temperature at its core increase until nuclear fusion can begin - this marks the birth of a star.
The balance between the gravitational collapse of the star and the outward pressure from fusion maintains the star’s size for millions to billions of years. This period is called the main sequence.

Our Sun is an example of a main sequence star.
The Sun has been in the main sequence stage for about 5 billion years, and it will stay in this stage for about 5 billion more.
Once all the hydrogen in the core has been fused into helium, the star leaves the main sequence.

For stars similar in size to our Sun, the core will contract and the outer layers expand, creating a red giant.
The outer layers of red giants eventually drift off into space, leaving behind a dense, hot core called a white dwarf.
White dwarfs, once they have cooled and stopped emitting heat and light, become black dwarfs.

Stars many times larger than our Sun undergo a more dramatic transformation.
These massive stars become red supergiants which can start to fuse heavier elements until iron is formed in the core.
Iron cannot be fused, so once a core of iron has formed, the star will quickly collapse and explode in an event known as a supernova.

During a supernova, layers of the star are blown off in a massive explosion, and the core is compressed into a neutron star.

If the star was exceptionally large, the core may collapse so much that it forms a black hole, an area of spacetime from which nothing, not even light, can escape due to intense gravitational forces.