Asterism (astronomy)
Encyclopedia
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, an asterism is a pattern of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s recognized on Earth's night sky. It may form part of an official constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth. The mostly simple shapes and few stars make these patterns easy to identify, and thus particularly useful to those learning to familiarize themselves with the night sky
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
.
Background
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., OrionOrion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
and Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
. 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
Babylonian star catalogues
Babylonian astronomy collated earlier observations and divinations into sets of Babylonian star catalogues, during and after the Kassite rule over Babylonia. These star catalogues, written in cuneiform script, contained lists of constellations, individual stars, and planets...
.
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 catalogue were Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
(1572–1625) and Nicolas Louis de Lacaille
Nicolas Louis de Lacaille
Abbé Nicolas Louis de Lacaille was a French astronomer.He is noted for his catalogue of nearly 10,000 southern stars, including 42 nebulous objects. This catalogue, called Coelum Australe Stelliferum, was published posthumously in 1763. It introduced 14 new constellations which have since become...
(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
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
(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 magnitudeApparent magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
, 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 clusterStar clusterStar clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
(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 visible near midnight, when observing from the northern hemisphere. Their component stars are bright and mark out simple geometric shapes.- Spring. Spring is marked by the Diamond of Virgo consisting of Arcturus, SpicaSpicaSpica is the brightest star in the constellation Virgo, and the 15th brightest star in the nighttime sky. It is 260 light years distant from Earth...
, DenebolaDenebolaDenebola is the second brightest star in the constellation Leo. It is an A-class star that is about distant from earth, and has a luminosity about twelve times that of the sun. Its apparent magnitude is 2.14...
, and Cor CaroliCor CaroliCor Caroli is the brightest star in the northern constellation Canes Venatici...
. 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 , Denebola , Spica triangle is given the name Spring TriangleSpring TriangleThe Spring Triangle is an astronomical asterism involving an imaginary triangle drawn upon the celestial sphere, with its defining vertices at Arcturus, Spica, and alternately Denebola or Regulus. This triangle connects the constellations of Boötes, Virgo, and Leo...
. Together these two triangles form the Diamond. Formally, the stars of the Diamond are located in the constellations BoötesBoötesBoötes is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere. The name comes from the Greek Βοώτης, Boōtēs, meaning herdsman or plowman...
, VirgoVirgo (constellation)Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...
, LeoLeo (constellation)Leo is one of the constellations of the zodiac. Its name is Latin for lion. Its symbol is . Leo lies between dim Cancer to the west and Virgo to the east.-Stars:...
, and Canes VenaticiCanes VenaticiCanes Venatici is one of the 88 official modern constellations. It is a small northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for "hunting dogs", and the constellation is often depicted in illustrations as representing the dogs of Boötes the...
.
- Summer. The Summer TriangleSummer TriangleThe Summer Triangle is an astronomical asterism involving an imaginary triangle drawn on the northern hemisphere's celestial sphere, with its defining vertices at Altair, Deneb, and Vega, being the brightest stars in the three constellations of Aquila, Cygnus, and Lyra.The English term was...
of DenebDenebDeneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...
, Altair, and VegaVegaVega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
— α CygniCygnus (constellation)Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
, α AquilaAquila (constellation)Aquila is a stellar constellation. Its name is Latin for 'eagle' and it is commonly represented as such. In mythology, Aquila was owned by the Roman god Jupiter and performed many tasks for him....
e, and α LyraLyraLyra is a small constellation. It is one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations recognized by the International Astronomical Union. Its principal star, Vega — a corner of the Summer Triangle — is one of the brightest...
e — 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 WayMilky WayThe Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
which marks the galactic equator, and are in the direction of the galactic center.
- Fall. The Great Square of PegasusPegasus (constellation)Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
is the quadrilateral formed by the stars α PegasiAlpha PegasiAlpha Pegasi is the third brightest star in the constellation Pegasus and one of the four stars in the asterism known as the Great Square of Pegasus. It has the traditional name Markab .Markab is a relatively average star nearing the end of its stellar evolution on the main sequence...
, β PegasiBeta PegasiBeta Pegasi is a star in the constellation Pegasus. Its traditional name is Scheat, a name that has also been used for Delta Aquarii....
