Algol

Wikipedia:

Algol (β Per, Beta Persei), known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered. Algol is actually a three-star system (Beta Persei A, B, and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every two days, 20 hours and 49 minutes during the roughly 10-hour long partial eclipses. There is also a secondary eclipse when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically. Algol gives its name to its class of eclipsing variable, known as Algol variables.

Observation history

In a 2012 paper, a claim is made that ancient Egyptians made detailed observations of the periodic variability of Algol, over 3200 years ago, in a papyrus document called the Cairo Calendar.

The variability of Algol was first recorded in 1667 by Geminiano Montanari, but the periodic nature of its brightness variations was not recognized until more than a century later by the British amateur astronomer John Goodricke (who also proposed a mechanism for the star's variability). In May 1783 he presented his findings to the Royal Society, suggesting that the periodic variability was caused by a dark body passing in front of the star (or else that the star itself has a darker region that is periodically turned toward the Earth.) For his report he was awarded the Copley Medal.

In 1881, the Harvard astronomer Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary. This was confirmed a few years later, in 1889, when the Potsdam astronomer Hermann Carl Vogel found periodic doppler shifts in the spectrum of Algol, inferring variations in the radial velocity of this binary system. Thus Algol became one of the first known spectroscopic binaries. Dr. Joel Stebbins at the University of Illinois Observatory used an early selenium cell photometer to produce the first-ever photoelectric study of a variable star. The light curve revealed the second minimum and the reflection effect between the two stars.

Despite many claims in the modern literature that its ancient association with a demon-like creature (Gorgon in the Greek tradition, ghoul in the Arabic tradition) strongly suggests that its variability was known long before the 17th century, there is no conclusive evidence for this.

System

From the point of view of the Earth, Algol A and Algol B form an eclipsing binary because their orbital plane contains the line of sight to the Earth. To be more precise, Algol is a triple-star system: the eclipsing binary pair is separated by only 0.062 astronomical units (AU) from each other, whereas the third star in the system (Algol C) is at an average distance of 2.69 AU from the pair, and the mutual orbital period of the trio is 681 Earth days. The total mass of the system is about 5.8 solar masses, and the mass ratios of A, B and C are about 4.5 to 1 to 2.

Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can be seen.

This system also exhibits variable activities in the forms of x-ray and radio wave flares. The former is thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer. The radio wave emissions might be created by magnetic cycles similar to those of sunspots, but simce the magnetic fields of these stars are up to ten times stronger than the field of the Sun, these radio flares are more powerful and more persistent.

Algol is located about 92.8 light years from the Sun, but about 7.3 million years ago it passed within 9.8 light years of the Solar System and its apparent magnitude was about -2.5, which is considerably brighter than the star Sirius is today. Because the total mass of the Algol system is about 5.8 solar masses, at the closest approach this might have given enough gravity to perturb the Oort cloud of the Solar System somewhat and hence increase the number of comets entering the inner solar system. However, the actual increase in net cometary collisions is thought to have been quite small.

Etymology of the name

The name Algol derives from Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) (see "ghoul"). The English name "Demon Star" is a direct translation of this .

In Hebrew folklore, Algol was called Rōsh ha Sāṭān or "Satan's Head", as stated by Edmund Chilmead, who called it "Divels head" or Rosch hassatan. A Latin name for Algol from the 16th century was Caput Larvae or "the Spectre's Head". Algol was also linked with "Lilith". Hipparchus and Pliny made this a separate, though connected, constellation.

In Chinese astronomy, Algol is called 大陵五 (the Fifth Star of the Mausoleum), and the star also bore the grim name of Tseih She (叠尸 - die2 shi1 in Modern Pinyin), meaning "piled up corpses".

Cultural significance

In the Tetrabiblos, the 2nd century astrological text of the Alexandrian astronomer Ptolemy, Algol is referred to as "the Gorgon of Perseus" and associated with death by decapitation: a theme which mirrors the myth of the hero Perseus’ victory over the snake-headed Gorgon Medusa. Astrologically, Algol is considered the most unfortunate star in the sky, and was listed as one of the 15 Behenian stars. Historically, the star has received a strong association with bloody violence across a wide variety of cultures.