A picture of stars, with a group of appearingly bright blue and white stars. The bright stars together are identified as the asterism
Coathanger resembling a
coathanger, in the constellation
An asterism is an
observed pattern or group of
stars in the
sky. Asterisms can be any identified pattern or group of stars, and therefore are a more general concept than the
formally defined 88constellations. Constellations are based on asterisms, but unlike asterisms, constellations outline and today completely divide the sky and all its celestial objects into regions around their central asterisms. For example, the asterism known as the
Big Dipper comprises the seven brightest stars in the constellation
Ursa Major. Another is the asterism of the
Southern Cross, within the constellation of
Asterisms range from simple shapes of just a few stars to more complex collections of many stars covering large portions of the sky. The stars themselves may be bright naked-eye objects or fainter, even telescopic, but they are generally all of a similar brightness to each other. The larger brighter asterisms are useful for people who are familiarizing themselves with the night sky.
The patterns of stars seen in asterisms are not necessarily a product of any physical association between the stars, but are rather the result of the particular perspectives of their observations. For example the
Summer Triangle is a purely observational physically unrelated group of stars, but the stars of
Orion's Belt are all members of the
Orion OB1 association and five of the seven stars of the Big Dipper are members of the
Ursa Major Moving Group. Physical associations, such as the
Pleiades, can be asterisms in their own right and part of other asterisms at the same time.
Background of asterisms and constellations
In many early civilizations, it was already common to associate groups of stars in
connect-the-dotsstick-figure patterns; some of the earliest records are those of ancient India in the
Vedanga Jyotisha and the
Babylonians. This process was essentially arbitrary, and different cultures have identified different constellations, although a few of the more obvious patterns tend to appear in the constellations of multiple cultures, such as those of
Scorpius. As anyone could arrange and name a grouping of stars there was no distinct difference between a constellation and an asterism. e.g.
Pliny the Elder (23–79 AD) in his book
Naturalis Historia refers and mentions 72 asterisms.
A general list containing 48 constellations likely began to develop with the astronomer
Hipparchus (c. 190 – c. 120 BC ), and was mostly accepted as standard in Europe 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 any leftover stars to create and squeeze in a new grouping among the established constellations.
Furthermore, exploration by Europeans to other parts of the globe exposed them to stars unknown to them. Two astronomers particularly known for greatly expanding the number of southern constellations were
Johann Bayer (1572–1625) and
Nicolas Louis de Lacaille (1713–1762). Bayer had listed twelve figures made out of stars that were too far south for Ptolemy to have seen; Lacaille created 14 new groups, mostly for the area surrounding
South Celestial Pole. Many of these proposed constellations have been formally accepted, but the rest have historically remained as asterisms.
In 1928, the
International Astronomical Union (IAU) precisely divided the sky into
88 official constellations following geometric boundaries encompassing all of the stars within them. Any additional new selected groupings of stars or
former constellations are often considered as asterisms. However, depending on the particular literature source, any technical distinctions between the terms 'constellation' and 'asterism' often remain somewhat ambiguous.
Asterisms consisting of first-magnitude stars
Some asterims consist completely of bright
first-magnitude stars, which mark out simple geometric shapes.
The Summer Triangle of
Vega – α
Aquilae, and α
Lyrae – is prominent in the northern hemisphere summer skies, as its three stars are all of the 1st magnitude. The stars of the Triangle are in the band of the
Milky Way which marks the galactic equator, and are in the direction of the galactic center.
The Great Diamond consisting of Arcturus, Spica,
Cor Caroli, the latter two not being first-magnitude stars. 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. Together these two triangles form the Diamond. Formally, the stars of the Diamond are in the constellations
Other asterisms consist partially of multiple first-magnitude stars.
The Big Dipper, also known as The Plough or Charles's Wain, is composed of the seven brightest stars in
Ursa Major. These stars 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.
The Southern Cross is an asterism by name, but the whole area is now recognised as the constellation
Crux. The main stars are
Delta, and arguably also
Epsilon Crucis. Earlier, Crux was deemed an asterism when Bayer created it in Uranometria (1603) from the stars in the hind legs of
Centaurus, decreasing the size of Centaur. These same stars were probably identified by
Pliny the Elder in his
Naturalis Historia as the asterism 'Thronos Caesaris.'