, γ PegasiGamma PegasiGamma Pegasi is a star in the constellation of Pegasus. It also has the traditional name Algenib; confusingly however, this name is also used for Alpha Persei....
, and α AndromedaeAlpha AndromedaeAlpha Andromedae , which has the traditional names Alpheratz and Sirrah , is the brightest star in the constellation of Andromeda. Located immediately northeast of the constellation of Pegasus, it is the northeastern star of the Great Square of Pegasus...
, 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.APINMUL.APINMUL.APIN is the conventional title given to a Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and astrology....
cuneiformCuneiformCuneiform can refer to:*Cuneiform script, an ancient writing system originating in Mesopotamia in the 4th millennium BC*Cuneiform , three bones in the human foot*Cuneiform Records, a music record label...
tablets from about 1100 to 700 BC.
- Winter. The winter midnight sky of the northern hemisphere is dominated by OrionOrion (constellation)Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
, in the direction opposite the galactic center. Also, fully one-third of the 1st-magnitude stars visible in the sky (seven of twenty-one) are in the Winter HexagonWinter HexagonThe Winter Hexagon or Winter Circle/Oval is an asterism appearing to be in the form of a hexagon with vertices at Rigel, Aldebaran, Capella, Pollux/Castor, Procyon, and Sirius. It is mostly upon the Northern Hemisphere's celestial sphere...
with SiriusSiriusSirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, ProcyonProcyonProcyon is the brightest star in the constellation Canis Minor. To the naked eye, it appears to be a single star, the seventh brightest in the night sky with a visual apparent magnitude of 0.34...
, PolluxPollux (star)Pollux is an orange giant star approximately 34 light-years from the Earth in the constellation of Gemini . Pollux is the brightest star in the constellation, brighter than Castor...
- toss in 2nd-magnitude CastorCastor (star)Castor is the second brightest star in the constellation Gemini and one of the brightest stars in the night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum...
- CapellaCapella (star)Capella is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in...
, AldebaranAldebaranAldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
, and RigelRigelRigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...
on the periphery, and BetelgeuseBetelgeuseBetelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...
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 regard it as a Heavenly 'G'.
Other examples
One of the best-known asterisms is the Big DipperBig Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...
or Plough. It is composed of the seven brightest stars in Ursa Major
Ursa Major
Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...
, where they delineate the Bear's hindquarters and exaggerated tail. With its longer tail, Ursa Minor
Ursa Minor
Ursa Minor , also known as the Little Bear, is a constellation in the northern sky. Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, whence the name Little Dipper...
hardly appears bearlike at all, and is widely known by its pseudonym, the Little Dipper.
Constellation aliases
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 CygnusCygnus (constellation)Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
. The upright runs from DenebDenebDeneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...
(α Cyg) in the Swan's tail to AlbireoAlbireoAlbireo is the fifth brightest star in the constellation Cygnus. Although it has the Bayer designation beta, it is fainter than Gamma Cygni, Delta Cygni, and Epsilon Cygni. Albireo appears to the naked eye to be a single star of magnitude 3 but through a telescope, even low magnification views...
(β Cyg) in the beak. The transverse runs from GienahGienahGienah can be a star:* Gienah in Corvus, or γ Corvi, is a star in the Corvus constellation* Gienah in Cygnus, or ε Cygni, is a star in the Cygnus constellationGienah derives from the Arabic, Al-Janah, meaning "The wing"....
(ε Cyg) in one wing to Delta CygniDelta CygniDelta Cygni is the third-magnitude star in the constellation Cygnus. It has the traditional name Rukh. It will be the "North Star" for at least four centuries around 11,250....
(δ Cyg) in the other. - The Fish Hook is the traditional Hawaiian name for ScorpiusScorpiusScorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
. The image will be even more obvious if the chart's lines from AntaresAntaresAntares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...
(α Sco) to GraffiasBeta ScorpiiBeta Scorpii is a star system in the constellation Scorpius. It has the traditional names Acrab, Akrab or Elacrab, all come from al-'Aqrab, the Scorpion, for the whole constellation, as well as Graffias, a name it shares with Xi Scorpii...
