Asterisms and constellations, what’ the difference between them, other than that one is from Greek (astêr) and the other from Latin (stella)?
Both are perceived groupings of stars. The stars may be at greatly different distances, in which case they are not physical groups in space, like star clusters.
By “constellation” we now mean one of the recognized areas into which we divide the sky: the 88 constellations defined by the International Astronomical Union. Or the constellations recognized by ancient or ethnic cultures, such as Egyptian, or Chinese, or the Navajo constellations I described in the cover picture story for Astronomical Calendar 2006.
By “asterism” we mean any perceived grouping of stars on the celestial sphere, whether it is within a constellation or across constellations. Here is my entry in Albedo to Zodiac:
.”.. any apparent grouping of stars, including groupings that are not official constellations. Examples are the Big Dipper (officially only a part of the constellation Ursa Major); the Teapot (a way of seeing the prominent stars of the constellation Sagittarius), also the Teaspoon that has been added above it; parts of constellations, such as the three stars forming the Belt of Orion; and groupings larger than constellations, such as the Summer Triangle, consisting of the three bright stars in Lyra, Cygnus, and Aquila. For other examples see Astronomical Companion, CONSTELLATIONS.”
And you can make up your own asterisms. They can help you in star-hopping to something you want to find with your telescope.
I think the classic asterism is the Coathanger, a group only just over a degree wide in Vulpecula (the “fox”), one of the small constellations in the Milky Way between Cygnus amd Aquila.
For one thing, it has many designations, because it caught the attention of generations of observers and compilers of sky surveys: Al Sufi’s Cluster, Brocchi’s Cluster, Collinder 399, Cr 399. “Coathanger” is not a catalogued designation but ranks only as a nickname, and yet I think this is what most astrophiles call it. It consists of 10 stars of magnitudes between 5 (about the naked-eye limit) and 7. The stars are at distances ranging from 235 to 1735 light years, so they are not gravitationally bound to each other.
Six of then lie in an east-west line, the bar of the coathanger, and they others make the hook.
Coathangers, or coat hangers (both punctuations seem to be allowed, though the one-word version looks phonetically ambiguous), are, I think, as abundantly used for hanging shirts as coats. They come in two forms: with an without the horizontal bar, which serves to stiffen and to enable hanging of trousers. The hooks get annoyingly tangled if you have too many clothes in too narrow a closet, one of my motives for trying to cull clothes. A coathanger can be a single piece of shaped wire, or partly or all have wood or plastic. The famous other use of the wire is to straighten it out and reach through narrow spaces for lost objects.
The device is an artificial pair of shoulders. Or a skeletal pair of shoulders. An entrepreneur could manufacture a novelty coathanger of two collarbones.
Can you tell us of personal asterisms you have imagined while looking, telescopically or otherwise, at the starry canopy?
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This is not an original asterism. Tony Flanders wrote about it in Sky and Telescope some years ago, I don’t know if he dreamed it up or got it from somebody else. The great square of Pegasus, the three bright stars of Andromeda, and Algol in Perseus make a really big dipper, very similar in shape to the more famous but smaller big dipper in Ursa Major.
That’s funny, I see the stars around Sadr as a heart-shaped outline, with the bright little open cluster NGC 6910 in the fold at the top of the heart.
The Terebellum asterism in eastern Sagittarius is one of my favorites. I read that the IAU has officially designated one of the four stars in that group as Terebellum, so the name no longer applies to the asterism. Another unusual name for this group was brought to my attention when I noticed that Robert Victor sometimes denoted the area on his Sky Calendar as the “Territory of Dogs”. I believe that refers to an ancient Chinese description of the four stars.
I have a couple of asterisms of my own.
The first is the Double Triangle, made up of Rasalhague, Altair, and Vega for the larger triangle, and Beta, Zeta, and Delta Herculis for the smaller one. The other asterism is pretty small, the Flat Iron, located between Alcor/Mizar and Alioth.
Nice! After a while we may be able to compile a map of Personal Asterisms.
I’m pleased to see you highlight the Coat Hanger asterism, it’s one of my favorite little stops when cruising through the Summer Triangle with binoculars. For me, a personal asterism that I don’t recall reading about anywhere is the loosely square-ish set of stars around Sadr (Gamma Cygni), the centerpiece of the Northern Cross, or Cygnus the Swan. I perceive these as the border of a “box of stars” centering on Sadr. It’s not very obvious (or else it would be written about elsewhere), but I recall noticing it back in the early 90s when I first started stargazing in earnest, and it’s been a favorite asterism ever since.
The Coathanger is one of the great objects for amateurs to “discover” using binoculars while looking for other things You stumble across it and go “whoah, what’s _that_!” A little research leads to the Collinder catalog. One of the little things which make amateur astronomy so rewarding.
I like Thuban’s Dumbell – it gives an easy way to find Thuban by looking for it in between the nearly matching pairs of the guard stars Kochab and Pherkad and the stars of the big dipper Alioth and Mizar.
From a southern hemisphere point of view, I’d say that the most recognisable asterism is the Saucepan (or Pot), which comprises the belt and sword of Orion.