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
Beta Scorpii (β Sco) and
Pi Scorpii (π Sco) are replaced with a line from Beta through
Delta Scorpii (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.
Boötes is sometimes known as the Ice Cream Cone. It is also known as the Kite.
The stars of
Cassiopeia form a W which is often used as a nickname.
The Great Square of
Pegasus is the quadrilateral formed by the stars
Alpheratz, representing the body of the winged horse. The asterism was recognized as the constellation ASH.IKU "The Field" on the
MUL.APINcuneiform tablets from about 1100 to 700 BC. Alpheratz is now only considered a part of the constellation Andromeda whereas formerly the star was a part of both constellations.
The Bowl of Virgo is formed by the stars Beta, Gamma, Delta, Epsilon and Eta Virginis. Together with
Spica, they form a Y shape.
Three Leaps of the Gazelle asterism
The Three Leaps of the Gazelle consists of three pairs of stars in
Ursa Major aligned in a row spanning about 30 degrees. In Arabic lore, the star pairs are pictured as the hoof prints of a gazelle startled from a pond by
Leo the lion. (The "pond" is pictured as the
Coma Star Cluster.) The first pair of stars are Xi and Nu, second pair Upsilon and Lambda, third pair Kappa and Iota Ursa Majoris. The pairs also mark three of the bear's paws.
Some asterisms refer to portions of traditional constellation figures. These include:
The Water Jar or Urn of
Aquarius is a Y-shaped figure centered upon
Zeta Aquarii and includes Gamma, Eta and Pi. It pours water in a stream of more than 20 stars terminating with the star
The Crab Breast of
Cancer is a quadrilateral formed by the four stars Gamma, Delta, Eta and Theta Cancri which make up the carapace (inner shell) of the Crab. Contained within is the
Beehive Cluster (Messier 44) which includes Epsilon Cancri.
The Snake Head is the westernmost portion of
Hydra consisting of the stars Delta, Epsilon, Zeta, Eta, Rho and Sigma Hydrae.
The Bull's Face of
Taurus is a V-shaped figure formed by prominent members of the
Hyades cluster, including stars Gamma, Delta¹, Delta², Delta³, Epsilon, Theta Tauri, as well as the bright star Alpha Tauri (
Aldebaran) which forms the red eye of the Bull.
Merak (Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of the
Big Dipper are often called the Pointers: a line from β to α and continued for about five times the distance between them arrives at the North Celestial Pole and the star
Polaris (α UMi/Alpha Ursae Minoris), the North Star.
Rigil Kentaurus (α Centauri) and
Hadar (β Centauri) are the Southern Pointers leading to the Southern Cross and thus helping to distinguish
Crux from the False Cross.
Asterisms across multiple constellations
Other asterisms that are formed from stars in more than one constellation.
The Egyptian X is a 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. 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 diamond-shaped False Cross is composed of the four stars
Alsephina (δ Velorum),
Markeb (κ Velorum),
Avior (ε Carinae), and
Aspidiske (ι Carinae). 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 is sometimes mistaken, causing errors in
astronavigation. Like the Southern Cross, three of its main four stars are whitish and one orange.
From latitudes above
40 degrees north especially, a prominent upper-case Y is formed by
Arcturus (α Boötis),
Seginus (γ Boötis) and
Izar (ε Boötis), and
Alphecca (α Coronae Borealis). Alphecca is far brighter than either Delta or Beta Boötis, diminishing the "kite" or "ice-cream cone" shape of Boötes. 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 the other stars of Boötes and Corona Borealis are not.
The Serpent Bowl is a large curved asterism spanning 3.5 hours of right ascension, from mid-northern latitudes best seen in July and August evenings. From west to east, it includes the stars Delta, Alpha and Epsilon Serpentis, Delta, Epsilon, Upsilon, Zeta and Eta Ophiuchi, Xi Serpentis, Nu and Tau Ophiuchi, Eta and Theta Serpentis.
The Eagle Tail Corona is a flattened curved figure in the tail of
Aquila and extending into
Scutum. It consists of the stars 14, 15, Lambda and 12 Aquilae, Eta Scuti, HD 174208, R and Beta Scuti. The compact open cluster
Messier 11 is also aligned with the curve.
The "37" or "LE" of
NGC 2169, in
Orion. It is visible with binoculars.
Asterisms range from the large and obvious to the small, and even telescopic.