(β Sco) and Pi ScorpiiPi ScorpiiPi Scorpii is a triple star system in the constellation Scorpius. It is approximately 459 light years from Earth.Pi Scorpii A is a contact binary of the Beta Lyrae type. Both its members are hot, blue-white B-type dwarfs. Together they have a mean apparent magnitude of +2.89. Their rotation...
(π Sco) are replaced with a line from Graffias through DschubbaDelta ScorpiiDelta Scorpii is a star in the constellation Scorpius. It has the traditional name Dschubba Delta Scorpii (δ Sco, δ Scorpii) is a star in the constellation Scorpius. It has the traditional name Dschubba Delta Scorpii (δ Sco, δ Scorpii) is a star in the constellation Scorpius. It has the...
(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 HerculesHercules (constellation)Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
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 BoötesBoötesBoötes is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere. The name comes from the Greek Βοώτης, Boōtēs, meaning herdsman or plowman...
. It is even better known as the Kite. - The stars of CassiopeiaCassiopeia (constellation)Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
form a W which is often used as a nickname. - In Australia, "Frying PanFrying panA frying pan, frypan, or skillet is a flat-bottomed pan used for frying, searing, and browning foods. It is typically in diameter with relatively low sides that flare outwards, a long handle, and no lid. Larger pans may have a small grab handle opposite the main handle...
" for ChamaeleonChamaeleonChamaeleon is a small constellation in the southern sky. It is named after the chameleon, a form of lizard. It was first defined in the sixteenth century.-History:...
, is an aid to finding south by the stars.
Sectional asterisms
An asterism may also be a section of a constellation that refers to the traditional figuring of the whole. Thus, for example, there are:- the Belt of Orion
- The Y in AquariusAquarius (constellation)Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-bearer" or "cup-bearer", and its symbol is , a representation of water....
(historically called "the Urn"), - Hercules' ClubHercules (constellation)Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
.
There are many others.
Non-sectional asterisms
Other asterisms are also composed of stars from one constellation, but do not refer to the traditional figures.- The four central stars in HerculesHercules (constellation)Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
, Epsilon (ε Her), Zeta (ζ Her), Eta (η Her), and PiPi HerculisPi Herculis is a fourth-magnitude star in the constellation Hercules. As one of the four stars in the Keystone asterism is one of the more easily recognized in the constellation.- Properties :...
(π Her), form the well-known Keystone. - The curve of stars at the front end of the Lion from Al AshfarEpsilon LeonisEpsilon Leonis is the fifth brightest star in the constellation Leo, consistent with its Bayer designation Epsilon...
(ε Leo/Epsilon Leonis) to RegulusAlpha LeonisAlpha Leonis is an American indie rock band from Portland, Oregon carried on Planet Ceres Enterprises. The band began as the solo project of Atticus Reynard, who released the album "Shadows of Men" on January 5, 2010.-Members:Current...
(α Leo/Alpha Leonis), looking much like a mirror-image question mark, has long been known as the SickleLeo (constellation)Leo is one of the constellations of the zodiac. Its name is Latin for lion. Its symbol is . Leo lies between dim Cancer to the west and Virgo to the east.-Stars:...
. - The bow and arrow of the Archer also make a well-formed TeapotSagittarius (constellation)Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
. (There is even a bit of nebulosity near the "spout" to serve as steam).
- AlphaAlpha DelphiniAlpha Delphini is a multiple star in the constellation Delphinus. It also has the name Sualocin, which was given to it as a practical joke by the astronomer Niccolò Cacciatore; the name is the Latinized version of his given name, spelled backwards.In Chinese, , meaning Good Gourd, refers to an...
, BetaBeta DelphiniBeta Delphini is a binary star in the constellation of Delphinus. As a practical joke, the astronomer Niccolò Cacciatore gave it the name Rotanev, which is a reversal of his Latinized family name, Venator.In Chinese, , meaning Good Gourd, refers to an asterism consisting of β Delphini, α...
, GammaGamma DelphiniGamma Delphini is a binary star system approximately 101 light-years away in the constellation of Delphinus. The star marks one corner of the asterism "Job's Coffin"...
, and DeltaDelta DelphiniDelta Delphini is a giant star in the constellation Delphinus.-References:*...
