Stars can live for billions of years. A star is born when an enormous cloud of hydrogen gas collapses until it is hot enough to burn nuclear fuel (producing tremendous amounts heat and radiation). As the nuclear fuel runs out (in about 5 billion years), the star expands while the core contracts; it becomes a giant star that eventually explodes and turns into a dim, cool object (a black dwarf, neutron star, or black hole, depending on its initial mass). The largest stars have the shortest life span (still billions of years); more massive stars burn hotter and faster than their smaller counterparts (like the Sun).

Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright. In most stars, the primary reaction converts hydrogen atoms into helium atoms, releasing an enormous amount of energy.

In the universe, most stars occur in groups of at least two stars. Two stars that are locked in an elliptical orbit around their center of mass (their barycenter) are called a binary star system. About half of all stars are in a binary star system.

There are larger groups of stars, called clusters, that are relatively unorganized collections of stars. Huge, organized collections of stars are called galaxies. Our Solar System is located in the Milky Way Galaxy, a spiral galaxy. All groups of stars are held together by gravitational forces.

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