Main-sequence stars are fusing hydrogen into helium. These nuclear fusion take place deep in the cores of stars. Stars spend about 90% of their lives in this stage. Our Sun is about 5,000 million years into its 10,000 million-year main-sequence.

One of the fundamental forces in the Universe is gravity. It is the force that holds us on Earth. It also keeps planets in orbit around stars and stars in orbit around the centre of our galaxy.

All stars form in nebulae, which are huge clouds of gas and dust. Though they shine for many thousands, and even millions of years, stars do not last forever. The changes that occur in a star over time and the final stage of its life depend on a star's size.