Algol, traditionally known as the Demon Star, is the second-brightest star in the constellation Perseus and one of the best-known variable stars in the night sky. Located approximately 93 light-years away, it is a three-star system whose two inner stars repeatedly eclipse each other, causing Algol to fade noticeably every 2.87 days.
Easy to Find in the Constellation Perseus
Algol lies in Perseus, approximately nine degrees south of Mirfak, the constellation’s brightest star. From mid-northern latitudes, Perseus is especially prominent on autumn and winter evenings and can be found below the familiar W-shaped constellation Cassiopeia. Algol, Mirfak and Almach in neighboring Andromeda form a conspicuous triangle, with Algol normally appearing as a moderately bright naked-eye star.
Its Fading Can Be Seen Without a Telescope
Algol normally shines at magnitude 2.1 but falls to approximately magnitude 3.4 during its deepest eclipses, losing about 70% of its apparent brightness. The change is large enough to follow with the naked eye from a reasonably dark location. Almach appears similar to Algol at normal brightness, while nearby Rho Persei provides a useful comparison when Algol is near minimum. Observing the stars several times over one evening can reveal the Demon Star fading and then recovering.
One Star Eclipses Another Every 2.87 Days
Most of Algol’s visible light normally comes from Algol A, its hot B-type main-sequence star, whose surface temperature is approximately 12,550 kelvins. The system fades dramatically when Algol B, a larger but much dimmer K-type subgiant with a temperature near 4,900 kelvins, passes in front and blocks much of Algol A’s light. The two complete an orbit every 2 days, 20 hours and 49 minutes, with each main eclipse and recovery lasting approximately 10 hours. A much shallower secondary eclipse occurs when Algol A passes in front of Algol B, but this cannot be detected by eye.
In 1783, John Goodricke correctly proposed that an orbiting body caused Algol’s periodic fading, more than a century before spectroscopy confirmed its binary nature. Algol later became the prototype for an entire class of eclipsing variable stars known as Algol variables.
Algol Is Actually a Three-Star System
The system consists of the hot star Algol A (Aa1), the cool subgiant Algol B (Aa2) and the more distant Algol C (Ab). Algol A and B are separated by only about 9.3 million kilometers (5.8 million miles), allowing them to orbit each other in less than three days. Algol C, an F-type main-sequence star with approximately 1.6 times the Sun’s mass, circles the close pair every 680 days at an average distance of about 2.7 times that between Earth and the Sun. The system is too distant and compact for its individual stars to be separated through an ordinary amateur telescope.
The Less Massive Star Looks Older
Algol A has approximately 3.4 times the Sun’s mass, while Algol B has only about 0.8 solar masses. Yet Algol B is much further along in its evolution. This seems to contradict the usual rule that massive stars consume their fuel and evolve faster than smaller stars born at the same time. The apparent contradiction became known as the Algol paradox.
Mass Transfer Solved the Algol Paradox
Algol B was originally the more massive of the two stars, so it evolved and expanded first. As its outer layers reached the region where Algol A’s gravity became stronger, large amounts of gas flowed onto its companion. Algol B was stripped down to its present low mass, while Algol A grew into the 3.4-solar-mass star seen today. Gas is still moving between them, but their reversed masses preserve the evidence of this earlier transformation.
The Cooler Star Produces Powerful X-Ray Flares
Despite supplying relatively little visible light, Algol B produces most of the system’s X-rays and radio flares. Tidal forces have synchronized its rotation with the 2.87-day orbit, making the swollen star rotate unusually quickly. This rapid rotation strengthens its magnetic activity, creating an extremely hot outer atmosphere and sometimes releasing enormous stellar flares. To our eyes, Algol looks like an ordinary point of light, but at X-ray and radio wavelengths it reveals itself as a magnetically violent and flare-prone system.
Why Algol Is Called the Demon Star
The name Algol comes from the Arabic phrase raʾs al-ghūl, meaning “head of the ghoul” or “head of the ogre.” In Greek sky lore, the star marks the severed head of Medusa carried by the hero Perseus. Historical star-name lists also record the Hebrew Rōsh ha Sāṭān, or “Satan’s Head,” and the Latin Caput Larvae, meaning “the Spectre’s Head.” Chinese astronomers knew Algol less dramatically as the “Fifth Star of the Mausoleum,” referring to its position within a larger asterism. These names helped give Algol its enduring sinister reputation, although they do not prove that ancient observers recognized its regular fading.