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The Question

(Submitted February 25, 1997)

I would just like to know what the life cycles of the stars are. Where they came from, and their life cycles, and an explanation of the Hertzsprung-Russell diagram.

The Answer

We primarily study the compact remnants of stars, not the evolution of stars. However, we can give a brief overview. Stars form from dense gas, usually in molecular clouds. After a star forms, it burns hydrogen into helium. It does this until the hydrogen begins to run out and then further stages of burning occur, i.e., helium burns into heavier elements. If the star is less massive than several times the mass of the Sun, it will eventually become a white dwarf. If it is more massive than this it will first implode and then explode in a supernova explosion. While the star is still burning hydrogen it is on the Main Sequence of the H-R diagram, which is a plot of temperature versus luminosity for stars. When the star is done burning hydrogen, it enters other regions of the H-R diagram such as the "Horizontal Branch", "Giant Branch" and "Asymptotic Giant Branch". For more info, check out


Andy Ptak
for the Ask an Astrophysicist team

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