Star of the Month
Beta Persei (Algol)
Position: 03 hrs 08.2 min 40 deg 57 min.
Due south at 22:42 (GMT) on 15 November 2019
Taken from the Simbad website
We have already met two variable stars, Delta Cephei (Double Star of the Month October 2019) and Omicron Ceti (Star of the Month November 2019). This month, the star of the month is the most famous eclipsing binary, Algol or Beta Persei. Unlike Mira, the main star is completely stable, it is a very hot B8 spectral class star which is white or bluish white. But it has a close companion star which is a much dimmer yellow K0 star. When the dim companion passes in front of the bright main star, some of the main star’s light is cut off and as seen from Earth, the star becomes dimmer. Once the dim star passes away again, the star returns to its normal brightness. It is like an exo-planet passing in front of a star, the companion star is just a lot bigger (and also gives off light itself). Hence the dim period is both regular and short. The overall period is 58 hours and 49 minutes, but the period of dimming is only about ten hours. The normal brightness is 2.1 and the dimmed brightness is 3.4, so there is a quite visible difference. Here is another piece of good news, Algol is easy to find. In December about 22.00 GMT look for the Square of Pegasus in the south-west sky, then look for a straight line of fairly bright white stars stretching between the top left corner of the Square towards the very bright star Capella, at an angle of roughly 45 degrees. Then look for a similarly bright white star underneath
the left-most of these stars (the one closest to Capella) namely Alpha Persei (Mirphak) and the star in this line-up to the right of Mirphak, namely Gamma Andromedae (Almaak or Almach), forming a more or less equilateral triangle. This is Algol and here is more good news. Mirphak is slightly brighter than Algol at 1.75, but Almaak is exactly the same as Algol at 2.05 and the star in the line-up to the right of Almaak, Beta Andromedae (Mirach), is also exactly the same at 2.15. So if Algol is dimmed, it will be clearly dimmer than Almaak or Mirach, but normally it will exactly the same as them. If Mira is the wonderful star, Algol (al-ghoul) is the demon star. But contrary to popular etymology, it is unlikely that its “bad” reputation has anything to do with its variability. In the classical pictorial depiction of the constellations (still used in the nineteenth century), Perseus is holding the head of Medusa the snake- haired Gorgon, and Beta Persei (Algol) and Rho Persei were her terrible eyes. The variability of Algol was first noted by the Italian astronomer Geminiano Montanari in 1667, but the periodic nature of this variability was first pointed out by the deaf English astronomer John Goodricke in 1783. He also proposed the correct mechanism for the variation and was awarded the Royal Society’s most prestigious award, the Copley Medal at the age of only eighteen, two years after William Herschel. This was fortunate as he died only three years later of pneumonia.