They could account for some of the dark matter suspected to exist in the universe. Variable Stars - A variable star is a star that changes in brightness. These fluctuations can range from seconds to years depending on the type of variable star. Stars usually change their brightness when they are young and when they are old and dying. They are classified as either intrinsic or extrinsic.
Intrinsic variables change their brightness because of conditions within the stars themselves.
Extrinsic variable change brightness because of some external factor, like an orbiting companion star. These are also known as eclipsing binaries. A binary star is a system of two stars that are gravitationally bound to each other. They orbit around a common point, called the center of mass. It is estimated that about half of all the stars in our galaxy are part of a binary system.