So how is a star formed? All stars begin as a ball of gas and dust in nebulae. A nebula is made primarily of hydrogen (97%) and helium (3%). Gravity pulls the gas and dust together into a sphere. This sphere of gas and dust is called a protostar. Hydrogen atoms combine to form helium, and energy is given off. At the exact moment when nuclear fusion begins, the star is said to be "born."

The star will continue to accumulate mass based on the amount of matter available in the nebula. Once the star stops accumulating mass, the star is called a main sequence star. Main sequence stars fuse hydrogen into helium, so it is sometimes said that main sequence stars burn hydrogen as a fuel. The hydrogen is not really burned; it is changed into helium by the process of nuclear fusion.

The length of time a star is considered a main sequence star depends on its initial mass. Is it better for stars to have more or less mass? Learn more about the star life cycle here.

The path the star life cycle takes depends on the initial mass of the star. Study the diagram below. It shows all stars forming from nebulae and developing into protostars. You can see that the top part of the diagram shows the sequence of events for a medium sized star while the bottom shows what happens to larger and giant stars.

Interactive exercise. Assistance may be required.