Star Facts: Algol

Image Credit: Tom J Martinez

Algol (Beta Persei), known informally as the Demon Star, is a triple star system in which the components are designated Beta Persei Aa1, Aa2, and Ab, respectively. The Algol system is not only the first, and perhaps the best known, eclipsing binary system to be discovered, it is also among the first non-nova variable stars to be found, and today it serves as the prototype for the Algol-class of variable stars. While the variability of the primary star in the system is well-known, there is also a “second minimum”, which occurs when the bright primary component occults the less luminous secondary star, but note that the second minimum cannot be detected visually.

Quick Facts

• Constellation: Perseus
• Coordinates: RA 3h 8m 10s|Dec +40° 57′ 20″
• Distance: 90 light-years
• Star Type: Aa1: B8V /Aa2: K0IV /Ab: A7m
• Mass: (Aa1) 3.17 sol
• Radius: (Aa1) 2.73 sol
• Apparent Magnitude: (Aa1) Variable from 2.12 to 3.39
• Luminosity: (Aa1) 182 sol
• Surface Temperature: (Aa1) 13,000K
• Rotational Velocity: 49 km/sec
• Age: (Aa1) 570 million years
• Other Designations: Gorgona, Gorgonea Prima, Demon Star, El Ghoul, ß Persei, ß Per, 26 Persei, BD+40°673, FK5 111, GC 3733, HD 19356, HIP 14576, HR 936, PPM 46127, SAO 38592


The Algol system is located within the constellation Perseus, which is visible from latitudes of between +90° and -35°, although best seen at about 9 PM (Local Time) during the month of December, when it is highest in the sky. As can be seen from the image above, Capella in Auriga and the Pleiades in Taurus form a huge celestial triangle with Mirfak, the brightest star in Perseus, with Algol easily located just a few degrees to its right.

Physical Properties

Triple Star System

The secondary component (Aa2) in the Algol system is a magnitude 2.9 K-type star that is separated from the primary star (Aa1) by only 0.062 AU, giving it an orbital period of 2 days, 20 hours, and 49 minutes. The third star in the system (Ab) is a magnitude 2.3 A-type star that is separated from the primary pair by a distance of 2.96 AU, giving the system a mutual orbital period of 681 days. Note, however, that a further seven faint stars are listed in the Washington Double Star Catalog as being members of the Algol system.

Algol Paradox

The most interesting aspect of the Algol system involves the Algol Paradox, which had confounded investigators for a long time. At issue is the fact that while the more massive star (Algol Aa1) in the system is still on the main sequence, the less massive secondary component (Algol Ab) is a sub-giant star that is at a much later evolutionary stage. Clearly, this seems to be contrary to established models of stellar evolution, which states that the more massive a star is, the sooner it must reach the next stage in its evolution. This therefore means that the more massive star in a binary system, and not the less massive, must reach the sub-giant stage first.

However, the problem is easily solved by the mechanism of mass transfer between two stars in close proximity to each other. As the more massive star expands during the normal course of its evolution, it sometimes fills and exceeds its Roche lobe, and thus some of its mass is transferred onto the less massive star. The additional mass accelerates the evolution of the (previously) less massive star, which means that it can then reach the sub-giant stage before the (previously) more massive star, which remains on the main sequence since it has lost a significant percentage of its mass to the other star. In some double star systems that are similar to the Algol system, this process of mass transfer can actually be observed.

X-Ray Emitter

The Algol system is also strong emitter of X-rays and radio wave flares, although the exact origins and/or causes of the emissions are not known for certain. In the case of the X-ray emissions, most investigators believe that these are caused by the magnetic fields of the primary stars interacting with the mass being transferred between them, while the radio flares are thought by some investigators to be caused by magnetic cycles on these stars that are somewhat similar to sunspots on the Sun. However, since the primary stars in the system have magnetic fields that are at least ten times more powerful than the Sun’s magnetic field, the observed radio flares from the Algol system is several times more powerful and persistent than similar emissions from the Sun.


While the Algol system is now about 92.8 light years away, there was a time about 7.3 million years ago when the system passed within about 9.8 light years of our sun. At this time, the system would have had an apparent visual magnitude of about -2.5, which is several times brighter than the star Sirius is today. Some investigators believe that during its closest approach to the Sun, the Algol system’s mass of 5.8 solar masses might have perturbed the Oort cloud sufficiently to dislodge a number of comets that might have entered the inner solar system. However, the actual increase in cometary collisions in the inner solar system following the Algol system’s passing is generally believed to be relatively low as compared to other periods in the region’s history.


In traditional astrology, Algol is considered to be one of the “unluckiest stars in the sky” and throughout history it has also been associated with demons, bloody violence, death, and general ill-luck of various kinds in nearly all major cultures and mythologies. In fact, the name “Algol” derives from the Arabic “raʾs al-ghūl “, which translates into “[the] head of the ogre”, with the English name for Algol as Demon Star being a direct translation of this Arabic phrase.

Other names for Algol include “Rōsh ha Sāṭān”, meaning “Satan’s Head” in old Hebrew folklore, and, “Caput Larvae” , meaning “[The] Spectre’s Head”, a Latin name from the 16th century. In China, Algol is known by another grim name- “The Fifth Star of Mausoleum”, on account of its position in an asterism known as “Mausoleum”, which is made up of a number of other stars in the Perseus constellation.

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