Variable Stars

Variable stars are stars that undergo significant changes in magnitude. About 50,000 of these have been discovered, and they are separated into two main groups. One type only appears to change in luminosity because it is eclipsed by another star in a binary system. Even planets have created optical variables by passing in front of stars.

Real variable stars actually swell and shrink. Usually, variable stars have periods that repeat over and over. In a lot of cases, these periods are a few days or weeks. But some have periods that last years, and they may increase or decrease by over 10 magnitudes. Others don't even have a period at all. These stars are very unstable (like stars in the process of a nova) and some explode without warning. Yet other variable stars are eruptive. Many of these are giants and supergiants, which flare easily because of unstable outer layers. Flare stars like these may brighten by two magnitudes in seconds, and then be gone just as fast.