Asterism (astronomy)

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This picture of Brocchi's Cluster (the Coathanger), an asterism in the constellation Vulpecula, was taken through binoculars Collinder 399.jpg
This picture of Brocchi's Cluster (the Coathanger), an asterism in the constellation Vulpecula, was taken through binoculars

In observational astronomy, an asterism is a popularly-known pattern or group of stars that can be seen in the night sky. This colloquial definition [lower-alpha 1] makes it appear quite similar to a constellation, [1] but they differ mostly in that a constellation is an officially recognized area of the sky, while an asterism is a visually obvious collection of stars and the lines used to mentally connect them; as such, asterisms do not have officially determined boundaries and are therefore a more general concept which may refer to any identified pattern of stars. This distinction between terms remains somewhat inconsistent, varying among published sources. An asterism may be understood as an informal group of stars within the area of an official or defunct former constellation. [2] Some include stars from more than one constellation.

Observational astronomy part of astronomical science concerned with recording data

Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical models. It is the practice and study of observing celestial objects with the use of telescopes and other astronomical instruments.

Star An astronomical object consisting of a luminous spheroid of plasma held together by its own gravity

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023) stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

Night sky dark sky in the deep twilight or at night

The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.


Asterisms range from simple shapes of just few stars to more complex collections of many bright stars. They are useful for people who are familiarizing themselves with the night sky. For example, the asterisms known as The Plough (Charles' Wain, the Big Dipper, etc.) comprises the seven brightest stars in the International Astronomical Union (IAU) recognised constellation Ursa Major. Another is the asterism of the Southern Cross, whose recognised constellation is Crux (crux is an area of the night sky in which the Southern Cross is located).

International Astronomical Union Association of professional astronomers

The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them.

Ursa Major Constellation in the northern celestial hemisphere

Ursa Major is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater she-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. Today it is the third largest of the 88 modern constellations.

Crux constellation in the southern celestial hemisphere

Crux is a constellation centred on four stars in the southern sky in a bright portion of the Milky Way. It is among the most easily distinguished constellations as its hallmark (asterism) stars each have an apparent visual magnitude brighter than +2.8, even though it is the smallest of all 88 modern constellations. Its name is Latin for cross, and it is dominated by a cross-shaped or kite-like asterism that is commonly known as the Southern Cross.

Background of asterisms and constellations

In many early civilizations, it was already common to associate groups of stars in connect-the-dots stick-figure patterns; some of the earliest records are those of 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 Orion and 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. [3]

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. The constellations were probably collected from various other sources. The earliest catalogue, Three Stars Each, mentions stars of Akkad, of Amurru, of Elam and others.

Orion (constellation) Constellation straddling the celestial equator

Orion is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous and recognizable constellations in the night sky. It was named after Orion, a hunter in Greek mythology. Its brightest stars are the supergiants: blue-white Rigel and red Betelgeuse.

Scorpius Zodiac constellation in the southern celestial hemisphere

Scorpius is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . Scorpius is one of the 48 constellations identified by the Greek astronomer Ptolemy in the second century. It is an ancient constellation that pre-dated the Greeks. It lies between Libra to the west and Sagittarius to the east. It is a large constellation located in the southern hemisphere near the center of the Milky Way.

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.

Hipparchus ancient Greek scholar

Hipparchus of Nicaea was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his incidental discovery of precession of the equinoxes.

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.

Johann Bayer German astronomer

Johann Bayer was a German lawyer and uranographer. He was born in Rain, Lower Bavaria, in 1572. At twenty, in 1592 he began his study of philosophy and law at the University of Ingolstadt, after which he moved to Augsburg to begin work as a lawyer, becoming legal adviser to the city council in 1612.

Celestial pole two imaginary points in the sky where the Earths axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to an observer at the Earth's North Pole and South Pole, respectively. As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day.

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. e.g. Both the open clusters The Pleiades or Seven Sisters and The Hyades in Taurus are sometimes considered as an asterisms, but this depends on the source.

Open cluster group of "sibling" stars that are loosely gravitationally bound to one another

An 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. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

Pleiades open cluster in the constellation of Taurus

The Pleiades, also known as the Seven Sisters and Messier 45, are 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.

Hyades (star cluster) star cluster

The Hyades is the nearest open cluster and one of the best-studied star clusters. Located about 153 light-years away from the Sun, it consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical characteristics, and motion through space. From the perspective of observers on Earth, the Hyades Cluster appears in the constellation Taurus, where its brightest stars form a "V" shape along with the still-brighter Aldebaran. However, Aldebaran is unrelated to the Hyades, as it is located much closer to Earth and merely happens to lie along the same line of sight.

