Colloquial usage does not draw a sharp distinction between "constellation" in the sense of an asterism (pattern of stars) and "constellation" in the sense of an area of the sky surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept. For example, the asterism known as the Big Dipper comprises the seven brightest stars in the IAU constellation that is called Ursa Major.
Like constellations, asterisms are in most cases composed of stars which, although visible in the same general area, are often located at very different distances from Earth.
Simple shapes composed of a few stars make asterisms easy to identify. Thus they are particularly useful to people who are familiarizing themselves with the night sky.
Even before[when?] 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 Babylonians.
Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 90–c 168). His list of 48 constellations was accepted as the standard for 1,800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use the leftover, 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 catalog 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.
- 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
Component stars of seasonal asterisms are bright and mark out simple geometric shapes.
- Spring. Spring is marked by the Great Diamond 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, and another with Spica to the South. The Arcturus, Regulus, Spica triangle is given the name Spring Triangle. Together these two triangles form the Diamond. Formally, the stars of the Diamond are located in the constellations Boötes, Virgo, Leo, and Canes Venatici.
- Summer. The Summer Triangle of Deneb, Altair, and Vega — 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, and are in the direction of the galactic center.
- Autumn. The Great Square of Pegasus is the quadrilateral formed by the stars α Pegasi, β Pegasi, γ Pegasi, and α Andromedae, representing the body of the winged horse. It may be glimpsed in its entirety on autumn nights. The asterism was recognized as the constellation ASH.IKU "The Field" on the MUL.APIN cuneiform tablets from about 1100 to 700 BC.
- Winter. The winter midnight sky of the northern hemisphere is dominated by Orion, in the direction opposite the galactic center. Also, one-third of the 1st-magnitude stars visible in the sky (seven of twenty-one) are in the Winter Hexagon with Sirius, Procyon, Pollux, with 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 large that it cannot be taken in all at once, thus making the lack of true circularity less noticeable. (The projection in the chart exaggerates the stretching.) Some prefer to regard it as a Heavenly 'G'. The Procyon, Betelgeuse and Sirius triangle is given the name Winter Triangle.
One of the best-known asterisms is the Big Dipper or Plough. It is composed of the seven brightest stars in Ursa Major, where they delineate the Bear's hindquarters and exaggerated tail, or alternatively, the "handle" forming the upper outline of the bear's head and neck. With its longer tail, Ursa Minor hardly appears bearlike at all, and is widely known by its pseudonym, 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 Fish Hook is the traditional Hawaiian 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.
- In Australia, "Frying Pan" for Chamaeleon, is an aid to finding south by the stars.
An asterism may also be a section of a constellation that refers to the traditional figuring of the whole. Thus, for example, there are:
- Orion's Belt/The Belt Of Orion
- The Y in Aquarius (historically called "the Urn")
- Hercules's Club
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, Epsilon (ε Her), Zeta (ζ Her), Eta (η Her), and Pi (π Her), 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, has 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).
- Alpha, Beta, Gamma, and Delta Delphini form Job's Coffin.
- The Terebellum is a small quadrilateral of four faint stars (Omega, 59, 60, 62) in Sagittarius' hindquarters.
- Four other stars (Beta — Miaplacidus, Upsilon, Theta, and Omega Carinae) form a well-shaped diamond — the Diamond Cross.
- The Saucepan or Pot can be:-
- The same stars as the Belt and Sword of Orion. The end of the handle is at ι Ori, with the far rim at η Ori.
- In Australia, part of Pavo.
- Just south of Pegasus, the western fish of Pisces is home to the Circlet formed from Gamma, Kappa, Lambda, TX, Iota, and Theta Piscium.
- 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 Ursae Minoris), the North Star.
- Alpha and Beta Centauri are the Southern Pointers leading to the Southern Cross and thus helping to 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, Naos (ζ Pup), and Phakt (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 somewhat larger and better shaped than the Southern Cross, for which it was often mistaken by ships' navigators.
- From latitudes above 40 degrees north especially, a prominent upper-case Y is formed by Arcturus, Gamma and Epsilon Bootis, and Alpha Coronae Borealis (Alphecca or Gemma). Alpha Coronae Borealis is far brighter than either Delta or Beta Bootis, distorting the "kite" or "ice-cream cone" shape of Bootes. From the United Kingdom in particular, where there is serious light pollution in many areas and also twilight all night for much of the time these constellations appear, this "Y" is often visible while β and δ Bootis and the other stars in Corona Borealis are not.
Asterisms range from the large and obvious to the small, and even telescopic.
