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Algol, The Demon Star.

Updated: Jul 1, 2023

In this weeks post, I thought it would be of interest to direct your attention to a well known naked eye 2nd magnitude star called Algol in the constellation of Perseus. Algol (Beta Persei) is the Arabic name for "ghoul" or "demon" hence it's moniker. This star certainly lives up to it's name as it is an eclipsing binary star that appears to wink (dims) at regular intervals, from magnitude 2.1 to 3.4. These variations were first noted by the Italian astronomer, Geminiano Montanari in 1667.

It was the incredible observer John Goodricke (1764-1786) who correctly suggested that a fainter companion star was eclipsing Algol. Indeed, in 1782, he accurately timed the period to every 2 days, 20 hours, 48 minutes and 56 seconds that the star goes through a 10 hour eclipse, 5 hours to minimum and 5 hours back to maximum. John Goodricke is also credited with discovering the periodic variation of Beta Lyrae and Delta Cephei, the prototypical example of Cepheid variable stars. John, who was deaf, (a horrible disability that I can identify with), was elected a Fellow of the Royal Society on 16 April 1786. Unfortunately, he never learned of this honour, as he died four days later from pneumonia. What a complete waste of talent. However, there is at least a plaque in honour to this dedicated and patient young man, in the grounds of York Minster.

Algol itself is a class B (B8) star that shines with a faintly bluish white light. It is a main sequence star that is 3.5 times more massive than our Sun. The companion to Algol is a much dimmer yellow-orange class G or K giant star. Every orbit, when the dimmer, larger G/K star passes in front of Algol, we see a deep eclipse. The eclipse covers about 80% of Algol, so some of the light of Algol still shines through. Between the deep "primary" eclipses is a smaller dip when Algol passes partially in front of the dim one. There is also a third star and possibly a fourth star in this system, though these do not play a part in the eclipses. The whole Algol system is about 98 light years away, though incredibly about 7.3 million years ago it passed within 9.8 light-years of our Solar System and its apparent magnitude was about −2.5, which is considerably brighter than the star Sirius is today. What a sight that must have been!

The above graphic shows how Algol and it's companion star may look like. I will finish this post by saying at the moment, Algol and Perseus are well placed in the Autumn/Winter sky. As Algol is a naked eye observation, it is very easy to see, and a quick look on the internet will give you the expected times to observe the dimming and brightness of the eclipse. Now that the longer nights are here, why not check it out yourself? Can you observe it for a full 10 hours, as John did? Have you got that patience? Hope you enjoyed this post and keep looking upwards to the truth...

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