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.