Star

Not all stars are as stable as the four discussed above. Many stars show periodic changes in brightness that are greater than the tiny variations a star like the Sun exhibits. Stellar variability has many causes.

Some stars pulsate, expanding and contracting repeatedly. As they get larger, they brighten, and as they contract they get dimmer. They produce a light curve such as the one in Figure 3. This is the record of luminosity variations in the star Mira, a cool, red star that shows pronounced pulsation with a period of about 330 days.

Stars may also be variable if they belong to a binary or multiple system, in which two or more stars are in orbit around one other. (Most stars belong to multiple systems; the Sun is in a minority in this respect.) An important class of stars are the eclipsing binaries, which produce a light curve as one star passes in front of the other, blocking out its light and causing the whole system to appear dimmer. It is possible to determine the radii and masses of stars in eclipsing binaries—a very difficult or Antares (Alpha Scorpionis) is the bright star dominating this photograph. It is a conspicuous red supergiant, the brightest star in the constellation Scorpius. The bright object to the right of Antares is the M4 (NGC 6121) globular star cluster. © Ronald Royer/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. impossible task with single stars. Figure 4 shows the light curve of the famous eclipsing binary Algol.