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Astronomy/How do Stars Shine?


I know that stars shine because of a process called fusion, but I don't know what fusion is?

Fusion is the combination of smaller atomic nuclei to make larger ones. The energy source for most stars is the merging of four hydrogen nuclei (in several steps) to make a single helium nucleus. In the process, about 0.7% of the mass of the hydrogen atoms is converted into energy, according to Einstein's formula E = m c-squared. In the Sun, 600 million tons of hydrogen is fused into helium every second. This results in the conversion of 4 million tons of mass to energy, in the form of high-energy gamma-ray photons. Over the course of a million years, those photons gradually leak through the interior of the Sun, until they are radiated at the surface at a rate of 400 trillion trillion watts (watts being a measure of energy radiated per second).

For more massive, brighter stars, the process is essentially the same, but with small differences. THe brighter the star, the faster hydrogen is fused to helium, and the more mass is converted to energy, at a rate directly proportional to the greater brightness. So a star a million times brighter than the Sun would fuse a million times more hydrogen per second, and produce a million times more energy per second. As it turns out, such a star would also have a slight difference in the exact steps used to convert the four hydrogen atoms into a single helium atom. But the beginning and end results are the same, and the net energy produced per unit of mass is essentially the same, so those details aren't really important for your question.

If you would like a little more detail about the fusion process, refer to Energy Production in the Core (of the Sun) at

Ir you would like a little more detail about how the gamma-radiation produced in the core of the star ends up as visible light near the surface, refer to Heat Flow in the Solar Interior (at ), and to Review of Heat Flow Inside Stars (at )

(Note: As stated at the start of some of those pages, my students were supposed to read parts of their text as background material before reading the web pages; but the discussion on the web pages is reasonably clear all by itself -- just perhaps not quite as clear as if it had been meant to stand on its own.)


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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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