Astro Bob: How to use asterisms to help you find the constellations
"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at astrobob.areavoices.com.
I often mention asterisms here. An asterism is a bright pattern of stars shaped like something familiar that’s not an official constellation. The most famous is Orion’s Belt with its three bright stars in a line. It’s as easily recognizable as the Big Dipper, another asterism. These bright, familiar patterns serve as easy entry points into the constellations. While you may not be able to find Ursa Major the Great Bear off the bat, you can easily find the Big Dipper and use it as your base of operations. By connecting additional fainter stars adjacent to the dipper you’ll soon make out the constellation in its entirety. When you do, you might be surprised at just how bear-like Ursa Major looks despite its unusually long tail.
Sometimes asterisms are buried within asterisms. For instance, the two stars in the bottom of the Dipper’s bucket are called the Pointers because they point to Polaris, the North Star. Very handy. The North Star sits at the tail end of the Little Dipper, yet another asterism. Find it and connect the dots into a smaller dipper. The W of Cassiopeia, located very low in the northern sky in June, is likewise an asterism, the brightest part of a constellation that depicts a queen sitting on her throne.
A small quadrangle of stars representing the head of Draco the dragon is shaped like a rhombus and nicknamed the Lozenge. To find it, extend a balled fist to the sky and look a little more than 3 fists to the right of the top of the Big Dipper’s handle or use the bright star Vega to find it. Once there, complete the dragon’s outline by linking the stars trailing the dragon’s head into a long, sinuous tail. To me it looks like a garden hose. Draco is a relatively faint constellation but the Lozenge will start you off on the right foot.
Let’s return to the Big Dipper again. In back of the Dipper off to the left (west) are three pairs of fainter 3rd and 4th magnitude stars that make one of my very favorite asterisms called the Three Leaps of the Gazelle . An ancient Arabic star group, the three leaps represent three sets of hoof prints. The myth relates that a gazelle was startled by a lion and leapt from “The Pond,” the neighboring constellation of Coma Berenices, leaving its impressions in the mud.
You can also use Vega, the brilliant star halfway up in the eastern sky at nightfall this time of month to locate the Lozenge and the Keystone of Hercules. The Lozenge lies a fist and a half to the upper left (west) of Vega while the Keystone asterism shines two fists to the upper right. The Keystone, another rhombus-shaped figure like the Lozenge, forms the heart of the faint constellation Hercules the strongman and provides the ideal entry point to get to know him better.
Sometimes asterisms are composed of stars from more than one constellation joined into eye-catching geometric figures. A great example is the Spring Triangle. Look high in the southern sky at nightfall and locate the brilliant, orange star Arcturus. If you connect to Spica (lower in the south) and Denebola, off to the west, you’ll see this immense triangle. Each of these three stars in turn serves as a guide to finding their respective constellations — Boötes the herdsman, Virgo the virgin and Leo and lion.
Speaking of Boötes (pronounced boh-OH-teez), three stars in that constellation connect with the brightest star, Gemma, in neighboring Corona Borealis, the northern crown, to make an obvious letter-shaped asterism simply called the Y. Gemma (also called Alphecca) will help you trace the crown, while you’ll soon discover that Arcturus sits at the pointy end of an ice cream cone-shaped constellation.
Off in the eastern sky you can’t miss the splendid Summer Triangle outlined by Vega, Deneb and Altair, the three brightest luminaries in their respective constellations, Lyra the harp, Cygnus the swan (also called the Northern Cross, itself an asterism!) and Aquila the eagle. Vega leads the Summer Triangle and is also its brightest star. The brightest part of the northern Milky Way is neatly framed by the figure, so if you can find the figure you’ll also know exactly where to look for the Milky Way. Be sure to point your binoculars at the Coathanger, a telescopic asterism. Although faintly visible without optical aid it’s amazing to see in binoculars or a small telescope and looks just like its name.
These are just a few of a 2-3 dozen easy, naked-eye asterisms waiting for you in the night sky. Let them be your introduction to the constellations. They are keys that open doors to a better understanding of the night sky’s geography.