With the naked eye, however, we only see approximately 3,000 of those stars at any one time. In gazing up at the night sky it is immediately obvious that some of those 3,000 stars are much brighter than others. Sirius, for example, has a magnitude of -1.46, while the faintest stars visible to the naked eye are around magnitude 6. Polaris, the North Star, shines at a magnitude of 2.0, and is considered a moderately bright star. However, because of the way the magnitude scale is set up, a star that is 5 magnitudes brighter than another star will actually be 100 times brighter.
Some stars actually change in magnitude and are aptly called variable stars.
Most stars, unlike our sun, are double star systems with two or more stars revolving around each other. In some cases, variable stars vary in brightness because one star in such a double star system eclipses the other. The double star Algol, or the Demon Star, in Perseus is one such variable star. Algol's companion star revolves around it in 2.9 days, causing it to change from a 2.2 magnitude star to a 3.4 magnitude star in just 20 minutes. True variable stars vary in brightness because they actually change in size.