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Why is the sun so hot and bright?
Answer 1:

The sun is powered by the thermonuclear fusion of hydrogen into helium, the same as an atomic bomb. The fuel in an atomic bomb has a diameter of a few centimeters. The sun has a diameter a 109 times that of the Earth! Even 92 million miles away, you probably expect it to be hot and bright.

So I assume you ask how the sun stays so hot... As you probably know, gravity pulls things together. All the gas that makes up the sun tries to fall into the center, making it very hot (25 million Fahrenheit) and very dense. Under these circumstances, hydrogen (which makes up 93% of the sun) nuclei fuse together to form helium.

One helium turns out to weigh a little less than the two hydrogen atoms, so where did the extra mass go? Well, mass and energy are conserved, but can change into each other according to Einstein's famous formula E=mc^2 (energy=mass times the speed of light squared).

Now here's a little exercise to appreciate how much energy a little mass can make. If I could take one gram of matter and convert it into pure energy, how much would it be worth? remember that a dollar buys about 30 million watts.

It turns out that a gram of hydrogen used for fusion can produce about a small fraction of that amount, but that's okay because the sun has about 2 octillion grams left to go :)

Answer 2:

The main factor determining the brightness and surface temperature of an average "main sequence" star is the stellar mass.In general, stars more massive than the Sun are hotter and brighter and less massive stars are cooler and less bright. Any good basic astronomy textbook will have much more information about stars.

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