A picture of stars, with a group of appearingly bright blue and white stars. The bright stars together are identified as the asterismCoathanger resembling acoathanger, in the constellationVulpecula.
Anasterism is anobserved pattern or group ofstars in thesky. Asterisms can be any identified star pattern, and therefore are a more general concept than the88 formally definedconstellations. Constellations are based upon asterisms, but unlike asterisms, constellations are defined regions with official boundaries which together encompass the entire sky.[1][2]
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 theSummer Triangle is a purely observational, physically unrelated group of stars, but the stars ofOrion's Belt are all members of theOrion OB1 association and five of the seven stars of the Big Dipper are members of theUrsa Major Moving Group. Physical associations, such as theHyades orPleiades, can be asterisms in their own right and also part of other asterisms.
In many early civilizations, it was common to associate groups of stars inconnect-the-dotsstick-figure patterns. Some of the earliest records are those of ancient India in theVedanga Jyotisha and theBabylonians.[citation needed] Different cultures identified different constellations, although a few of the more obvious patterns tend to appear in the constellations of multiple cultures, such as those ofOrion andScorpius. As anyone could arrange and name a grouping of stars there was no distinct difference between aconstellation and anasterism. For example,Pliny the Elder mentions 72 asterisms in his bookNaturalis Historia.[3]
A general list containing 48 constellations likely began to develop with the astronomerHipparchus (c. 190 – c. 120 BCE). 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.[citation needed]
Exploration by Europeans to other parts of the globe exposed them to stars previously unknown to them. Two astronomers particularly known for greatly expanding the number of southern constellations wereJohann Bayer (1572–1625) andNicolas Louis de Lacaille (1713–1762). Bayer had listed twelve figures made out of stars that were too far south forPtolemy to have seen. Lacaille created 14 new groups, mostly for the area surroundingSouth Celestial Pole. Many of these proposed constellations have been formally accepted, but the rest have remained as asterisms.[citation needed]
In 1928, theInternational Astronomical Union (IAU) precisely divided the sky into88 official constellations following geometric boundaries encompassing all of the stars within them. Any additional new selected groupings of stars orformer constellations are often considered as asterisms. However, technical distinctions between the terms 'constellation' and 'asterism' often remain somewhat ambiguous.[citation needed]
Some asterisms consist completely of brightfirst-magnitude stars, which mark out simple geometric shapes.
TheSummer Triangle ofDeneb,Altair, andVega – αCygni, αAquilae, and αLyrae – is prominent in the northern hemisphere summer skies, as its three stars are all of the 1st magnitude.[4] The stars of the Triangle are in the band of theMilky Way which marks the galactic equator, and are in the direction of theGalactic Center.
TheGreat Diamond consisting of Arcturus, Spica,Denebola andCor Caroli, the latter two not being first-magnitude stars.[7] 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 constellationsBoötes,Virgo,Leo, andCanes Venatici.
Other asterisms consist partially of multiple first-magnitude stars.
TheBig Dipper, also known asThe Plough orCharles's Wain, is composed of the seven brightest stars inUrsa Major.[5] 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, theLittle Dipper.
TheSouthern Cross is an asterism by name, but the whole area is now recognised as the constellationCrux. The main stars areAlpha,Beta,Gamma,Delta, and arguably alsoEpsilon Crucis. Earlier, Cruxwas deemed an asterism when Bayer created it inUranometria (1603) from the stars in the hind legs ofCentaurus, decreasing the size of Centaur. These same stars were probably identified byPliny the Elder in hisNaturalis Historia as the asterism 'Thronos Caesaris.'[3]
TheFish Hook is the traditional Hawaiian name forScorpius. The image will be even more obvious if the chart's lines fromAntares (α Sco) toBeta Scorpii (β Sco) andPi Scorpii (π Sco) are replaced with a line from Beta throughDelta 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 ofHercules will complete the figure of the Butterfly.[8]
Boötes is sometimes known as theIce Cream Cone.[9] It is also known as theKite.[10]
The stars ofCassiopeia form a W which is often used as a nickname.[11]
TheGreat Square ofPegasus is the quadrilateral formed by the starsMarkab,Scheat,Algenib, andAlpheratz, representing the body of the winged horse.[12] The asterism was recognized as the constellation ASH.IKU "The Field" on theMUL.APINcuneiform tablets from about 1100 to 700 BC.[13] Alpheratz is now only considered a part of the constellation Andromeda whereas formerly the star was a part of both constellations.
