A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light, except for the sun. The sun looks like a ball because it is much closer to Earth than any other star. The sun and most other stars are made of gas and a hot, gas-like substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.
Stars form when clouds of interstellar gas collapse under the influence of gravity. During their lifetimes, stars pass through a series of stages, with the sequence and timing depending crucially on the mass of the star. As a star passes through these stages, different elements are created, again depending on the star's mass. When stars have completed their development, they shed their material back into the interstellar medium, enriching the matter from which future generations of stars will form. Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, life span, and eventual fate.
Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old—the observed age of the universe. The more massive the star, the shorter its life span, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.
When a star has finished burning hydrogen in its core, it will start burning its outer layers in a series of concentric shells. The star will expand as the source of heat moves outward and its outer layers cool. Stars with very low mass will eventually fade and cool; Sun-like stars will evolve into red giants; and high mass stars will become supergiants. Once a star has used up all its available nuclear fuel, it will deflate, because there is no longer any power source to replace the energy lost from its surface. As it collapses, if it has enough mass, its helium core stars to burn and change into carbon. Once the fuel in its core is used up again, helium-shell burning begins in the star's atmosphere and the star expands. In very massive stars, this process is repeated until iron is produced. When a Sun-like star has used up all of its fuel, it will lose its outer atmosphere in a spectacular planetary nebula and collapse to become a white dwarf. A high-mass star will explode as a supernova and leave behind a neutron star or black hole.
During a star's life, it passes through many phases, but most of its time will be spent on the main sequence. Main-sequence stars are those that convert hydrogen into helium in their cores by nuclear reactions. This means that the chances of seeing any star are greatest during its main-sequence life time. In fact, about 90 percent of all observed stars are on the main sequence, during which time they are very stable. Although main-sequence stars are spread throughout the Milky Way, they appear predominantly in its plane and central bulge.
The form a star takes in the ultimate stage of its life is called a stellar end point. Such end points include some of the most exotic objects in the Milky Way. The fate of a star is dictated by its mass, with lower-mass tars becoming white dwarfs, and the highest-mass stars becoming black holes, from which not even light can escape. Between these are neutron stars, including spinning pulsars.
A multiple star is a system of two or more stars bound together by gravity. Systems with two stars are called binary or double stars. Binary stars orbit each other at a great variety of distances, with orbital periods ranging from a few hours to millions of years. Not all multiple systems consist of just two stars in mutual orbit, it may be a complex system of three or more stars. Most of the stars in the Milky Way are members of either binary or multiple systems, single stars like the Sun are more unusual. Most multiples are so close together that we know about them only from their spectra. They also vary widely in size and color, stars of any age and type can be members of a multiple star system.
A nova is a binary system, consisting of a giant star that is being orbited by a smaller white dwarf. The giant star has grown so large that its outer material is no longer gravitationally bound to the star and instead falls onto the white-dwarf companion.