DelphiniDelphinusDelphinus is a constellation in the northern sky, close to the celestial equator. Its name is Latin for dolphin. Delphinus was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains among the 88 modern constellations recognized by the International Astronomical...
form Job's Coffin. - The TerebellumTerebellum (astronomy)The Terebellum, by Ptolemy called τετράπλευρον , is a quadrilateral of stars in the constellation Sagittarius. It is formed of four 4th magnitude stars, all within two degrees of each other:* Omega Sagittarii, at the northeast corner....
is a small quadrilateral of four faint stars (OmegaOmega SagittariiOmega Sagittarii is a G-type subgiant star in the constellation of Sagittarius. It has an apparent visual magnitude of approximately 4.70. It is the northeast corner of the asterism called the Terebellum and at approximately 78 light years from Earth, it is the nearest of the four stars in...
, 5959 Sagittarii59 Sagittarii , also known by its Bayer designation b Sagittarii, is a K-type bright giant star in the constellation of Sagittarius. It has an apparent visual magnitude of approximately 4.544...
, 6060 Sagittarii60 Sagittarii , or A Sagittarii , is a G-type giant star in the constellation of Sagittarius. It is approximately 340 light-years from the Earth. It is the northwest corner of the asterism called the Terebellum and, with an apparent magnitude of approximately 4.851, it is the dimmest...
, 6262 Sagittarii62 Sagittarii or c Sagittarii is an M-type giant star in the constellation of Sagittarius. It is the southwest corner of the asterism called the Terebellum. It is an irregular variable whose apparent visual magnitude varies between 4.45 and 4.64, and, at its brightest, it is the...
) in SagittariusSagittarius (constellation)Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
' hindquarters. - Four other stars (Beta — MiaplacidusBeta CarinaeBeta Carinae is the second brightest star in the constellation Carina and one of the brightest stars in the night sky, with apparent magnitude 1.68. It is the brightest star in the south polar asterism known as the Diamond Cross, marking the southwestern end of the asterism. Beta Carinae also has...
, UpsilonUpsilon CarinaeUpsilon Carinae is a binary star in the constellation Carina. It is part of the Diamond Cross asterism in southern Carina....
, ThetaTheta CarinaeTheta Carinae is a star in the constellation Carina. It is the brightest star in the open star cluster IC 2602, and it marks the northeastern end of the Diamond Cross asterism....
, and OmegaOmega CarinaeOmega Carinae is a star in the constellation Carina. With a declination greater than 70 degrees south of the celestial equator, it is the most southerly of the bright stars of Carina , and it is part of a popular southern asterism known as the Diamond Cross. ω Carinae is a blue-white B-type giant...
CarinaCarina (constellation)Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was formerly part of the larger constellation of Argo Navis until that constellation was divided in three.-Stars:...
e) form a well-shaped diamond — the Diamond CrossDiamond CrossThe Diamond Cross is an asterism in the southern constellation Carina. The Diamond Cross is composed of four bright stars: Beta, Theta, Upsilon and Omega Carinae. These four bright stars create an almost perfect diamond shape, hence the name "Diamond Cross". The entire asterism is visible to all...
. - The Saucepan 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 PavoPavo (constellation)Pavo is a constellation in the southern sky. Its name is Latin for peacock. It is one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman and it first appeared on a 35-cm diameter celestial globe published in 1597 in...
.
- Just south of Pegasus, the western fish of PiscesPisces (constellation)Pisces is a constellation of the zodiac. Its name is the Latin plural for fish, and its symbol is . It lies between Aquarius to the west and Aries to the east...
is home to the Circlet formed from GammaGamma PisciumGamma Piscium is a star approximately 138 light years away from Earth, in the constellation Pisces. It is a yellow star with a spectral type of G9III, meaning it has a surface temperature of 5,000 to 6,000 degrees and is a normal giant star. It is slightly cooler than our Sun, yet it is 11 solar...
, KappaKappa PisciumKappa Piscium is a multiple star approximately 162 light years away from Earth, in the constellation Pisces. Appearing as a single point in the sky, it is easily split when viewed with a pair of binoculars, and consists of three components...
, LambdaLambda PisciumLambda Piscium is a star approximately 101 light years away from Earth in the constellation Pisces. It is a bluish-white star of A7V spectral classification, meaning it has a surface temperature of 7,500 to 11,000 kelvins. It is much brighter and hotter in surface temperature than our sun, yet is...
