Last week we looked at the summer triangle, more correctly what’s inside the summer triangle. If you recall, the summer triangle is made from stars of three constellations.
So, is the summer triangle a constellation? No. It is an asterism. The sky is full of asterisms and constellations. OK, humans define them; on their own they are simply stars minding their own business.
Humans love to categorize, to create recognizable patterns everywhere, in science, art, religion, medicine, everything. Patterns help us.
There are 88 constellations recognized by the International Astronomical Union (IAU). These 88 constellations cover the entire celestial sphere. Look somewhere in the sky and you are looking at one or more constellations. Every constellation has a boundary, defined by the IAU, and abuts one or more neighboring constellation(s). No room for more constellations.
Many constellations are made in part by asterisms, that is, a group of stars we recognize. The asterism usually has its own name, not associated with the constellation’s name. Most constellations make me scratch my head – how do they see a fox in Vulpecula? Oh, and the fox is carrying away a young goose named Anser. There is a recognizable group of stars in Vulpecula however, an asterism named The Coathanger, and it really looks like a Coathanger! So, Vulpecula is a constellation, and it contains the Coathanger asterism, also known as Collinder 399. At first the Coathanger was thought to be a star cluster, like the Pleiades, and named Brocchi’s Cluster, but its stars are not associated with each other, so it’s not a cluster.
Here are some constellations and asterisms to check out…
Ursa Major (the great bear) – you can find it by finding the Big Dipper. The Big Dipper is Ursa Major’s hind quarter and tail. It’s an asterism, comprising only about 25% of Ursa Major. The rest of Ursa Major is pointed down, toward the northwestern horizon.
Boötes (the Plowman) – find it by following the Big Dipper’s handle (Ursa Major’s tail) away from the cup and continue the curve. The curve lands on bright star Arcturus, the alpha (α) star in Boötes. Arcturus is the bottom star of a kite or ice cream cone shaped asterism. The kite or ice cream cone is pointed to the right.
Cygnus (the Swan) – find it by locating Deneb in the Summer Triangle. Deneb is the alpha (α) star and is the tail of the swan. Deneb is also the tip of an asterism named the Northern Cross. Follow an imaginary line slightly upper right from Deneb past a somewhat bright star, gamma (γ) Cygni, to another somewhat bright star Albireo, beta (β Cygni) the bottom of the cross, also forming the swan’s eye. The cross members of the Northern Cross go through gamma Cygni. Can you make them out? Light pollution makes it a little challenging.
While there is no room for more constellations, asterisms are added all the time.
What’s in the Sky?
07/27; 10:24 pm; starting north northwest: The ISS skims along the northern horizon before disappearing (enters Earth’s shadow) in the misshapen W (an asterism) of Cassiopeia, the Queen.