Perseus with Cassiepoeia LOLocated close to the prominent W-shaped constellation Cassiopeia, Perseus depicts the legendary Greek hero of the same name. When seen from mid-northern latitudes, both Cassiopeia and Perseus are located at or near the overhead point during evenings in late-autumn and throughout the winter months, and the distinctive shape of Cassiopeia is really quite unmistakeable. Once you’ve identified Cassiopeia, the inverted Y-shape of the constellation Perseus can be seen extending away towards the south as shown here.

One of Perseus’ main claims to fame was that he slew the hideous Medusa, one of the three Gorgons, which was no mean achievement bearing in mind that Medusa had serpents instead of hair and that anyone looking at her would be instantly turned into stone. Having received assistance from the gods, however, Perseus managed to complete his task safely.

While on his way back home to Seriphos, he came across the beautiful princess Andromeda, daughter of King Cepheus and Queen Cassiopeia, chained to a rock and awaiting sacrifice to a particularly ferocious sea monster. Perseus promptly saved the day by swooping down and killing the monster, following which he freed Andromeda and whisked her to safety. Naturally, and in the best Greek tradition, Perseus married Andromeda and returned with her to Seriphos.

After a long and prosperous life Perseus died and was placed among the stars alongside his wife Andromeda and his in-laws Cepheus and Cassiopeia, the latter of which is depicted by the neighbouring constellation Cassiopeia. Both Cepheus and Cassiopeia, together with the bright star Algenib in Perseus (see below) are depicted on the chart of North Circumpolar Stars.

The brightest star in Perseus is Algenib, a white supergiant which shines from a distance of nearly 600 light years, the light we are seeing having set off on its journey towards us around the time that Henry V was on the English throne. The name of this star is derived from the Arabic ‘al-janb’ which means ‘the Side or Flank’, an obvious reference to its location within the figure of Perseus.

A little way to the southwest of Algenib is Algol, a star deriving its name from the Arabic ‘ra’s al-ghul’ meaning ‘the Demon’s Head’ and which, according to legend, depicts the severed head of Medusa the Gorgon held by Perseus. Algol is one of the most famous variable stars in the entire sky, the earliest recorded observations of its variability being made by the Italian astronomer Geminiano Montanari in 1667. Montanari noticed that Algol was shining at less than its usual brightness, his subsequent series of observations confirming that Algol was indeed variable. However, it was the York-based astronomer John Goodricke who made the first accurate measurements of its period of variability. It was also Goodricke who first suggested that Algol was a binary and that the variations in brightness were due to the occasional eclipse of a brighter star by a fainter companion.

Algol is what astronomers refer to as an eclipsing binary and, at a distance of 93 light years, is the closest object of its kind. It is indeed a system of two stars in orbit around each other, with one star considerably fainter than its companion. The plane of their orbit is almost exactly lined up with our position in space and the fainter star regularly passes in front of, or eclipses, its brighter companion. When this occurs, the overall light output from Algol decreases from magnitude 2.1 to 3.4 before climbing again. The entire sequence takes around 10 hours with a well-determined period between successive times of minimum brightness (minima) of 2.867 days (2 days, 20 hours, 48 minutes, 56 seconds). By making a series of random checks on Algol over several nights, you should eventually be able to detect its fluctuations from which point you can follow its changes in magnitude by comparing it to nearby stars.

The times of Algol’s variations can be predicted accurately, and forthcoming minima include those of January 10th 2015 (22h 42m GMT), January 13th 2015 (19h 36m GMT), January 31st 2015 (00h 30m GMT), February 2nd 2015 (21h 18m GMT), February 22nd 2015 (23h 00m GMT) and March 17th 2015 (21h 30m GMT). Start checking Algol’s brightness every half hour or so from around three hours before the predicted minimum and you should notice that the star gets slowly dimmer. You can detect the changes in magnitude by comparing it to nearby stars. Algol remains at its minimum brightness for about 20 minutes as the fainter star passes across the brighter one after which it gradually returns to normal.

A little to the north of Algenib is the magnitude 2.91 yellow giant star Gamma Persei, which shines from a distance of around 250 light years. Miram is the northernmost of the brighter stars in Perseus, this magnitude 3.77 orange giant being located at a distance of nearly 900 light years. Relatively close in comparison is magnitude 3.79 Misam, the light from this yellow giant star having set off on its journey towards us just 113 years ago. Located towards the southern end of the constellation, one of the most distant stars in Perseus is Menkib, the light from this magnitude 3.98 blue giant star has taken around 1,200 years to reach us.

Although Algenib and many of the other stars that form Perseus are single stars, most of the stars known to astronomers are members of what we call star clusters, of which there are many examples in the sky. A number of these are bright enough to be visible to the naked eye, including a pair of open star clusters which lie at the northern end of Perseus, near the border with the neighbouring constellation Cassiopeia. Their official designations are NGC 869 and NGC 884, although they are popularly and collectively known as the Sword Handle Double Cluster. They mark the position of the sword held by Perseus, and are just about visible to the naked eye under really clear, dark skies.

Attention was drawn to them as long ago as the 2nd century AD by the Greek astronomer Claudius Ptolemaeus (better known as Ptolemy) who referred to them as being: ‘At the tip of the right hand (of Perseus) and is misty [nebulosa]’ It is worth remembering that the night skies under which Ptolemy carried out his observations were significantly darker than those we generally experience today. Consequently, you may have difficulty picking them out if there is any form of light pollution around. However, a pair of binoculars will bring them out quite well, revealing the Sword Handle as a distinctive pair of glowing patches of light.

To find them, first of all identify Algenib and then follow the short line of stars from Algenib through Gamma and Miram. The Sword Handle lies just to the north west of Miram in the general direction of Cassiopeia. What you are seeing is the collective glow from the large number of stars that form these two clusters. Some of these stars may be visible in binoculars, and even a small telescope bring out the splendour of these two beautiful objects and reveal some of the individual stars within them.

Both NGC 869 and NGC 884 shine from a distance of around 7,500 light years which means that the light we are seeing actually predates the final stages of the construction of Stonehenge! The Sword Handle Double Cluster lies in a rich section of the Milky Way and time spent carefully sweeping this area with binoculars or a telescope at low magnification is well rewarded.

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