Algol

Lore & Background

The variability of Algol was noted in 1667 by Italian astronomer Geminiano Montanari, but the periodic nature of its brightness changes was not recognized until more than a century later. In May 1783, British amateur astronomer John Goodricke presented findings to the Royal Society suggesting the variability was caused by a dark body passing in front of the star, for which he was awarded the Copley Medal. In 1881, Harvard astronomer Edward Charles Pickering presented evidence that Algol was an eclipsing binary, confirmed in 1889 when Hermann Carl Vogel found periodic Doppler shifts in its spectrum, making it one of the first known spectroscopic binaries.

Reader's Guide

Algol is a triple-star system consisting of Beta Persei Aa1, Aa2, and Ab. The close pair, a B8 main-sequence star and a K0 subgiant, orbit every 2.9 days and undergo eclipses that cause the star's brightness to vary. The third star, an F1 main-sequence star, orbits the pair every 680 days. Studies of Algol led to the Algol paradox in stellar evolution: the more massive component is still on the main sequence while the less massive one is a subgiant, resolved by mass transfer. The system exhibits X-ray and radio flares, and its distance has been measured via very-long-baseline interferometry at 94 light-years. About 7.3 million years ago it passed within 9.8 light-years of the Solar System, possibly perturbing the Oort cloud.

Did You Know?

• Algol's name derives from Arabic 'raʾs al-ghūl', meaning 'head of the ogre'.
• The star was called 'Satan's Head' in Hebrew folklore and 'Caput Larvae' (the Spectre's Head) in Latin.
• In Chinese astronomy, Beta Persei is called 大陵五 (Dà Líng wu), the Fifth Star of the Mausoleum.
• The Algol paradox—where the less massive star is more evolved—is resolved by mass transfer between the components.