, TX, IotaIota PisciumIota Piscium is a star 45 light years away from Earth, in the constellation Pisces. The star is a yellowish dwarf star of spectral type F7V, which is somewhat larger and brighter than our sun, but still within the range considered to have the potential for Earth-like planets. It has a surface...
, and ThetaTheta PisciumTheta Piscium is a star approximately 159 light years away from Earth in the constellation Pisces. It is an orange-red star of the spectral type K1III, meaning it is a normal giant star with a surface temperature of 3,500 to 5,000 kelvins, and is brighter yet cooler than our sun....
Piscium. - Dubhe and MerakBeta Ursae MajorisBeta Ursae Majoris is a star in the constellation of Ursa Major. It has the traditional name Merak.It is more familiar to northern hemisphere observers as one of the "pointer stars" in the Big Dipper, and a straight line connecting it with nearby Alpha Ursae Majoris extends to Polaris, the north...
(Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of the Big DipperBig DipperThe Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...
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 PolarisPolarisPolaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
(α UMi/Alpha Ursae Minoris), the North Star. - AlphaAlpha CentauriAlpha Centauri is the brightest star in the southern constellation of Centaurus...
and Beta CentauriBeta CentauriBeta Centauri , also known as Hadar or Agena, is the second brightest star in the constellation Centaurus and the tenth brightest star in the night sky. Beta Centauri is a B1 III blue-white giant star that the astrometric Hipparcos satellite calculates as about 161 parsecs or 525 light-years from...
are the Southern Pointers leading to the Southern Cross and thus helping to distinguish CruxCruxCrux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...
from the False Cross.
Cross-border groups
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. SiriusSiriusSirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
(α CMa), ProcyonProcyonProcyon is the brightest star in the constellation Canis Minor. To the naked eye, it appears to be a single star, the seventh brightest in the night sky with a visual apparent magnitude of 0.34...
(α CMi), and BetelgeuseBetelgeuseBetelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...
(α Ori) form one to the North (Winter TriangleWinter HexagonThe Winter Hexagon or Winter Circle/Oval is an asterism appearing to be in the form of a hexagon with vertices at Rigel, Aldebaran, Capella, Pollux/Castor, Procyon, and Sirius. It is mostly upon the Northern Hemisphere's celestial sphere...
) while Sirius, NaosZeta PuppisZeta Puppis is a star in the constellation of Puppis. It is also known by the traditional names Naos and Suhail Hadar in Arabic....
(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 DracoDraco (constellation)Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...
and one - Iota Herculis - in the foot of HerculesHercules (constellation)Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. - The False Cross is composed of the four stars DeltaDelta VelorumDelta Velorum is a star system in the constellation Vela. It is approximately 79.7 light years from Earth. It is sometimes given the name Koo She, Chinese for 'Bow and Arrows', along with ω Velorum and stars in Canis Major....
and Kappa VelorumKappa VelorumKappa Velorum is a binary star in the constellation Vela. It also has the traditional name Markab, often spelled Markeb to distinguish it from similarly named stars such as Alpha Pegasi....
(δ and κ Vel) and EpsilonEpsilon CarinaeEpsilon Carinae is a star in the constellation Carina. At apparent magnitude +1.86 it is one of the brightest stars in the night sky, but is not visible from the northern hemisphere....
and Iota CarinaeIota CarinaeIota Carinae is a star in the constellation Carina. It is one of the brighter stars in the night sky.It has the traditional names Aspidiske, Scutulum and Turais...
(ε and ι Car). Although its component stars are not quite as bright as those of the Southern CrossCruxCrux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...
, 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 north40th parallel northThe 40th parallel north is a circle of latitude that is 40 degrees north of the Earth's equatorial plane. It crosses Europe, the Mediterranean Sea, Asia, the Pacific Ocean, North America, and the Atlantic Ocean....
especially, a prominent upper-case Y is formed by Arcturus, GammaGamma BoötisGamma Boötis is a star in the constellation Boötes. It has the traditional name Seginus of uncertain origin...
and EpsilonEpsilon BoötisEpsilon Boötis is a double star in the constellation Boötes. It has the traditional names Izar and Pulcherrima....