TheBowl of Virgo is formed by the stars Beta, Gamma, Delta, Epsilon and Eta Virginis. Together withSpica, they form a Y shape.
Three Leaps of the Gazelle asterism
TheThree Leaps of the Gazelle consists of three pairs of stars inUrsa 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 byLeo the lion. (The "pond" is pictured as theComa 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:
TheWater Jar orUrn ofAquarius is a Y-shaped figure centered uponZeta Aquarii and includes Gamma, Eta and Pi. It pours water in a stream of more than 20 stars terminating with the starFomalhaut.
TheCrab Breast ofCancer (mentioned byPtolemy) 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 theBeehive Cluster (Messier 44) which includes Epsilon Cancri.
TheSnake Head is the westernmost portion ofHydra consisting of the stars Delta, Epsilon, Zeta, Eta, Rho and Sigma Hydrae.
Orion's Sword which hangs from Orion's Belt consists of the stars 42 Orionis, Theta Orionis, Iota Orionis, and centered upon M42, theOrion Nebula.
TheBull's Face ofTaurus is a V-shaped figure formed by prominent members of theHyades 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.
TheSaucepan orPot, being the same stars as the Belt and Sword of Orion. The end of the handle is atι Orionis, with the far rim atη Orionis.[citation needed]
The four central stars inHercules,Epsilon (ε Her),Zeta (ζ Her),Eta (η Her), andPi (π Her), form theKeystone.[4] The bright globular clusterMessier 13 lies along the western segment, between Zeta and Eta.
The curve of stars at the front end of the Lion fromEpsilon (ε Leo) toRegulus (α Leo), looking much like a mirror-image question mark, has long been known as theSickle.[5]
Just south of Pegasus, the western fish ofPisces is home to theCirclet formed fromGamma (γ Piscium),Kappa (κ Piscium),Lambda (λ Piscium),TX Piscium,Iota (ι Piscium), andTheta (θ Piscium).[4][5]
Dubhe andMerak (Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of theBig Dipper are often called thePointers:[18] a line from β to α and continued for about five times the distance between them arrives at the North Celestial Pole and the starPolaris (α UMi/Alpha Ursae Minoris), the North Star.
Rigil Kentaurus (α Centauri) andHadar (β Centauri) are theSouthern Pointers leading to the Southern Cross[19] and thus helping to distinguishCrux from the False Cross.
Other asterisms that are formed from stars in more than one constellation.
TheEgyptian 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), andBetelgeuse (α Ori) form one to the North (Winter Triangle) while Sirius,Naos (ζ Pup), andPhakt (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.[citation needed]
The diamond-shapedFalse Cross is composed of the four starsAlsephina (δ Velorum),Markeb (κ Velorum),Avior (ε Carinae), andAspidiske (ι Carinae).[14] Although its component stars are not quite as bright as those of theSouthern Cross, it is somewhat larger and better shaped than the Southern Cross, for which it is sometimes mistaken, causing errors inastronavigation. Like the Southern Cross, three of its main four stars are whitish and one orange.[20]
TheNorthern Y is formed by four prominent stars,Arcturus (α Boötis),Seginus (γ Boötis),Alphecca (α Coronae Borealis), and centered onIzar (ε Boötis). From theUnited Kingdom in particular, where there is seriouslight pollution in many areas and alsotwilight much of the night when these constellations appear, this "Y" is often visible while other stars of Boötes and Corona Borealis are not.[citation needed]
TheLightning Bolt, aligned north to south, consists of the starsEpsilon Pegasi,Alpha Aquarii,Beta Aquarii andDelta Capricorni. (It should not be confused with the so-called David Bowie "constellation".) Easily visible to naked eyes even in light polluted skies, the asterism is useful for orienting among three constellations.[21]
TheSerpent 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.
TheEagle Tail Corona is a flattened curved figure in the tail ofAquila and extending intoScutum. It consists of the stars 14, 15, Lambda and 12 Aquilae, Eta Scuti, HD 174208, R and Beta Scuti. The compact open clusterMessier 11 is also aligned with the curve.
^Rogers, J. H. (1 February 1998). "Origins of the ancient constellations: I. The Mesopotamian traditions".Journal of the British Astronomical Association.108:9–28.Bibcode:1998JBAA..108....9R.
