Asterism (astronomy)

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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 Vulpecula. Cr399.jpg
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 Vulpecula.

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 88 formally defined constellations. 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. [1] [2] 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 Crux.

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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 Hyades or 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 common to associate groups of stars in connect-the-dots stick-figure patterns. Some of the earliest records are those of ancient India in the Vedanga Jyotisha and the Babylonians.[ 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 of Orion and Scorpius. As anyone could arrange and name a grouping of stars there was no distinct difference between a constellation and an asterism. For example, Pliny the Elder mentions 72 asterisms in his book Naturalis Historia. [3]

A general list containing 48 constellations likely began to develop with the astronomer Hipparchus (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 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 remained as asterisms.[ citation needed ]

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, technical distinctions between the terms 'constellation' and 'asterism' often remain somewhat ambiguous.[ citation needed ]

Asterisms consisting of first-magnitude stars

Some asterisms consist completely of bright first-magnitude stars, which mark out simple geometric shapes.

Other asterisms consist partially of multiple first-magnitude stars.

All other first-magnitude stars are the only such stars in their asterisms or constellations, with Canopus in the Argo Navis asterism south of Sirius, visually east of the Carina Nebula and near the Large Magellanic Cloud (both being first-magnitude deep-sky objects), Achernar in the Eridanus constellation east of Canopus, Fomalhaut in the Southern Fish constellation east of Achernar and Antares in the Scorpius constellation visually near the Galactic Center.

Some major asterisms on a celestial map (the projection exaggerates the stretching) Major asterisms.png
Some major asterisms on a celestial map (the projection exaggerates the stretching)

Constellation-based asterisms

The Big Dipper asterism Plough big dipper.svg
The Big Dipper asterism
Three Leaps of the Gazelle asterism Three Leaps of the Gazelle.jpg
Three Leaps of the Gazelle asterism

Some asterisms refer to portions of traditional constellation figures. These include:

Other particular asterisms

The "Teapot" asterism in Sagittarius. The Milky Way appears as "steam" coming from the spout. Sagittarius-teapot-asterism.jpg
The "Teapot" asterism in Sagittarius. The Milky Way appears as "steam" coming from the spout.

Other asterisms are also composed of stars from one constellation, but do not refer to the traditional figures.

Asterisms across multiple constellations

Other asterisms that are formed from stars in more than one constellation.

Telescopic asterisms

The "37" or "LE" of NGC 2169, in Orion. It is visible with binoculars. NGC 2169.jpg
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.

See also

Related Research Articles

<span class="mw-page-title-main">Aquarius (constellation)</span> Zodiac constellation straddling the celestial equator

Aquarius is an equatorial constellation of the zodiac, between Capricornus and Pisces. Its name is Latin for "water-carrier" or "cup-carrier", and its old astronomical symbol is (♒︎), a representation of water. Aquarius is one of the oldest of the recognized constellations along the zodiac. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is found in a region often called the Sea due to its profusion of constellations with watery associations such as Cetus the whale, Pisces the fish, and Eridanus the river.

<span class="mw-page-title-main">Boötes</span> Constellation in the northern celestial hemisphere

Boötes 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 Latin: Boōtēs, which comes from Greek: Βοώτης, translit. Boṓtēs 'herdsman' or 'plowman'.

<span class="mw-page-title-main">Crux</span> Constellation in the southern celestial hemisphere

Crux is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name Crux is Latin for cross. Even though it is the smallest of all 88 modern constellations, Crux is among the most easily distinguished as its four main stars each have an apparent visual magnitude brighter than +2.8. It has attained a high level of cultural significance in many Southern Hemisphere states and nations.

<span class="mw-page-title-main">Carina (constellation)</span> Constellation in the southern celestial hemisphere

Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was the southern foundation of the larger constellation of Argo Navis until it was divided into three pieces, the other two being Puppis, and Vela.

<span class="mw-page-title-main">Canis Major</span> Constellation in the southern celestial hemisphere

Canis Major is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemy's 48 constellations, and is counted among the 88 modern constellations. Its name is Latin for "greater dog" in contrast to Canis Minor, the "lesser dog"; both figures are commonly represented as following the constellation of Orion the hunter through the sky. The Milky Way passes through Canis Major and several open clusters lie within its borders, most notably M41.

<span class="mw-page-title-main">Canis Minor</span> Constellation in the northern celestial hemisphere

Canis Minor is a small constellation in the northern celestial hemisphere. In the second century, it was included as an asterism, or pattern, of two stars in Ptolemy's 48 constellations, and it is counted among the 88 modern constellations. Its name is Latin for "lesser dog", in contrast to Canis Major, the "greater dog"; both figures are commonly represented as following the constellation of Orion the hunter.

<span class="mw-page-title-main">Centaurus</span> Constellation in the southern celestial hemisphere

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. In Greek mythology, Centaurus represents a centaur; a creature that is half human, half horse. Notable stars include Alpha Centauri, the nearest star system to the Solar System, its neighbour in the sky Beta Centauri, and V766 Centauri, one of the largest stars yet discovered. The constellation also contains Omega Centauri, the brightest globular cluster as visible from Earth and the largest identified in the Milky Way, possibly a remnant of a dwarf galaxy.

<span class="mw-page-title-main">Chamaeleon</span> Constellation in the southern celestial hemisphere

Chamaeleon is a small constellation in the deep southern sky. It is named after the chameleon, a kind of lizard. It was first defined in the 16th century.

