Asterisms for Autumn Nights

This week’s moon, and the mild temperatures of early autumn, are fine for viewing the patterns formed by the brightest stars in the night sky.

An asterism is a set of usually prominent stars that form an obvious simple shape in the sky. In many instances the asterism borrows stars from adjacent constellations, or consists of the major parts of a larger constellation. The most famous of these is The Big Dipper, which consists of seven bright stars inside the larger constellation of Ursa Major (the Big Bear). The Big Dipper can be found low over the northern horizon in October.

A conveniently placed new autumn asterism is in Pegasus (the Flying Horse). Face east after dark and look for a gigantic square of modest stars tilted with one corner pointing down. All four stars are roughly the same brightness, about magnitude 2.5. At 8 pm local time, it is sitting about one-third of the way up the eastern sky, and the square measures two fist widths corner to corner. This asterism is known as the Great Square of Pegasus.

Clockwise from the top, the four corners of the square are the stars Scheat, Markab, Algenib, and Alpheratz. Scheat is a cool red giant star about 200 light years distant, and is about one hundred times the diameter of our Sun. Visually, its red colour is readily detectable. Markab is a hot, blue-white star 140 light-years away that emits intense ultraviolet radiation. This rapidly spinning star is nearing the end of its time as a blue star and will soon evolve into an orange giant. Algenib is an extremely hot blue star with a luminosity of about 4,000 times that of our Sun! At 335 light-years distant, it is the farthest of the four. Finally, Alpheratz, which is actually part of neighbouring Andromeda, is a hot blue sub-giant star located at a distance of only 97 light-years.

Enjoy your autumn stargazing with Star Walk 2!