MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Animation of the eclipsing binary star Algol (Beta Persei). Algol is a variable star as seen from Earth: usually it has a magnitude of 2.1, but every 2.87 days it dims rapidly to magnitude 3.4 for a few hours, before brightening again. This is shown by the light curve at upper right. This occurs as Algol is an eclipsing binary star - it is made of two components that orbit each other very closely, and in the same plane as the Earth. This means that they eclipse each other regularly. The stars are a smaller, much brighter blue-white star (Algol A) and a larger, much dimmer orange sub-giant (Algol B). When the small bright star is eclipsed by the B star, the visual magnitude falls sharply. There is a much smaller dip when the bright star eclipses the fainter one, but that is not visible with the naked eye. The Algol system lies some 93 light years from Earth in the constellation Perseus.
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