What Are the Stages in the Cycle of an Average Star Like the Sun?

Giant Molecular Clouds

• Stars form from giant molecular clouds. These huge expanses of dust and gases can be hundreds of light-years across, and can give birth to many stars. The force of gravity slowly causes the molecules within the cloud to collapse into many different clumps or fragments. Each fragment becomes a protostar.

Protostar

• A protostar is a fast-spinning mass of superheated gases. As the protostar accumulates more and more mass and its gravity becomes stronger, the temperature and pressure grows until hydrogen nuclei fuse into helium atoms. Once nuclear fusion begins, the star's life has officially begun.

Main Sequence

• As nuclear fusion releases energy and creates an outward force to counteract gravitational collapse, the star enters the longest, most stable part of its life cycle. Stars at this point in their development are called main sequence stars. The sun is thought to be about 4.5 billion years into its main sequence phase, with another 4.5 or 5 billion years before it runs out of nuclear fuel and begins the next stage.

Red Giant

• Once most of the hydrogen fuel in the star's core has been turned into helium, fusion slows down and there is no longer enough energy to fight the force of gravity. The inner parts of the star collapse, and the energy generated causes the outer layers to expand. They cool as they do so, turning red and forming a red giant, sometimes hundreds of times the size of the sun as it exists today.

White Dwarf

• The outer layers of the red dwarf continue to expand and cool until all that's left is the remnant of the star's core, which is now called a white dwarf. A star around the size of the sun may briefly undergo helium fusion during the red giant stage to form carbon, but this process occurs very quickly and once it is complete, no more fusion occurs. The white dwarf, approximately the size of the Earth but far, far denser, spends the next several million years radiating off its remaining heat and energy.

Black Dwarf

• Once a white dwarf has radiated most of its energy and finally ceases to emit heat or light, it is referred to as a black dwarf. Black dwarfs are purely hypothetical -- the universe is thought to be too young for any stars to have reached this stage yet.