A star is a gigantic mass of glowing gas- mainly hydrogen and helium. A star's life span depends on how fast it turns its supply of hydrogen into helium. The pressure inside a star is so intense that the nuclei of hydrogen atoms start smashing into each other. This process,called nuclear fusion, throws off energy in the form of light and heat. It also produces helium. >
Stars come in all colors and sizes. Some stars are thousands of times brighter than others. The Sun is an average star, but it stands out in one way: It has no companion (most stars occur in pairs, triples, or clusters). The Sun's closest neighbor, Proxima Centauri, belongs to a triple-star group that shines bright just 4.3 light-years from Earth. Stars vary in size from tiny neutron stars just 30 km (19 mi) across to red supergiants nearly 1 billion km (602 million mi) in diameter. Sirius B, a white dwarf, has a diameter of 53,000 km (32,000 mi)-hardly bigger than Uranus. The red supergiant Betelgeuse, by contrast, is so huge that astronomers can make out features on its surface despite the fact that it is 500 light-years away.How bright a star looks depends on three things-how bright the star really is (absolute brightness), how far away it is from Earth, and how much dust lies between the star and Earth. Two stars that look equally bright may in fact be a bright distant star next to a dim, close star. The two stars have the same apparent brightness, but one has a greater absolute brightness. If two stars are equally far away, the star with the greater absolute brightness will have the greater apparent brightness too.
The Hertzsprung-Russell diagram shows the link between the a star's temperature and its luminosity-the amount of light the star gives off. More than 90 percent of all stars, including the Sun, fall within a narrow band known as the main sequence. Other stars, including white dwarfs, red giants, and red supergiants, fall outside the main sequence-a clue that they are at later stages in their lives.