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 KingdomUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
in particular, where there is serious light pollutionLight pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
in many areas and also twilightTwilightTwilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...
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.
Telescopic patterns
Asterisms range from the large and obvious to the small, and even telescopic.- A renowned object, the CoathangerBrocchi's ClusterCollinder 399 is a random grouping of stars located in the constellation Vulpecula near the border with Sagitta. Collinder 399 is known as Al Sufi's Cluster or Brocchi's Cluster...
, formally open clusterOpen clusterAn open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
Collinder 399, in VulpeculaVulpeculaVulpecula is a faint constellation in the northern sky. Its name is Latin for "little fox", although it is commonly known simply as the fox. It was identified in the seventeenth century, and is located in the middle of the Summer Triangle .-Stars:There are no stars brighter than 4th magnitude in...
may be recognized by the eye. - Kemble's CascadeKemble's CascadeKemble's Cascade , located in the constellation Camelopardalis, is an asterism — a pattern created by unrelated stars. It is an apparent straight line of more than 20 colorful 5th to 10th magnitude stars over a distance of approximately five moon diameters, and the open cluster NGC 1502 can be...
in CamelopardalisCamelopardalisCamelopardalis is a large but faint constellation in the northern sky. The constellation was introduced in 1612 by Petrus Plancius. Some older astronomy books give an alternative spelling of the name, Camelopardus.-Etymology:...
is a chain of stars (the waterfall) that ends in open cluster NGC 1502. - Approximately 40 stars in MonocerosMonocerosMonoceros is a faint constellation on the celestial equator. Its name is Greek for unicorn. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south and Hydra to the east...
, including a portion of NGC 2264NGC 2264NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object:*the Cone Nebula,*the Christmas Tree Cluster,Two other objects are within this designation but not officially included:...
, fill out the triangular Christmas Tree ClusterNGC 2264NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object:*the Cone Nebula,*the Christmas Tree Cluster,Two other objects are within this designation but not officially included:...
.
Former asterisms
Argo is a special case. Argo NavisArgo Navis
Argo Navis was a large constellation in the southern sky that has since been divided into three constellations. It represented the Argo, the ship used by Jason and the Argonauts in Greek mythology...
, ("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
Carina (constellation)
Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was formerly part of the larger constellation of Argo Navis until that constellation was divided in three.-Stars:...
, Puppis
Puppis
Puppis is a constellation in the southern sky. Its name is the Latin word for the poop deck of a ship, and Puppis represents the deck of the ship and its deckhouses...
, and Vela
Vela (constellation)
Vela is a constellation in the southern sky. Its name is Latin for the sails of a ship, and it was originally part of a larger constellation, the ship Argo Navis, which was later divided into three parts, the others being Carina and Puppis.-Stars:...
), 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
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...
. Crux was an asterism when Bayer created it in Uranometria
Uranometria
Uranometria is the short title of a star atlas produced by Johann Bayer.It was published in Augsburg, Germany, in 1603 by Christophorus Mangus under the full title Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. This translates to...
(1603) from stars in the hind legs of Centaurus
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...
. It was given constellation status in 1930, thereby mutilating the Centaur.
Centaurus had been reduced in size once before. Lupus
Lupus (constellation)
Lupus is a constellation in the southern sky. Its name is Latin for wolf. Lupus was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations...
was originally considered to be merely a sectional asterism (as an unspecified Wild Beast in the Centaur's grasp). Hipparchus
Hipparchus
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...
split it off in the 200s BC, and Ptolemy's list confirmed its independent status.
In its original figuration, Leo
Leo (constellation)
Leo is one of the constellations of the zodiac. Its name is Latin for lion. Its symbol is . Leo lies between dim Cancer to the west and Virgo to the east.-Stars:...
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
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...
's acceptance of Coma
Coma Berenices
Coma Berenices is a traditional asterism that has since been defined as one of the 88 modern constellations. It is located near Leo, to which it formerly belonged, and accommodates the North Galactic Pole...
, Bayer recorded it and refigured the Lion. The IAU confirmed Coma's status as a constellation.
