Brian Jones: The Swan
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The constellation of Cygnus, also known as the Northern Cross, can be seen straddling the Milky Way high in the south western sky during October evenings, its leading star Deneb lying a little to the east of the bright star Vega in Lyra (see my last blog) as shown below. Records show that ancient astronomers have likened this constellation to a flying eagle, a partridge and even a pigeon! The astronomers of Ancient Greece and Arabia identified the group as a hen, and Deneb in fact derives its name from the Arabic for ‘the Hen’s Tail’.
Depicting the tail of Cygnus, Deneb lies at the top of a large cruciform pattern of stars, and from here the rest of Cygnus can be picked out easily. Deneb is classed by astronomers as a supergiant star and this object is indeed a cosmic giant. With a true luminosity of around 60,000 times that of our own Sun and a diameter of over 50 million miles, Deneb would truly dwarf our parent star!
The light we are seeing from Deneb set off around 1,600 years ago which means we are seeing the star as it was at around the time the Romans left Britain. Somewhat closer to us is Albireo, a star whose name is derived from the Arabic for ‘the Hen’s Beak’ and which shines from a distance of over 400 light years. Albireo is a magnificent double star and even a small telescope will show its yellowish and bluish components. Sadir is the star marking the point of intersection of the body and wings of the swan and the area of sky between this point and Albireo is very rich in stars and well worth sweeping with binoculars.
Lying close to Deneb is the pair of stars Omicron1 and Omicron2 which form a nice wide double when seen through binoculars. Binoculars will also reveal another star lying very close to Omicron1 as shown here. This is the star 30 Cygni which has a bluish tint and contrasts with the orange hues of Omicron1 and Omicron2.
If you like a challenge, try and spot the open star cluster M39 which lies just to the west of a line joining the two stars Rho and Pi2. If you follow the line of stars from Deneb through Xi, 68, Rho and Pi2 as shown on the main chart (all are easily visible through binoculars) you will arrive at the area of sky near M39. Now, using Rho as a ‘guide star’, you should be able to locate the cluster M39 using the accompanying finder chart. Discovered by the French astronomer Charles Messier in 1764, M39 lies at a distance of around 900 light years and is a loose cluster of around 30 stars. A pair of binoculars will reveal a few of its member stars and a small telescope will show the cluster as a conspicuously V-shaped gathering. A dark, moonless sky, together with a little patience, will help you to track down this pretty star cluster.
Cygnus spans the Milky Way and indeed conjures up the image of a huge celestial bird flying down towards the southern horizon. An old Polish hymn talks of ‘a sleeping swan’s white plumage fringed with gold’ – the glow of the Milky Way surrounding the heavenly bird. On a clear, dark, moonless night the effect is striking. So, why not spend some time under the October night sky and see if you agree? Next month we’ll check out the constellation Aries the Ram. Until then, happy stargazing!