Large or bright asterisms

Component stars of asterisms are bright and mark out simple geometric shapes.

Constellation based asterisms

The Big Dipper asterism Plough big dipper.svg
The Big Dipper asterism

Some asterisms may also be part of a constellation referring to the traditional figuring of the whole outline. For example, there are:

There are many others. [9]

Commonly recognised asterisms

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

Cross-border asterisms

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

Telescopic asterisms

Asterisms range from the large and obvious to the small, and even telescopic.

See also


  1. In usage, 'constellation' is sometimes a synonym of 'asterism', which reflects the fact that the respective words in Latin (constellatio) and Greek (ἀστερισμός, asterismos), from which the English terms are derived, are synonymous. While a terminological distinction may be present in English, such a distinction does not necessarily occur in other languages.

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Boötes 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 the Greek Βοώτης, Boōtēs, meaning “herdsman” or “plowman”.

Capricornus Zodiac constellation in the southern celestial hemisphere

Capricornus is one of the constellations of the zodiac. Its name is Latin for "horned goat" or "goat horn" or "having horns like a goat's", and it is commonly represented in the form of a sea-goat: a mythical creature that is half goat, half fish. Its symbol is .

Carina (constellation) 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 formerly part of the larger constellation of Argo Navis until that constellation was divided into three pieces, the other two being Puppis, and Vela.

Delphinus Constellation in the northern celestial hemisphere

Delphinus Eng. oth: ) 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 Union. It is one of the smaller constellations, ranked 69th in size.

Pisces (constellation) Zodiac constellation straddling the celestial equator

Pisces is a constellation of the zodiac. Its name is the Latin plural for fish. It lies between Aquarius to the west and Aries to the east. The ecliptic and the celestial equator intersect within this constellation and in Virgo. Its symbol is .

Sagittarius (constellation) Zodiac constellation in the southern celestial hemisphere

Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur pulling back a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east.

Virgo (constellation) Zodiac constellation straddling the celestial equator

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 and the largest constellation in the zodiac. It can be easily found through its brightest star, Spica.

Scutum (constellation) constellation in the southern celestial hemisphere

Scutum is a small constellation introduced in the seventeenth century. Its name is Latin for shield.

Beta Andromedae star in the northern constellation of Andromeda

Beta Andromedae, officially named Mirach, is a prominent star in the northern constellation of Andromeda. It is northeast of the Great Square of Pegasus and is theoretically visible to all observers north of 54° S. It is commonly used by stargazers to find the Andromeda Galaxy. The galaxy NGC 404, also known as Mirach's Ghost, is seven arc minutes away from Mirach.

The Ursa Major Moving Group, also known as Collinder 285 and the Ursa Major association, is a nearby 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. It is rich in bright stars including most of the stars of the Big Dipper.

Imai (star) star in the constellation Crux

Imai, Delta Crucis, is a star in the southern constellation of Crux, and is the faintest of the four bright stars that form the prominent asterism known as the Southern Cross. This star has an apparent magnitude of 2.79, and its proper name was adopted by the astronomical community on 10 August 2018. Imai is a massive, hot and rapidly rotating star that is in the process of evolving into a giant, and is located at a distance of about 345 light-years from the Sun.

Delta Serpentis is a star system in the constellation Serpens, in its head.

Delta Andromedae multiple star system in the northern constellation of Andromeda

Delta Andromedae is a multiple star system in the northern constellation of Andromeda. Based upon parallax measurements, it is located at a distance of approximately 105.5 light-years from the Earth.

Big Dipper grouping of stars, part of 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.

Great Diamond asterism

The Great Diamond is an asterism. Astronomy popularizer Hans A. Rey called it the Virgin's Diamond. It is composed of the following stars:

Epsilon Piscium star

Epsilon Piscium is the Bayer designation for a star approximately 182 light-years away from the Earth, in the constellation Pisces. It is a yellow-orange star of the G9 III or K0 III spectral type, meaning it has a surface temperature around 5,000 kelvins. This is a normal giant star, slightly cooler in surface temperature, yet brighter and larger than the Sun. It is a suspected occultation double, with both stars having the same magnitude, separated by 0.25 arcsecond.

Psi<sup>1</sup> Piscium star

Psi1 Piscium is binary star in constellation of Pisces. It is approximately 280 light years from Earth, based on its parallax.


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