<p>Photo: Stars.</p>

Photograph courtesy NASA

Stars are cosmic energy engines that produce heat, light, ultraviolet rays, x-rays, and other forms of radiation. They are composed largely of gas and plasma, a superheated state of matter composed of subatomic particles.

Though the most familiar star, our own sun, stands alone, about three of every four stars exist as part of a binary system containing two mutually orbiting stars.

No one knows how many stars exist, but the number would be staggering. Our universe likely contains more than 100 billion galaxies, and each of those galaxies may have more than 100 billion stars.

Yet on a clear, dark night Earth's sky reveals only about 3,000 stars to the naked eye. Humans of many cultures have charted the heavens by these stars.


Some stars have always stood out from the rest. Their brightness is a factor of how much energy they put out, which is called their luminosity, and also how far away from Earth they are.

Stars in the heavens may also appear to be different colors because their temperatures are not all the same. Hot stars are white or blue, whereas cooler stars appear to have orange or red hues.

Stars may occur in many sizes, which are classified in a range from dwarfs to supergiants. Supergiants may have radii a thousand times larger than that of our own sun.

Hydrogen is the primary building block of stars. The gas circles through space in cosmic dust clouds called nebulae. In time, gravity causes these clouds to condense and collapse in on themselves. As they get smaller, the clouds spin faster because of the conservation of angular momentum—the same principle that causes a spinning skater to speed up when she pulls in her arms.

Building pressures cause rising temperatures inside such a nascent star, and nuclear fusion begins when a developing young star's core temperature climbs to about 27 million degrees Fahrenheit (15 million degrees Celsius).

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'Live From Space' March 14

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