Popular Astronomy

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Perseus

Most constellation names are Latin for something or other, like Leo the Lion, but Perseus is... well, Perseus. He was a great hero in Greek myths, and got up to all sorts of amazing deeds of the sort that you can only put up with in myths, like killing an absolutely detestable woman called Medusa who had snakes for hair and could turn anyone to stone with a single glance. Remind you of anyone you know? Well, there he is in the sky after all that effort. His constellation is right next to Cassiopeia, who also features in the myths as a queen who got on everyone's nerves because of her vanity, and her beautiful daughter (well, she had to be beautiful, didn't she) Andromeda, whom he saved from being eaten by the sea monster, Cetus. The whole story is a lot more complicated but that's the executive summary.

Map of Perseus
For hints on understanding the star map, please click here.

Things to look for in Perseus

His stars form a sort of T-shape, the brightest being Mirfak, at the centre of the crossbar of the T. At the bottom of the T is the star Algol, aka the Demon Star. This marks the head of Medusa, which Perseus carries on his belt as an awful warning to other ugly women not to start getting ideas. This is a very famous star, because it is one of the first variable stars to be discovered. Every few days it dips in brightness as a result of a dimmer star in orbit around it hiding it from view, so we mostly see the dimmer one for a few hours. You might think that if stars are made of gas the brighter one would still shine through the dimmer one, but there's so much gas in a star that this doesn't happen.

These dips – known as minima – happen every 2 days 21 hours so you might think you'd see Algol looking faint quite often. But if you want to see one at a sensible time in the evening there are only about one or two each month which you will see. Sky and Telescope magazine have an online calculator which you can use to see when there will next be one at a suitable time for observation.

Double Cluster, credit Robin Scagell There are some nice star clusters in Perseus, including one BOGOF – that's Buy One Get One Free. This is the Double Cluster, at the top right of the constellation, and you don't need much imagination to work out from the name that there are two star clusters side by side. They are rich in faint stars and though they're visible from city skies using binoculars you get the best views from darker skies and with either high-power binoculars or a small telescope magnifying only about 25 times, so that you get both clusters in the same field of view. With large telescopes you can usually only see one at a time.

Another cluster, M34, near Algol, is a little larger than either member of the double cluster, but has fewer stars. And NGC 1528 is smaller and has fewer stars but still worth a look. With all these clusters, you have to hold binoculars really steady to see the individual stars.

If you want a really bright cluster, there's one in Perseus that many people overlook. It's the one surrounding Mirfak, and it's a really lovely sight in binoculars. It's known as the Alpha Persei Cluster, and although the stars are quite widely scattered they really are a cluster. Look for an S-shape of faint stars winding away from Mirfak.

M34, credit Robin Scagell NGC 1528, credit Robin Scagell Alpha Persei Cluster, credit Robin Scagell

Text by Robin Scagell