History and life of a star

• How are stars formed?

  1. Start as a nebula of dust and gas
  2. The temperature rises and becomes concentrated and forms a protostar (baby star)
  3. Fusion occurs, hydrogen fuses to form helium and the star is much hotter and brighter. This is called main sequence
• After main sequence for smaller stars, hydrogen runs out and helium nuclei are fused, which releases more energy. This expands to form a red giant. When it stops fusing helium, a white dwarf is formed and cools, turning brown and then black.
• After main sequence for larger stars, hydrogen runs out and other elements are fused. The star becomes a red super giant. Once the star stops carrying out nuclear fusion, the star explodes, called a supernova.
• The heaviest element that can be made in fusion in stars is iron. When a supernova occurs, the temperature is high enough to produce elements heavier than iron.
• After a supernova, the remains of the star can form one of two objects. The first is called a neutron star. It consists of neutrons densely packed together. The second is called a black hole. This has so much gravity that not even light can escape.