- A renowned object, the Coathanger, formally open cluster Collinder 399, in Vulpecula may be recognized by the eye.
- Kemble's Cascade in Camelopardalis is a chain of stars (the waterfall) that ends in open cluster NGC 1502.
- Approximately 40 stars in Monoceros, including a portion of NGC 2264, fill out the triangular Christmas Tree Cluster.
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 Deck, 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.
Centaurus had been reduced in size once before. Lupus was originally considered to be merely a sectional asterism (as an unspecified Wild Beast in the Centaur's grasp). Hipparchus split it off in the 3rd century BC, and Ptolemy's list confirmed its independent status.
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.
The groups named here were sectional asterisms that have been promoted to constellation status. For a list of proposed constellations that were not accepted as anything beyond asterisms, and are now considered obsolete, see Former constellations.
In the formal sense used here, asterisms are groups of stars that have not been categorized as something else.[lower-alpha 1] Objects which do not fall within the bounds of this definition include the Milky Way, nebulae, and open clusters.
Dividing the night sky into two roughly equal hemispheres, the Milky Way appears as a hazy band of white light arching across the entire celestial sphere. Many cultures have myths about "the broad white road in heaven." That the glow originates from innumerable faint stars and other materials which lie within the galactic plane was one of Galileo's early telescopic discoveries. Similarly, the Magellanic Clouds are not asterisms, but galaxies in their own right.
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. (It may be noted that, with the addition of Aldebaran, which is in the same line of sight, the Hyades open cluster forms a V-shaped sectional asterism in Taurus).
The fine point of what constitutes an asterism may be seen in two examples. Theta Orionis (θ Ori) is embedded in, and illuminates, the Orion Nebula (M42). Looked at telescopically, it resolved into four stars arranged in a trapezoid, and they were nicknamed the Trapezium. The asterism retained this name even when it was discovered that there were yet more stars in the group. However, it has since been determined that the Orion Nebula is a stellar nursery and that the Trapezium is actually an Open Cluster. Thus it is no longer an asterism. On the other hand, M73 in Aquarius, which was thought to be an Open Cluster, turns out to be composed of unrelated stars, and may now be considered to be an asterism.
- While in common parlance, any fixed celestial feature may be called an asterism, it is not strictly proper to refer to a constellation, cluster, or galaxy as such, and thus reduce it to a mere asterism. The "aliases" are asterisms because they are simplified figures, leaving out the dimmer stars in the official constellations.
- AstronomyOnline.org — the Night Sky: Asterisms
- AstronomyOnline: Image of Big Dipper, Diamond of Virgo, The Sail, Sickle, and Asses and the Manger
- Spring triangle at Space.com . Accessed March 2011
- Warren Rupp Observatory: Table of Asterisms
- AstronomyOnline: Image of Cassiopeia, Square of Pegasus, The Circlet, and Y of Aquarius
- Origins of the ancient constellations: I. The Mesopotamian traditions, by J.H. Rogers 1998, page 19
- Asterisms at SEDS
- Little Dipper, Keystone, Kite, and the Lozenge in AstronomyOnline.org: Asterisms
- Space.com: Hercules: See the Celestial Strongman
- History of the Constellations: Bootes
- The W, Square of Pegasus, Circlet, and Y of Aquarius in AstronomyOnline.org: Asterisms
- Teapot, Fish Hook, and Bull of Poniatowski in AstronomyOnline.org: Asterisms
- Sagittarius in Star Names: Their Lore and Meaning
- Starry Night Skies Photography: "Southern Cross, False Cross & Diamond Cross"
- DavidDarling.info: UMa
- DavidDarling.info: Cen
- David Ratlegde's Virtual Home: Observing Asterisms
- A star hop through Monoceros
- Henry Wadsworth Longfellow, The Song of Hiawatha, Part III: "Hiawatha's Childhood"
- Allen, Richard Hinckley (1969). Star Names: Their Lore and Meaning. Dover Publications Inc. (Reprint of 1899 original). ISBN 0-486-21079-0.
- Burnham, Robert (1978). Burnham's Celestial Handbook (3 vols). Dover Publications Inc. ISBN 0-486-23567-X, ISBN 0-486-23568-8, ISBN 0-486-23673-0.
- Michanowsky, George (1979). The Once and Future Star. Barnes and Noble Books. ISBN 0-06-464027-2.
- Pasachoff, Jay M. (2000). A Field Guide to the Stars and Planets (4th ed.). Houghton Mifflin Co. ISBN 0-395-93431-1