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 conta . . .
Some stars have always stood out from the rest. Their brightness is a factor of how much energy they put out–known as luminosity–and 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 ha . . .
Young stars at this stage are called protostars. As they develop, they accumulate mass from the clouds around them and grow into what are known as main sequence stars. Main sequence stars like our own sun exist in a state of nuclear fusion during which they will emit energy for billions of years by converting hydrogen to helium. Stars evolve over b . . .
Structure The sun, like others stars, is a ball of gas. In terms of the number of atoms, it is made of 91.0% hydrogen and 8.9% helium. By mass, the sun is about 70.6% hydrogen and 27.4% helium. The sun's enormous mass is held together by gravitational attraction, producing immense pressure and temperature at its core. The sun has six regions . . .
The photosphere lie the tenuous chromosphere and the corona (crown), which make up the thin solar atmosphere. This is where we see features such as sunspots and solar flares. Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the moon covers the . . .
Potential for Life The sun itself is not a place conducive to living things, with its hot, energetic mix of gases and plasma. But the sun has made life on Earth possible, providing warmth as well as energy that organisms like plants use to form the basis of many food chains. . . .
Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other's work in a critical process called peer review. Defining the term Planet is important, beca . . .