"Normal" stars: These exists most of all sorts. They have an average diameter and an average surfacetemperature and strength of light.
Red giants: Red giants are stars, which have a relatively fresh surface and a very high strength of light. They are called red giants, because of their huge size.
White dwarfs: There is also a group, which contains very small, but also very hot stars. These are the white dwarfs. It are stars, which were once like the Sun, but are now in a late phase of their life.
Neutronstars: This are stars which used to have a weight over 7 times the weight of the Sun. When the kernel of an exploding cell collapses under the influence of gravitation, a neutronstar comes into being. While the kernel becomes smaller and smaller, the protons and electrons are compressed and form neutrons. The star collapses totally and gets an immense density: a teaspoon filled with this would weigh 100.000.000 ton!
Stars originate in the big gas- and matterclouds, spread by the galaxies. They start radiate light if their kernels burst into flames. A gigantic star, which radiate light powerful, will die early. Opposite, a smaller star will live longer. Stars like our Sun are going to die quiet. They will shrink to a compact white dwarf, as small as a planet. The heavy stars will explode with an imposing blast. After the explosion they radiate light as much as a whole galaxy.
Nobody knows exactly how a cloud of gasses and matter can change into a star. When a cloud collapses, energy can escape and heats the cloud. The centre of the cloud reaches a temperature of 10 million degrees or higher. Mainly the gasses consist of hydrogen. The hydrogenatoms fuse together and huge amounts of energy, like light, heat and other sorts of radiation, can escape. Because of this the collapsing cloud starts to radiate as a star. After a while the star calms down and starts radiate light invariable.
A star as large as our Sun radiates light during 10 billion years, till the hydrogen (his fuel) in his kernel is consumed. The star starts collapsing again by the gravitational force. The heat causes fusion of hydrogen around the kernel. Because of this the star becomes larger and radiates more light. The kernel continues to shrink and gets hotter. When the temperature of the shrinking kernel of a star reaches 100 million degrees, there can happen another fusion. By this reaction the kernels of heliumatoms changes into carbonatoms. Because of this there escapes enough energy to let the expanded star radiate light like a red giant. But on a certain moment all the helium in the kernel runs out and the star starts shrinking again. In the long term the substance will be packed up under the influence of gravitational force and becomes a white dwarf with a gigantic density.
Stars with a weight of a few times our Sun, will die on a spectacular way. First they grow out to a giant star and then they blow up themselves. This is called a "supernova". When a star changes into a supernova, it radiates light millions of times more. During a short time, the star can radiate more light than a whole galaxy (Click here for a picture)
What happens with a star after it exploded as a supernova, depends of the weight of the star. A star with a weight more than seven times the weight of our Sun becomes a neutronstar. Stars with a weight more than seven times the weight of our Sun become black holes.
