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Video:What Is the Definition of a Star?

with Michael Shara

If you're interested in learning the definition of a star, our expert will clear up any misunderstandings. In this informational video, you will learn the basic definition of a star.See Transcript

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Transcript:What Is the Definition of a Star?

Hi. I'm Michael Shara, Curator of Astrophysics at the American Museum of Natural History, here for About.com. Today I'm here to tell you about how stars are born and how they live and what they are.

What is a Star?

Stars are huge balls of gas, mostly hydrogen and helium that are really nuclear reactors. Gigantic nuclear reactors with very very hot hydrogen in their cores in their centers fusing to make helium.

Formation of Stars

The way a star is born is from a large cloud of interstellar gas and dust, that can be a light year across. Trillions of miles in diameter. That's starts to collapse under its own weight under its own gravity. As it does it starts to slowly spin, and, as it spins, most of the material in the collapsing cloud falls into the center to make the star. The stuff that gets left behind on the outside forms a ring or a donut shaped area, that breaks up and becomes planets.

So, astronomers now understand that planets are the natural afterbirth of stars. They come together. And, that's why the planets of our solar system aren't unique they're lots and lots, in fact, probably a trillion planets - just in our Milky Way Galaxy. As a star is being born it gets hotter and hotter in its center. Protons crash faster and faster into each other to release more and more energy. Finally, the star gets so luminesce and so bright. That it stops anymore in falling material from falling onto it. It's no longer a stellar fetus but it's become a stellar newborn object.

Baby Stars

A brand new baby star. As a star gets a little older and then a little older, it starts to change its brightness and its color and then settles into a long period of middle age, where it's almost the same brightness and temperature. And, our sun is in that phase today.

Red Giant Stars

Finally, when a star gets really old and had burned up most of its hydrogen it expands and becomes a red giant star before expelling it's outer envelope into space and dying as a white dwarf star surrounded by a shimmering cloud of dust and gas called a planetary nebula.

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