Stars as a Source of Light: Definition & Explanation

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  • 0:01 What is a Star?
  • 1:02 Why Do Stars Shine?
  • 1:35 The Moon & Planets
  • 2:35 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After reviewing this lesson, you will be able to explain how stars are a source of light, and why we see other objects in our solar system. A short quiz will follow.

What Is a Star?

A star is a fixed bright point in the night sky. That's what it looks like, but what actually is a star? Although stars might look tiny and insignificant, that's only because they are so far away. A star is actually a burning ball of hot gas and plasma like the sun. The sun is nothing more than one of trillions of stars spread across our universe. It only looks different because it's so much closer to us.

Stars come in many shapes and sizes, from regular, ordinary main sequence stars like the sun to huge, red super-giant stars, or tiny, dim white dwarfs, or average size, but extremely bright blue-tinted stars. Part of the reason for this is that stars also go through lots of stages in their lives. The sun won't always look the way it does now. One day, billions of years from now, it will swell to form a red giant. And, eventually, once it burns itself out, all that will be left is a cool and dim white dwarf star.

Why Do Stars Shine?

We already said that stars are burning balls of hot gas and plasma, and burning things do shine, just like how you can see around a campfire, but there's a bit more to it than that. Deep inside the sun (and every star), a special kind of reaction is happening. It's called a fusion reaction. At the very center, where it's millions of degrees hot, hydrogen atoms are being fused (or joined) together to form helium atoms. When this happens energy is released, and this amazing power source is what powers the sun for billions of years.

The Moon & Planets

Up in the night sky, there are lots of objects. Some of them are twinkling stars, but not all of them. If you see an unusually bright star, especially if you can't see it twinkle, there's a good chance it's actually one of the planets. We see Mars, Venus and Jupiter most often because they're usually the brightest to our eyes. You also see the moon. But why? They're not burning like the sun, so why do they shine?

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