Even so venerable a constellation as Libra
Libra (constellation)
Libra is a constellation of the zodiac. Its name is Latin for weighing scales, and its symbol is . It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east.-Notable features:]...
was once merely an asterism. Until the middle of the first millennium BC, the Zodiac
Zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...
consisted of only eleven constellations. The biblical
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...
reference to "the eleven stars" (Genesis 37:9) is more accurately "the eleven asterisms/constellations (of the Zodiac)." At the time, Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
' claws were pictured as extending to Zubenelgenubi, "the southern claw" and Zubeneschamali, "the northern claw" (Alpha and Beta Librae). Later, when Virgo
Virgo (constellation)
Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...
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
Former constellations
Former constellations are constellations that are no longer recognized by the International Astronomical Union for various reasons. Many of these constellations existed for long periods of time, even centuries in many cases, which means they still have a large historical value and can be found on...
.
Non-asterisms
In the formal sense used here, asterisms are groups of stars that have not been categorized as something else. 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
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
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
early telescopic discoveries. Similarly, the Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
are not asterisms, but galaxies in their own right.
Nebulae, clouds of gas and dust that dot the galaxy, whether emission
Emission nebula
An emission nebula is a cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star...
, such as the Pelican
Pelican Nebula
The Pelican Nebula is an H II region associated with the North America Nebula in the constellation Cygnus. The gaseous contortions of this emission nebula bear a resemblance to a pelican, giving rise to its name...
, or dark
Dark nebula
A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula or that it blocks out background stars . The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger...
, such as the Horsehead
Horsehead Nebula
The Horsehead Nebula is a dark nebula in the constellation Orion. The nebula is located just to the south of the star Alnitak, which is farthest east on Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. The Horsehead Nebula is approximately 1500 light years from Earth...
, are clearly not asterisms as they are not composed of stars.
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
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
(M45
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...
) and the Hyades
Hyades (star cluster)
The Hyades is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. The Hipparcos satellite, the Hubble Space Telescope, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly...
in Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...
and the Beehive
Beehive Cluster
The Beehive Cluster, also known as Praesepe , M44, NGC 2632, or Cr 189, is an open cluster in the constellation Cancer. It is one of the nearest open clusters to the Solar System, and it contains a larger star population than most other nearby clusters...
(M44
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...
) in Cancer
Cancer (constellation)
Cancer is one of the twelve constellations of the zodiac. Its name is Latin for crab and it is commonly represented as such. Its symbol is . Cancer is small and its stars are faint...
. (It may be noted that, with the addition of Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
, 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
Theta Orionis
Theta Orionis is the name of several astronomical objects, located at RA DEC :* Theta1 Orionis, the Trapezium Cluster, an open star cluster, the Orion OB Association 1d...
(θ Ori) is embedded in, and illuminates, the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
(M42). Looked at telescopically, it resolved into four stars arranged in a trapezoid, and they were nicknamed the Trapezium
Trapezium (astronomy)
The Trapezium, or Orion Trapezium Cluster is a tight open cluster of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galilei. On February 4, 1617 he sketched three of the stars , but missed the surrounding nebulosity...
. 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
Messier 73
Messier 73 is an asterism of four stars in the constellation of Aquarius. An asterism is composed of physically unconnected stars that appear close to each other in the sky as seen from Earth...
in Aquarius
Aquarius (constellation)
Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-bearer" or "cup-bearer", and its symbol is , a representation of water....
, 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.
See also
- Former constellationsFormer constellationsFormer constellations are constellations that are no longer recognized by the International Astronomical Union for various reasons. Many of these constellations existed for long periods of time, even centuries in many cases, which means they still have a large historical value and can be found on...
- NakshatraNakshatraNakshatra is the term for lunar mansion in Hindu astrology. A nakshatra is one of 27 sectors along the ecliptic...
- Chinese constellationChinese constellationChinese constellations are the way the ancient Chinese grouped the stars. They are very different from the modern IAU recognized constellations. This is because the IAU was based on Greco-Roman astronomy instead of Chinese astronomy....
- Australian Aboriginal astronomyAustralian Aboriginal astronomyAustralian Aboriginal astronomy is a name given to indigenous Australian culture relating to astronomical subjects — such as the Sun and Moon, the stars, planets, and the Milky Way, and their motions on the sky...