What causes the difference in heat and brightness in stars?

For most stars, called Main Sequence stars, their mass determines their brightness and temperature. The more massive the star, the harder gravity squeezes the gases inside the star, which makes the gases hotter, causing them to create more energy in their deep interior through the nuclear fusion of hydrogen to helium. That extra energy makes a massive star bigger, hotter and brighter than a less massive star. On the average, doubling the mass increases the brightness of the star by a factor of ten, so a star eight to ten times the mass of the Sun is about a thousand times brighter, while a star eight to ten times less massive than the Sun is about a thousand times fainter. (At the moment my website is "down" due to a cut fiber optic line, which is why it took so long for me to receive your message; but once it is "up" you can read a bit about that at The Mass-Luminosity Diagram and Main Sequence Lifetimes (at ).

Once stars start to die, their brightness and temperature change in more complex ways, but for a given stage of stellar life, more massive stars are still bigger and brighter (though not always hotter) than less massive stars.


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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