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.