<span class="mw-page-title-main">Pisces (constellation)</span> Zodiac constellation straddling the celestial equator

Pisces is a constellation of the zodiac. Its vast bulk – and main asterism viewed in most European cultures per Greco-Roman antiquity as a distant pair of fishes connected by one cord each that join at an apex – are in the Northern celestial hemisphere. Its old astronomical symbol is (♓︎). Its name is Latin for "fishes". It is between Aquarius, of similar size, to the southwest and Aries, which is smaller, to the east. The ecliptic and the celestial equator intersect within this constellation and in Virgo. This means the sun passes directly overhead of the equator, on average, at approximately this point in the sky, at the March equinox.

<span class="mw-page-title-main">Sagittarius (constellation)</span> Zodiac constellation in the southern celestial hemisphere

Sagittarius is one of the constellations of the zodiac and is located in the Southern celestial hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its old astronomical symbol is (♐︎). Its name is Latin for "archer". Sagittarius is commonly represented as a centaur drawing a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east.

<span class="mw-page-title-main">Triangulum Australe</span> Constellation in the southern celestial hemisphere

Triangulum Australe is a small constellation in the far Southern Celestial Hemisphere. Its name is Latin for "the southern triangle", which distinguishes it from Triangulum in the northern sky and is derived from the acute, almost equilateral pattern of its three brightest stars. It was first depicted on a celestial globe as Triangulus Antarcticus by Petrus Plancius in 1589, and later with more accuracy and its current name by Johann Bayer in his 1603 Uranometria. The French explorer and astronomer Nicolas Louis de Lacaille charted and gave the brighter stars their Bayer designations in 1756.

<span class="mw-page-title-main">Ursa Major</span> Constellation in the northern sky

Ursa Major is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater bear", referring to and contrasting it with nearby Ursa Minor, the lesser bear. In antiquity, it was one of the original 48 constellations listed by Ptolemy in the 2nd century AD, drawing on earlier works by Greek, Egyptian, Babylonian, and Assyrian astronomers. Today it is the third largest of the 88 modern constellations.

<span class="mw-page-title-main">Ursa Minor</span> Constellation in the northern celestial hemisphere

Ursa Minor, also known as the Little Bear, is a constellation located in the far northern sky. As with the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, hence the North American name, Little Dipper: seven stars with four in its bowl like its partner the Big Dipper. Ursa Minor was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Ursa Minor has traditionally been important for navigation, particularly by mariners, because of Polaris being the north pole star.

<span class="mw-page-title-main">Vela (constellation)</span> Constellation in the southern celestial hemisphere

Vela is a constellation in the southern sky, which contains the Vela Supercluster. 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. With an apparent magnitude of 1.8, its brightest star is the hot blue multiple star Gamma Velorum, one component of which is the closest and brightest Wolf-Rayet star in the sky. Delta and Kappa Velorum, together with Epsilon and Iota Carinae, form the asterism known as the False Cross. 1.95-magnitude Delta is actually a triple or quintuple star system.

<span class="mw-page-title-main">Orion (constellation)</span> Constellation straddling the celestial equator

Orion is a prominent set of stars visible during winter in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century astronomer Ptolemy. It is named for a hunter in Greek mythology.

<span class="mw-page-title-main">Cassiopeia (constellation)</span> Constellation in the northern celestial hemisphere

Cassiopeia is a constellation and asterism in the northern sky named after the vain queen Cassiopeia, mother of Andromeda, in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is easily recognizable due to its distinctive 'W' shape, formed by five bright stars.

<span class="mw-page-title-main">Auriga (constellation)</span> Constellation in the northern celestial hemisphere

Auriga is a constellation in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century astronomer Ptolemy. Its name is Latin for '(the) charioteer', associating it with various mythological beings, including Erichthonius and Myrtilus. Auriga is most prominent during winter evenings in the northern Hemisphere, as are five other constellations that have stars in the Winter Hexagon asterism. Because of its northern declination, Auriga is only visible in its entirety as far south as -34°; for observers farther south it lies partially or fully below the horizon. A large constellation, with an area of 657 square degrees, it is half the size of the largest, Hydra.

<span class="mw-page-title-main">Spring Triangle</span> Pattern of stars in the Northern Hemisphere

The Spring Triangle is an astronomical asterism involving an imaginary triangle drawn upon the celestial sphere, with its defining vertices at Arcturus, Spica, and Regulus. This triangle connects the constellations of Boötes, Virgo, and Leo. It is visible in the evening rising in the southeastern sky of the Northern Hemisphere between March and May and setting until August, while at morning rising and setting from November to the end of February.

<span class="mw-page-title-main">Ursa Major moving group</span>

The Ursa Major Moving Group, also known as Collinder 285 and the Ursa Major association, is the closest stellar moving group – a set of stars with common velocities in space and thought to have a common origin in space and time. In the case of the Ursa Major group, all the stars formed about 300 million years ago. Its core is located roughly 80 light years away and part of the Local Bubble. It is rich in bright stars including most of the stars of the Big Dipper.

<span class="mw-page-title-main">Big Dipper</span> Pattern of seven bright stars in the constellation Ursa Major

The Big Dipper or the plough is a large asterism consisting of seven bright stars of the constellation Ursa Major; six of them are of second magnitude and one, Megrez (δ), of third magnitude. Four define a "bowl" or "body" and three define a "handle" or "head". It is recognized as a distinct grouping in many cultures. The North Star (Polaris), the current northern pole star and the tip of the handle of the Little Dipper, can be located by extending an imaginary line through the front two stars of the asterism, Merak (β) and Dubhe (α). This makes it useful in celestial navigation.

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