Asterism (astronomy)

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Collinder 399, an asterism in the constellation Vulpecula that resembles a coathanger. Cr399.jpg
Collinder 399 , an asterism in the constellation Vulpecula that resembles a coathanger.

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 (as in, by the IAU) 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.

Contents

Asterisms range from simple shapes of just a 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 asterism 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.

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 ancient India in the Vedanga Jyotisha and the Babylonians.[ citation needed ] 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]

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.[ citation needed ]

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.[ 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, 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 asterisms, but this depends on the source.[ citation needed ]

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

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

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

See also

Notes

  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.

Related Research Articles

Arcturus Star in the constellation Boötes

Arcturus, designation α Boötis, is the brightest star in the constellation of Boötes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere. Together with Spica and Denebola, Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli. When viewed from Earth, it appears to be positioned almost at the north galactic pole of the Milky Way.

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 .

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.

Carina (constellation) Constellation in the southern celestial hemisphere

Carina is a constellation in the southern sky. Its name is Latin for the hull or 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.

Centaurus 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.

Delphinus Constellation in the northern celestial hemisphere

Delphinus is a small constellation in the Northern Celestial Hemisphere, close to the celestial equator. Its name is the Latin version for the Greek word for dolphin (δελφίνι). 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. It is one of the smaller constellations, ranked 69th in size. Delphinus' five brightest stars form a distinctive asterism symbolizing a dolphin with four stars representing the body and one the tail. It is bordered by Vulpecula, Sagitta, Aquila, Aquarius, Equuleus and Pegasus.

Pisces (constellation) Zodiac constellation straddling the celestial equator

Pisces is a constellation of the zodiac and is located in the Northern celestial hemisphere. 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 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 name is Latin for "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.

Scorpius Zodiac constellation in the southern celestial hemisphere

Scorpius is one of the constellations of the zodiac and is located in the Southern celestial hemisphere. 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-dates 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.

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.

Ursa Minor Constellation in the northern celestial hemisphere, containing the northern celestial pole

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, hence the North American name, Little Dipper: seven stars with four in its bowl like its partner the Big Dipper. It 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.

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. The ecliptic intersects the celestial equator within this constellation and Pisces. 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.

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 observers 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.

Beta Andromedae Star in the constellation 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.

Ursa Major Moving Group

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.

Big Dipper Large grouping of stars, part of Ursa Major constellation

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.

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:

Gemini (constellation) Zodiac constellation in the northern hemisphere

Gemini is one of the constellations of the zodiac and is located in the northern celestial hemisphere. It was one of the 48 constellations described by the 2nd century AD astronomer Ptolemy, and it remains one of the 88 modern constellations today. Its name is Latin for twins, and it is associated with the twins Castor and Pollux in Greek mythology. Its symbol is .

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Bibliography