The mild nights of late summer are great for stargazing, and one of the most popular constellations, Cygnus, the Swan, is almost overhead. Find it by looking high in the south for a big triangle of stars, called the Summer Triangle. The top of the triangle is marked by the very bright star Vega, and the other two are Deneb in Cygnus, to the left of Vega, and Altair, in Aquila, lower down.
Deneb is the main star in Cygnus, and you can see the main pattern from the map below (based on the free Stellarium software). The wonderful thing about Cygnus is that it looks very much like its name (a flying swan) and has the Milky Way running right through the centre of the cross-shaped pattern also known as the Northern Cross.
In Greek mythology, Cygnus represents the swan into which the god Zeus transformed himself for one of his romantic conquests and that the eggs which were laid as a result of the union hatched into the Gemini Twins, Castor and Pollux.
Deneb, marking the tail of the Swan, is one of the brightest stars in our part of the Galaxy. Although dimmer (at magnitude 1.3) than much closer Vega (which is 26 light years away and has an apparent magnitude of 0.0) it is, in fact about 2,600 light years away and it an absolutely giant super-hot blue-white star. It is the most distant of the first magnitude stars.
Scan the area surrounding Deneb with a pair of modest binoculars and, even in summer’s light skies, you’ll see hundreds of stars because you are looking straight through the plane of our own galaxy, the Milky Way. If you have dark enough skies you’ll also notice that the Milky Way appears to split into two lanes. In fact this is an illusion caused by a large dark cloud of dust and gas which blocks out some of the stars. You can trace this lane of the Milky Way all the way down to and beyond the third member of the Summer Triangle – Altair.
Now turn your attention from the tail of the swan to its shiny eye – the opposite end to the star Deneb. Here is probably one of the most beautiful objects in the night sky, and an easy target for the casual stargazer – the star Albireo or Beta Cygni. (The stars in a constellation have Greek letters, usually starting with the brightest as Alpha, then Beta, then… you get the picture.)
To the naked eye, Albireo seems to be a single yellow star of magnitude 3.1 which is actually dimmer than Gamma Cygni. But if you can look at it through a modest telescope it is revealed as a double star with a blue–green companion at magnitude 5.1. Separated by only 35 seconds of arc, the two companions provide one of the best, if not the best, contrasting double stars in the sky for small instruments – once described as a diamond and sapphire together. Actually, you can see them as two stars even with some binoculars, if you hold them steady enough, but a telescope is better.
For a long time they were thought of as being a genuine binary system (two stars revolving around each other) but more recent observations of their “proper motions” – their movement against more distant stars – have revealed that they are, in fact, more likely an optical double – that is they just happen to appear close to each other by virtue of line of sight with our Solar System.
It is calculated that the primary yellow companion is 434 light years away and the smaller dimmer blue companion is in fact closer at only 401 light years distant. Despite this, they are still a remarkable pairing and a true delight to show to people who are stargazing for the first time. A real must.
Check back soon for more about Cygnus.