The Square of Pegasus is not the most striking asterism in the sky. With corner stars of the second magnitude or fainter, and sides around 15° long, it can easily be overlooked. It is also so empty that one test of sky quality is to count the number of stars visible to the naked eye within it – if we can see four then our sky is average while even under superb conditions we might struggle to reach a dozen.
It is recognisable as a square, however, and is a useful starting point if we wish to glimpse the most distant object most of us can see with the unaided eye, the Andromeda Galaxy. At nightfall at present, the Square is tipped to its left with its centre some 30° high in the E, much as it appears in our chart which shows a window of sky roughly 50° wide. By about 23:15 BST, it is placed squarely around 60° high on Britain’s meridian.
The star at the top-left corner of the Square, Alpheratz, once enjoyed dual membership of both Pegasus and Andromeda. In 1930, though, the constellation boundaries were formalised and Alpheratz assigned solely to the latter. While the brightest star in Andromeda, it only just outshines Mirach and Almach which stand almost in line to its left.
Take the line from Alpheratz to Mirach, passing Delta en route, and then turn sharp right towards the fainter Mu and then Nu. The Andromeda Galaxy, also known as Messier 31 or M31, lies a little to the W of Nu, appearing as an oval smudge to the naked eye under reasonable skies. Binoculars show it easily while telescopes and photographs have it extending over 3°.
Its spiral structure appears similar to that of our own Milky Way, but it may contain more than twice as many stars - perhaps as many as a trillion. We see it tipped at an angle of 77° from a distance of 2.5 million light years but it is approaching our galaxy at 110 km per second as we head for a collision in some 4 billion years. This despite the overall expansion of the Universe; in fact, M31, the Milky Way and more than 50 other smaller galaxies belong to the Local Group of galaxies which is bound together by gravity.
There are several small satellite galaxies close to M31 while the third largest member of the Local Group, and the only other spiral, stands 15° to its S in the neighbouring constellation of Triangulum. M33, also called the Pinwheel Galaxy from its beautiful appearance in photographs, lies about 3 million light years away and is very much smaller than the Milky Way with fewer than a fifth of its stars. Under exceptional skies, it is just visible to the unaided eye.
