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Why Do Stars Shine?

Sirius. Image credit: Hubble

Sirius. Image credit: Hubble


Head outside on a dark night and look up into the night sky. If you’re away from the bright city lights and it’s a clear night, you should see beautiful stars shining in the night. Just think, the light from those stars has traveled light-years through space to reach your eyes. But why do stars shine at all? Where is the light coming from?

All stars, and our own Sun is just an example, are hot balls of glowing plasma held together by their own gravity. And the gravity of a star is very intense. Stars are continuously crushing themselves inward, and the gravitational friction of this causes their interiors to heat up. A star like the Sun is a mere 5,800 Kelvin at its surface, but at its core, it can be 15 million Kelvin – now that’s hot!

The intense pressure and temperature at the core of a star allow nuclear fusion reactions to take place. This is where atoms of hydrogen are fused into atoms of helium (through several stages). This reaction releases an enormous amount of energy in the form of gamma rays. These gamma rays are trapped inside the star, and they push outward against the gravitational contraction of the star. That’s why stars hold to a certain size, and don’t continue contracting. The gamma rays jump around in the star, trying to get out. They’re absorbed by one atom, and then emitted again. This can happen many times a second, and a single photon can take 100,000 years to get from the core of the star to its surface.

When the photons have reached the surface, they’ve lost some of their energy, becoming visible light photons, and not the gamma rays they started out as. These photons leap off the surface of the Sun and head out in a straight line into space. They can travel forever if they don’t run into anything.

When you look at a star like Sirius, located about 8 light-years away, you’re seeing photons that left the surface of the star 8 years ago and traveled through space, without running into anything. Your eyeballs are the first thing those photons have encountered.

So why do stars shine? Because they have huge fusion reactors in their cores releasing a tremendous amount of energy.

We have written many articles about stars here on Universe Today. Here’s an article about an artificial star that astronomers create, and here’s an article about a star that recently shut down nuclear fusion in its core.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
University of Illinois
NASA

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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