REVIEW: Stages of a Star & Our Sun

Stars go through several stages. What happens to a star depends on its mass.

Protostar
A protostar is a collection of gas that is beginning to condense. Eventually a protostar will reach nuclear fusion and its main sequence begins. Our sun formed a little over 4 billion years ago.

Main Sequence
The main sequence is the longest part of a star's life. During the main sequence the star burns hydrogen steadily for billions of years. The length of time depends on the mass. Larger stars burn out more quickly, while smaller stars burn longer. Our sun is currently in its main sequence.

Red Giant/Supergiant
After a star runs out of hydrogen, it begins to burn helium. This kind of nuclear fusion results in the star becoming larger and redder. As a result, these stars are often called "red giants." "Supergiants" are very large red giants. Our sun will become a red giant (it is not massive enough to become a supergiant).

Supernova & Planetary Nebula
After the red giant phase of an average size star (like our sun), the star will become a planetary nebula. As a planetary nebula, the outer layers of the star will be vented off into space, leaving the core of heavier elements behind.

Stars more massive than our sun will supernova - explode! During this explosion, the outer layers of the star (where the lightest elements are located) will be thrown out into space.

White Dwarf/Black Hole vs. Neutron Star and Black Hole
After a star runs out of helium, it either becomes a white/black dwarf, neutron star or a black hole. White dwarfs are small stars that burn carbon, but are never as hot or bright as it was in previous stages. A black dwarf is hypothesize to be the end of life for average size stars, like our sun. A black dwarf is a star where fusion is no longer taking place. This is how our sun will end its life.

After a supernova, a more massive star leaves behind a dense core of heavier elements - a neutron star. If a star is really massive, it collapses and becomes a black hole. Black holes have strong gravitational forces that suck anything nearby, including light.

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