Overview of a Star's Life

A gas cloud, if big enough, starts to shrink. The density and temperature increase so nuclear fusion can start. This is when Hydrogen is converted into Helium. The "burning" of Hydrogen stops the gas cloud from shrinking. At this point, the gas cloud becomes a star. This is the present state of our Sun.

After billions of years, most of the Hydrogen fuel has been "burned", and the star begins to shrink again. The star has to turn to another source of fuel, Helium.

The next stage in the life of a star is called a red giant. The star here is much bigger than it was initially. When the red giant star runs out of fuel, the star begins to shrink again. This contraction heats up the core of the star enough so that the heavier elements can be made. When the star runs out of this type of fuel, it has neared the end of its life.

The star begins to throw off layers because it can't support them anymore. This is called a planetary nebula. The core of the star becomes a white dwarf. This is an extremely dense star the size of a planet. Finally, when the white dwarf has used all its energy, it stops shining and becomes a "black dwarf", a dead star. This is expected to be the final state of our Sun.

For stars with higher masses than the Sun (up to about 40 times greater), the outer layers of the star may be thrown off with much more force. This is a supernova. This type of star collapses down to a very compact size. This is what is called a "neutron star".

Stars bigger then 40 times the Sun may collapse into a "black hole".

Some Terms to Remember

Birth - Stars start out in giant clouds of dust called nebulae. Gravity forces the dust to bunch together. As more and more dust bunches up, gravity gets stronger and it starts to get hot and becomes a protostar. Once the center gets hot enough, nuclear fusion will begin and a young star is born.

Main Sequence Star - Once a star, it will continue to burn energy and glow for billions of years. This is the state of the star for the majority of its life and is called the "main sequence". During this time a balance is met between gravity wanting to shrink the star and heat wanting to make it grow bigger. The star will remain this way until it runs out of hydrogen.

Red Giant - When the hydrogen runs out, the outside of the star expands and it becomes a red giant.

Collapse - Eventually the core of the star will start to make iron. This will cause the star to collapse. What happens to the star next depends on how much mass it had (how big it was). The average star will become a white dwarf star. Larger stars will create a huge nuclear explosion called a supernova. After the supernova it may become a black hole or a neutron star.

Life Cycle

Young stars at this stage are called protostars. As they develop they accumulate mass from the clouds around them and grow into what are known as main sequence stars. Main sequence stars like our own sun exist in a state of nuclear fusion during which they will emit energy for billions of years by converting hydrogen to helium.

Stars evolve over billions of years. When their main sequence phase ends they pass through other states of existence according to their size and other characteristics. The larger a star's mass, the shorter its lifespan will be.

As stars move toward the end of their lives much of their hydrogen has been converted to helium. Helium sinks to the star's core and raises the star's temperature—causing its outer shell to expand. These large, swelling stars are known as red giants.

The red giant phase is actually a prelude to a star shedding its outer layers and becoming a small, dense body called a white dwarf. White dwarfs cool for billions of years, until they eventually go dark and produce no energy. At this point, which scientists have yet to observe, such stars become known as black dwarfs.

A few stars eschew this evolutionary path and instead go out with a bang—detonating as supernovae. These violent explosions leave behind a small core that may become a neutron star or even, if the remnant is large enough, a black hole.