The Life of a Star All stars begin their lives as nebula.

A star is born when gas and dust from a nebula contract and it becomes very hot. At this point, nuclear fusion starts. The earliest stage of a star’s life is called a protostar.

How long does a star live? That depends on how much mass it has. In general, smaller stars burn longer than larger stars. In a way, they are more efficient. A medium-sized star will last for about 10 billion years.

When a star begins to run out of fuel, its center shrinks and the outer region expands. At his point, it will become either a white dwarf, a neutron star or a black hole. A white dwarf star is very dense. It is about the size of earth, but has a mass equal to that of the Sun. It is a white color, but has no fuel left.

A supernova is formed when a dying giant star explodes. After, this explosion occurs, some material is left behind.

This leftover material may form a neutron star. Neutron stars are even smaller and more dense than white dwarf stars.

The largest stars become black holes when they die. A black hole is so dense and its gravity is so strong that nothing can escape - not even light!

So how do scientists know that black holes exist? If there is gas near a black hole, it is pulled towards it So strongly that it rotates and spins very fast. The gas heats up and emits x-rays, which can be detected.

A quasar is a fairly recently discovery. It is a bright object that is very far away. It is not a star. Scientists think they are giant black holes in distant galaxies. The light is created by the mass of spinning gas.