![]() Algol C is a white star that is four times as bright as the sun. Yet another star, Algol C, circles the A and B pair from once every 1.86 years. Algol B is a yellow star that is "dim" when compared to the brighter blue star, yet compared to our sun it is three times more luminous.Īlgol A and Algol B are both several times larger than the sun. The dimmer star Algol B is the eclipsing body. The brighter star, Algol A is a bluish star about 90 times more luminous than our sun. The star is located about 90 light-years from Earth, and is actually made up of two stars in a binary system. ![]() Amazingly, he also correctly suggested the reason for the brightness variations: A large, dim body orbits the star and periodically blocks much of its light from our view. He observed the star systematically and determined its period. Īlgol's variability was rediscovered in 1782 by the English amateur astronomer John Goodricke, an 18-year-old deaf mute. Perhaps this is why few other astronomers paid much attention to Montanari's discovery at the time. At that time the only other variable star known (save for a few novae) was Mira, whose light varies over months instead of hours. The first person who definitely noticed Algol's winking behavior was the Italian professor of mathematics Geminiano Montanari of Bologna around the year 1667. (Image credit: Starry Night Software) First noticed 350 years ago These are the dates and times, in Universal Time, when Algol should be at its dimmest, magnitude +3.4 instead of +2.1.This table shows the times for mid-eclipse of Algol for North America. For your convenience, use this comparison star chart for estimating Algol’s magnitude. ![]() You should end up with a light curve looking like the one shown above. Good comparison stars are nearby Gamma Andromedae (magnitude +2.2) to Algol’s west, and Epsilon Persei (magnitude +2.9) to its east. To track Algol, compare it periodically with stars of known brightness in its immediate surroundings. The star stays at its faintest value for two hours centered on the time of mid-eclipse. Light curve of the eclipsing variable star Algol (Beta Persei). A second, minor dip in brightness occurs as the companion passes behind the primary, but the fading is so slight that it is undetectable to the eye. As the companion moves off the primary, Algol returns to peak brightness. The fading lasts for about ten hours as the companion passes in front of the primary along our line of sight. Its changes are very plain to the naked eye – in the middle of an eclipse it shines dimly at magnitude +3.4 instead of its usual +2.1. There is also a third star in the system, Algol C, but it does not participate in the eclipses.Īlgol fades and rebrightens like clockwork every 2 days 20 hours 49 minutes. Its companion, Algol B, is a larger but fainter orange star that covers about 80-percent of Algol A during the eclipses. The stars are too close together to be distinguished individually in a telescope, but analysis of the light from Algol reveals that the brightest of the pair, Algol A, is a hot star one hundred times as luminous as the Sun. Goodricke also suggested, correctly, that the variations were caused by eclipses.Īlgol is an eclipsing binary, consisting of two stars that regularly pass in front of each other as seen from the Earth. However, this remains unsupported by any other real evidence.Īlgol’s variations in light were first recognized by the Italian astronomer Geminiano Montanari in 1667, and the periodicity of the variations was first measured by the English amateur astronomer John Goodricke in 1783. ![]() ![]() To ancient and medieval astrologers Algol was the most dangerous and unfortunate star in the heavens, which seems to suggest that its strange variability might have been noticed in antiquity. You can check on it whenever you step outdoors on nights when Perseus is in view.Īlgol’s name derives from the Arabic Al Ra’s al Ghul, “The Demon’s Head”, and in Greek mythology it represents the head of Medusa held by Perseus. This is because it can be observed with the unaided eye and it has a relatively short period of less than three days. Beta Persei, better known as Algol, is the premier eclipsing binary star of the northern sky. ![]()
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