THE ECLIPSING VARIABLE STAR ALGOL (BETA PERSEI). When at MINIMUM magnitude, you will notice that the star Algol, is only the same brightness as the nearby star Rho Persei (see Illustration for their two positions in relation to each other). If you observe Algol,several hours later, you will see that Algol noticeably outshines Rho Persei, as Beta Persei brightens up again.
This star is an ECLIPSING VARIABLE STAR which changes in brightness from a visual magnitude of +2.3 to +3.7 ( = becomes three times fainter, or thereabouts), over a period of 2 days 20h and 49 minutes. The variability of this star was first noted by Montanari in 1672. However, it was a young, deaf and dumb, amateur astronomer called John Goodricke, from York, who realised the nature of this phenomenon. He observed Algol and suggested, correctly, that the variation in brightness might be caused by an eclipse or partial eclipse of a bright star by a fainter, less luminous one, causing the drop in magnitude.
We now know that the Algol system which lies at a distance of some 95 light years, consists of two stars, separated by some 7 million miles revolving about a common centre of gravity seen from Earth, the star shines brightly for 59 hours, then in the course of just 5 hours, it fades to minimum brightness, and stays at Minimum for about 20 minutes. It then rises to full brightness again in another 5 hours.
This illustration was created using 'STELLARIUM' software. The excellent constellation illustrations were designed by Johan Meuris.
In this classical representation of the constellation of the greek hero Perseus, son of Zeus, Algol and rho Persei mark the two eyes of the severed head of Medusa the Gorgon, whose baleful glare, turned all who had the misfortune to gaze at it, into stone!
The times when ALGOL is at MINIMUM brightness is to be found on the FRONT PAGE of 'John's Skynotes', and in the Summaries of Events on each month's notes.