Algol: β Persei
(1982)

Algol (β Persei; RA 03h 04.9m, Dec: +40° 46') is the second brightest star in the constellation of Perseus and is the prototype eclipsing binary. However, it is more than a double star. At times as many as five components have been suggested, but currently it is believed that there are three stars in the system, all of which have been observed spectroscopically. Over the past one hundred years or so, the system has been extensively studied: approximately one thousand papers have been published concerning it.

This essay [on this and other pages] presents a chiefly historical account of observations of the system, together with the various models put forward to explain the observed phenomena [up to January 1982].

An appendix provides definitions and explications of some terms which may be unfamiliar.

The Legend of Perseus and Medusa

To fulfil a rash promise to King Polydectes of Seriphos, the Greek hero Perseus (the son of Danaë, the daughter of King Acrisius of Argos, and Zeus, the supreme Olympian diet) was obliged to slay the Gorgon Medusa, a vile creature who had serpents for hair, huge teeth, a protruding tongue, and altogether so ugly a face that all who gazed at it were petrified with fright.

He was aided in this task by the goddess Athene, a sworn enemy of Medusa’s, who presented him with a brightly polished shield. The god Hermes also helped Perseus, giving him an adamantine harpe, or sickle, with which to cut off Medusa’s head. But the hero still needed a pair of winged sandals, a magic kibisis, or game-bag, to contain the decapitated head, and the dark helmet of invisibility that belonged to Hades: all these things were in the care of the Stygian Nymphs, from whom Perseus had to fetch them.

Perseus journeyed westward to the Land of the Hyperboreans, where he found the Gorgon asleep among the rain-worn shapes of men and wild beasts petrified by her gaze. He fixed his eyes on the reflexion in the shield, Athene guided his hand, and he cut off Medusa’s head with a single stroke of the harpe. Hurriedly thrusting the head into his kibisis he took flight; though Stheno and Euryale, Medusa’s immortal sisters, rose to pursue him, the helmet made Perseus invisible, and he escaped safely southward.

To commemorate his valour and heroism in this and other adventures, the gods of Olympus set the image of Perseus among the stars, with the head of the Gorgon Medusa marked by the star which we now know as Algol.

[Adapted from the account in Robert Graves’ The Greek Myths, Vol. II.]

Ancient observations

Algol, the proper name of β Persei, is a contracted form of the Arabic Ras Al Ghūl, which means “the head of the demon”. It may be that this was a translation from the Greek, but it is more likely to be of much earlier origin, for it is quite probable that the Greek constellation system and the cognate legends are primarily of Semitic or even pre-Semitic origin. In this context it may also be noted that the Hebrew name of the star is Rōshha-Ṣaṭan, or “the Devil’s head”. The significance of the name is uncertain: it may be that the early Arabian astronomers were aware that the star varied in brightness from time to time and, hence, ascribed to it a demoniacal nature which later led to its association with the monster in the Perseus legend.

Figure 1. The constellation of Perseus (RA 03h, Dec: +40°)

Any record of such an explanation, if it ever existed, may well have perished when the Library of Alexandria was destroyed by fire. Certainly, Chinese astronomers of the Han Dynasty had recorded the brightness changes of the star, which they knew as Tsi-Chi (“the piled-up corpses”). However, it was not until the latter half of the seventeenth century that Algol attracted the attention of European astronomers.