LIVES OF STARS A large cloud of dust and gases is called a nubula. It is the source of all stars. Gravity pulls the gases closer together and they heat up. This is called a protostar. A star is born when the gases and dust become hot enough for nuclear fusion to take place. Nuclear fusion is the process in which hydrogen is changed to helium. Other elements are formed but hydrogen to helium is by far the most common.

During fusion tremendous amounts of energy are released. The life of a star depends on its mass. The smaller the star, the longer it lasts. Small stars can live for about 200 billion years. Medium stars live for about 10 billion years. Large stars live for about 10 million years.

For most of its life, the star glows brightly as nuclear fusion uses up the hydrogen fuel, but it does not change size.

DEATHS OF STARS All stars eventually start to run out of hydrogen fuel and become red giants or supergiants, depending on how large they are. When fuel runs out, it becomes a white dwarf, neutron star, or black hole. Small and medium stars become red giants and then white dwarfs. These are the size of the Earth but have the mass of the sun, so it is very dense. A spoonful of white dwarf is equal to a truckload of earth. It has no fuel so it just glows faintly with left-over heat but eventually dies. It is then called a black dwarf.

A dying giant or super giant star can suddenly explode. The star becomes millions of times brighter. This is called a supernova. Some of the exploded material may enter a nebula which may form a new star. Our sun probably started this way. The material that is left behind forms a neutron star. Neutron stars are very small and dense. They may have three times the mass of our sun with a volume smaller than Pluto. The most massive stars become black holes.

After the supernova a mass of five times our sun may remain. The gases pull in and a mass of five times our sun is in a diameter the size of NYC. The gravity is so strong that even light cannot escape. This is called a black hole. We cannot detect a black hole directly. Any gas near it will be pulled so strongly that it will spin rapidly around the black hole. This will cause the gas to give off x-rays that we can detect. We can also detect black holes from the effect they have on the gravity of nearby stars.

The supernova 1987A. The left hand photo was taken before the explosion; the arrow shows the star that blew up.

This black hole is pulling matter from a nearby star into its center.

Quasars Discovered in the 1960s, quasars are very bright objects that are about 12 billion light years away. They are on the edge of the universe. Each one is a galaxy with a black hole in the center.

The eagle nebula

Orion Nebula