A star shines because its center is so hot and dense that hydrogen nuclei fuse together, creating tremendous energy. It lives for millions or billions of years while the inward pull from its own gravity is balanced by the outward pressure from nuclear fusion. Its life ends when the nuclear fuel has been used up. First the star swells, brightens and cools to become a red giant. Then it collapses into a compact stellar remnant, much smaller than our Sun but of similar mass.

Stars less than eight times heavier than the Sun die relatively peacefully. The outer layers are shed in a stellar wind, making the star temporarily visible as a planetary nebulae. The remnant is about the size of the Earth and is called a white dwarf. Heavier stars die in a spectacular supernova explosion. If the star was moderately heavy, the remnant is a neutron star: a dense ball of neutral elementary particles, squeezed into a space little more than 10 miles across. Extremely heavy stars (more than 25 times heavier than the Sun) have no means to withstand their own gravity as they die. They collapse completely to a black hole.

We can see examples of the life cycle of stars all around us in the sky. Our own Sun is a fairly typical medium-sized middle-aged star. The star Betelgeuse is a well-known red giant. Planetary nebulae and supernova remnants can both be spectacular sights, even through a small telescope. Good examples are NGC 7027 and the Crab nebula, respectively. Albireo is an example of a binary star system in which two stars orbit around each other. More than half of all stars live in such systems. If one of the stars in such a binary system evolves into a black hole, then the system can sometimes be observed as a bright X-ray source. In our own Milky Way galaxy this is the case for example in Cygnus X-1. More examples can seen in other nearby galaxies, such as in M33. The following pages describe the properties of these objects, and their connection to stellar evolution in general: