In astronomy, an asterism is a pattern of stars seen in Earth's sky which is not an official constellation. Like constellations, they are composed of stars which, while they are in the same general direction, are not physically related, often being at significantly different distances from Earth. An asterism may be composed of stars from one or more constellations. Their mostly simple shapes and few stars make these patterns easy to identify and thus particularly useful to those just learning to orient themselves when viewing the night sky.
The visible stars are strewn randomly about the sky. Even before the dawn of civilization, it became common to clump various stars together in connect-the-dots stick-figure patterns. The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius. Historically, without an "official" list, there was really no difference between a constellation and an asterism. Anyone could arrange and name a grouping which might or might not be generally accepted. Still, some of our own constellations go back at least as far as the Sumerians.
Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 85–c 165). His list of 48 constellations was accepted as the standard for 1800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use the left over, non-figure ("amorphic") stars to create and squeeze in a new grouping among the established constellations. Two astronomers particularly known for attempting to expand Ptolemy's catalogue were Johann Bayer (1572–1625) and Nicolas Louis de Lacaille (1713–1762). Bayer listed a dozen figures that had been suggested since Ptolemy's day; Lacaille created new groups, mostly for the area near the South Celestial Pole, unobserved by the ancients. Many of their proposed constellations have been accepted, the rest remaining asterisms, mostly obsolete. Clarification was necessary to determine which groupings are constellations and which stars belonged to them. The situation was finally regularized in 1930 when the International Astronomical Union (IAU) divided the sky into 88 official constellations with precise boundaries. Any other grouping is an asterism.
Points to bear in mind —
- The seasons indicated here are for the Northern Hemisphere. For the Southern Hemisphere, substitute the opposite season.
- The smaller the number of a star's magnitude, the brighter it is. Thus those of the 1st magnitude are brighter than those of the 2nd. Even negative magnitudes are possible, and the few so rated, though still called "first" magnitude, are the very brightest.
- A true star cluster (see below), whose stars are gravitationally related, is not an asterism.
Large seasonal asterisms
By happenstance, in each of the four seasons there is a large asterism overhead near midnight. Their component stars are bright and mark out simple geometric shapes.
- Spring is marked by the Diamond of Virgo consisting of Arcturus, Spica, Denebola, and Cor Caroli. An East-West line from Arcturus to Denebola forms an equilateral triangle with Cor Caroli to the North (Spring Triangle), and another with Spica to the South. Together these two triangles form the Diamond. Stretching from 38°N to 11°S, the Diamond is too large to be seen all at once. Formally, the stars of the Diamond are located in the constellations Boötes, Virgo, Leo, and Canes Venatici.
- The Summer Triangle of (Deneb, Altair, and Vega, or α Cygni, α Aquilae, and α Lyrae) is easily recognized as its three stars are all of the 1st magnitude. The stars of the Triangle are located in the band of the Milky Way which marks the galactic equator.
- The Great Square of Pegasus is the quadrilateral that forms the body of the winged horse. It may be glimpsed in its entirety on Autumn nights.
- Fully one-third of the 1st magnitude stars visible in the sky (seven of twenty-one) are in the Winter Circle with Sirius, Procyon, Pollux - toss in 2nd magnitude Castor - Capella, Aldebaran, and Rigel on the periphery, and Betelgeuse located off-center. Although somewhat flattened, and thus more elliptical than circular, the figure is so huge that it is impossible to take it all in at a single glance, thus making the lack of true circularity less noticeable. (The projection in the chart exaggerates the stretching.) Some prefer to consider the shape a Hexagon.
- Use the links for star-charts and photographs.
Undoubtedly, the best-known asterism is the Big Dipper or Plough. Composed of the seven brightest stars in Ursa Major where they delineate the Bear's hindquarters and exaggerated tail, the Dipper is often the first grouping of stars shown to beginners in the Northern Hemisphere. With its even more ridiculously lengthened tail, Ursa Minor hardly appears bearlike at all. It does much better under its pseudonym of the Little Dipper.
Ursa Minor is not the only constellation that does not look very much like what it represents. Very few do. This has led to nicknames for some of the constellations. These nicknames are another variety of asterism. A glance at the stick-figures shown in the charts under the constellation names will easily explain the origin of these asterisms.
- The best-known of this type is the Northern Cross in Cygnus. The upright runs from Deneb (α Cyg) in the Swan's tail to Albireo (β Cyg) in the beak. The transverse runs from Gienah (ε Cyg) in one wing to Delta Cygni (δ Cyg) in the other.
- The Great Hook is the traditional Polynesian name for Scorpius. The image will be even more obvious if the chart's lines from Antares (α Sco) to Graffias (β Sco) and Pi Scorpii (π Sco) are replaced with a line from Graffias through Dschubba (δ Sco) to Pi forming a large capped J.
- Adding vertical lines to connect the limbs at the left and right in the main diagram of Hercules will complete the figure of the Butterfly.
- Although hardly an ancient notion, it is easy to see why the Ice Cream Cone is sometimes applied to Boötes. It is even better-known as the Kite.
- The stars of Cassiopeia form a W which is often used as a nickname.
An asterism may also be a section of a constellation that refers to the traditional figuring of the whole. Thus, for example, there are:
There are many others.
Other asterisms are also composed of stars from one constellation, but do not refer to the traditional figures.
- The four central stars in Hercules, ε (Epsilon), ζ (Zeta), η (Eta), and π (Pi), form the well-known Keystone.
- The curve of stars at the front end of the Lion from Al Ashfar (ε Leo / Epsilon Leonis) to Regulus (α Leo / Alpha Leonis), looking much like a mirror-image question mark, have long been known as the Sickle.
- The bow and arrow of the Archer also make a well-formed Teapot. (There is even a bit of nebulosity near the "spout" to serve as steam.)
- Dubhe and Merak (Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of the Big Dipper are habitually called The Pointers: A line from β to α and continued for a bit over five times the distance between them, arrives at the North Celestial Pole and the star Polaris (α UMi / Alpha Ursa Minoris), the North Star.
- Alpha and Beta Centauri are the less-useful Southern Pointers as they lead to the Southern Cross which is easily spotted on its own. However using them does help distinguish Crux from the False Cross.
Like the Seasonal asterisms, there are others that are formed from stars in more than one constellation.
- There is another large asterism which, like the Diamond of Virgo, is composed of a pair of equilateral triangles. Sirius (α CMa), Procyon (α CMi), and Betelgeuse (α Ori) form one to the North (Winter Triangle) while Sirius, Phaet (ζ Pup), and Naos (α Col) form another to the South. Unlike the Diamond, however, these triangles meet, not base-to-base, but vertex-to-vertex, forming the Egyptian X. The name derives from both the shape and, because the stars straddle the Celestial Equator, it is more easily seen from south of the Mediterranean than in Europe.
- The Lozenge is a small diamond formed from three stars - Eltanin, Grumium, and Rastaban (Gamma, Xi, and Beta Draconis) - in the head of Draco and one - Iota Herculis - in the foot of Hercules.
- The False Cross is composed of the four stars Delta and Kappa Velorum (δ and κ Vel) and Epsilon and Iota Carinae (ε and ι Car). Although its component stars are not quite as bright as those of the Southern Cross, it is actually somewhat larger and better shaped than the official constellation.
Asterisms range from the large and obvious to the small, and even telescopic.
- Although the photograph displays the shape upside down, the Coathanger in Vulpecula is easy to discern.
- Kemble's Cascade in Camelopardalis appears like a multicolored toothpick in the night sky.
Argo is a special case. Argo Navis, ("the ship Argo"), was, by far, the largest of Ptolemy's constellations. Starting with Lacaille in his Coelum Australe Stelliferum (1763), it became common to refer to its various parts as the Keel, the Poop, and the Sails. In the 1930 IAU arrangement, Argo was deemed too large, and these old sectional asterisms were recognized as official constellations (Carina, Puppis, and Vela), thereby turning Argo, as a whole, into an asterism.
The Southern Cross is not an asterism, but merely a variation on the meaning of Crux. Crux was an asterism when Bayer created it in Uranometria (1603) from stars in the hind legs of Centaurus. It was given constellation status in 1930, thereby mutilating the Centaur.
In its original figuration, Leo included a spray of faint stars pictured as the tuft in the Lion's Tail which stretched straight out from its body. Antedating even Ptolemy by centuries, Conon of Alexandria created the asterism "Berenice's Hair" commemorating his queen in 243 BC. Following Tycho's acceptance of Coma, Bayer recorded it and refigured the Lion. The IAU confirmed Coma's status as a constellation.
Even so venerable a constellation as Libra was once merely an asterism. Until the middle of the first millennium BC, the Zodiac consisted of only eleven constellations. The biblical reference to "the eleven stars" (Genesis 37:9) is more accurately "the eleven asterisms/constellations (of the Zodiac)." At the time, Scorpius' claws were pictured as extending to Zubenelgenubi, "the southern claw" and Zubeneschamali, "the northern claw" (Alpha and Beta Librae). Later, when Virgo was reimagined as Astraea, the goddess of justice, the Claws became a set of scales held in her hand. By Ptolemy's day, Libra had become an independent constellation, unconnected with either of its neighbors. Still, the names of its stars reflect the time when it was the asterism of "The Claws" and its figuration is that of the old sectional asterism within Virgo.
Open clusters are groups of stars that are physically related — gravitationally bound together and moving through the galaxy in the same direction and speed. As these groupings are not human constructs, but real phenomena, they do not count as asterisms. Among the best-known and closest are the Pleiades (M45) and the Hyades in Taurus and the Beehive (M44) in Cancer.
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