Algol

Algol is Beta Persei, at about 94 light years, and it is the prototype of an entire class of variable star.

Its brightness drops from magnitude 2.1 to 3.4 and recovers, on a period of 2.867 days, about two days, twenty hours and forty-nine minutes. The dimming lasts a few hours and is obvious to an attentive naked eye: at minimum the star is visibly less bright than usual, and the change can be followed by comparison with neighbouring stars without any instrument.

The cause is eclipse. Algol is a multiple system with three confirmed components: a hot blue main-sequence star of type B8V, an orange subgiant of type K0IV, and a more distant third star of type F1V. The deep minimum occurs when the dimmer subgiant passes in front of the brighter blue star. This is what an eclipsing binary is, and Algol gives its name to the type: Algol variables.

The system also produced a genuine astrophysical puzzle, the Algol paradox. The less massive component is the more evolved one, which should be impossible if both formed together, since more massive stars evolve faster. The resolution is mass transfer: the subgiant was originally the more massive star, evolved first, expanded, and transferred much of its mass to its companion. The paradox and its solution were an important step in understanding binary evolution.

The Arabic ra's al-ghul means the head of the ghoul or demon; the star marks the severed head of Medusa held by Perseus.

One further detail bears on the tradition: the eclipse is partial, not total. The subgiant covers most but not all of the blue star's disc, which is why the minimum is a drop of about 1.3 magnitudes rather than a disappearance. The star never vanishes; it dims to roughly a third of its usual brightness and returns within about ten hours.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Arabs and the StarsPaul Kunitzsch

The ordering of events around Algol is the single most instructive thing about it, and it is regularly told backwards.

Algol's malefic reputation is ancient. It is the Gorgon's head, associated with decapitation, and it is the most consistently ill-omened star in the astrological tradition. Ptolemy gives it the nature of Saturn and Jupiter; the Saturn half carries the entire grim reputation, and the Jupiter half is so routinely forgotten that most modern accounts describe Algol as simply Saturnine.

The variability was documented far later. Geminiano Montanari noted that the star's brightness changed in 1667, without establishing periodicity. John Goodricke, a deaf English amateur astronomer then aged eighteen, determined the period and presented it to the Royal Society in 1783, in a paper titled A series of observations on, and a discovery of, the period of the variation of the light of the bright star in the head of Medusa, called Algol, published in the Philosophical Transactions. He proposed that a dark body passing in front of the star could account for it, which is correct, and which anticipated the concept of an eclipsing binary by a century. He was awarded the Copley Medal and died at twenty-one.

So the reputation predates the discovery of the variability by at least fifteen centuries. Algol was not called the Demon Star because it winks; it was called the Demon Star, and much later someone noticed that it winks.

The frequent modern claim that ancient observers knew of the variation and named the star for it is therefore not established by the naming. There is a specific proposal that an Egyptian calendar of lucky and unlucky days from around the thirteenth century BC encodes Algol's period, based on statistical analysis of its day ratings. That claim is interesting, it is published, and it has been questioned; it should be reported as contested and not as fact.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with TetrabiblosClaudius Ptolemy

Algol is a useful test of how much of a star's traditional character is actually attributable, because the gap between the source and the modern delineation is unusually wide.

Ptolemy gives two planets, Saturn and Jupiter. That is the whole of it. There is no mention of decapitation, of hanging, of violent death, of strangulation, or of any of the specific horrors that fill modern entries.

Those specifics accumulate through the medieval Arabic and Latin tradition, where the star is Caput Algol and is used in talismanic magic, and they reach their familiar English form in Robson in 1923. Robson's entry for Algol is the most alarming in his book and is quoted constantly. He was compiling: some of it is from Ptolemy, some from the Arabic material, some from Renaissance sources, and some from nineteenth-century writers whose own sources he does not give.

Agrippa includes Algol as the first of the Behenian fifteen, pairing it with diamond and with black hellebore, a genuinely poisonous plant, which is consistent with the star's character and is a good illustration of the internal logic of the Behenian scheme.

The Gorgon association is worth one observation. Perseus holds Medusa's severed head, and Algol marks the head. Whether the constellation figure was drawn to place a variable, ill-omened star at the point of severance, or whether the star acquired its character from an already-existing figure, is not determinable. The Greek constellation figure is itself inherited and reworked from Mesopotamian material, and the chain is not traceable that far back.