LIFE CYCLE OF A STAR

• Dust and gas (mainly hydrogen and helium) are pulled together by gravity to form a star.
• During 'main sequence' period of a star's life cycle, fusion of hydrogen nuclei releases energy to make helium nuclei in the core and a star is stable because the forces within it are balanced.
• The core (centre) of a star is where the temperature and density are greatest and where most nuclear fusion takes place.
• The more massive a star, the hotter its core and the heavier the nuclei it can create by fusion.
• A star's life cycle is determined by the mass of the star.
• A main sequence star uses nuclear reactions to produce light and heat. When it runs out of hydrogen, what happens next in its life cycle depends upon its mass.
• A larger star will swell to become a red supergiant, in which helium nuclei fuse to form carbon, followed by further fusion that produces heavier nuclei such as nitrogen and oxygen. It expands, cools and turns red. The outer layers then blast away as a supernova is formed. The core collapses and depending upon mass, it forms either a neutron star or a black hole.
• A smaller star, similar to our Sun, follows a different sequence, expanding to become a red giant. It then sheds out layers of gas, exposing the core as a white dwarf and finally cools to become a black dwarf.
• Fusion processes in stars are the source of energy and produce all of the naturally occurring elements.

  These elements may be distributed throughout the universe by the explosion of a massive star (supernova) at the end of its life.