Asterism (astronomy)

Updated on Nov 12, 2023

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In astronomy, an asterism is any pattern of stars recognized in the Earth's night sky. It may be part of an official constellation or it may be composed of stars from more than one constellation. Asterisms are composed of stars which, although visible in the same general area of the sky as viewed from Earth, are located at very different distances from Earth, at great distances from each other. Many asterisms are simple shapes composed of a few bright stars, making them easy to identify, and particularly useful to people who are familiarizing themselves with the night sky.

In colloquial usage, constellation is a synonym of asterism. But in astronomy, a constellation is an officially recognized area of the sky, surrounding certain asterisms. For example, the asterism known as the Big Dipper comprises the seven brightest stars in the International Astronomical Union (IAU) constellation Ursa Major. Asterism is the more general term, referring to any identified pattern of stars.

Background

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. Anyone could arrange and name a grouping, which could even overlap with other groupings, and since there was no "official" list, there was no difference between a constellation and an asterism.

An official list began to develop with Greco-Roman astronomer Claudius Ptolemy of Alexandria (c. 100 – c. 170), whose list of 48 constellations was 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. Furthermore, exploration by Europeans to other parts of the globe exposed them to stars unknown to ancient Mediterraneans. Two astronomers particularly known for expanding on Ptolemy's catalog were Johann Bayer (1572–1625) and Nicolas Louis de Lacaille (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. Many of their proposed constellations have been accepted, with the rest remaining as asterisms. In 1930, the International Astronomical Union (IAU) divided the sky into 88 official constellations surrounding recognized asterisms, with precise geometric boundaries that encompass all of the stars within them. Any grouping – whether it has an area of the sky identified by it or not – 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 magnitude, 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 cluster (see below), whose stars are gravitationally related, is not an asterism.

Large seasonal asterisms

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

Spring. Spring is marked by the Great Diamond consisting of Arcturus, Spica, Denebola, and Cor Caroli. 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, Regulus, Spica triangle is given the name Spring Triangle. Together these two triangles form the Diamond. Formally, the stars of the Diamond are in the constellations Boötes, Virgo, Leo, and Canes Venatici.

Summer. The Summer Triangle of Deneb, Altair, and Vega — α Cygni, α Aquilae, and α Lyrae — is easily recognized, as its three stars are all of the 1st magnitude. The stars of the Triangle are in the band of the Milky Way which marks the galactic equator, and are in the direction of the galactic center.

Autumn. The Great Square of Pegasus is the quadrilateral formed by the stars α Pegasi, β Pegasi, γ Pegasi, and α Andromedae, 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.APIN cuneiform tablets from about 1100 to 700 BC.

Winter. The winter midnight sky of the northern hemisphere is dominated by Orion, in the direction opposite the galactic center. Also, one-third of the 1st-magnitude stars visible in the sky (seven of twenty-one) are in the Winter Hexagon with Sirius, Procyon, Pollux, Rigel with 2nd-magnitude Castor - Capella, and Aldebaran, on the periphery, and Betelgeuse off-center. Although somewhat flattened, and thus more elliptical than circular, the figure is so large that it cannot be taken in all at once, 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'. The Procyon, Betelgeuse and Sirius triangle is given the name Winter Triangle.

Other examples

One of the best-known asterisms is the Big Dipper or Plough. It is composed of the seven brightest stars in Ursa Major, where they 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, 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 Cygnus. The upright runs from Deneb (α Cyg) in the Swan's tail to Albireo (β Cyg) in the beak. The transverse runs from Gienah (ε Cyg) in one wing to Delta Cygni (δ Cyg) in the other.

The Fish Hook is the traditional Hawaiian name for Scorpius. The image will be even more obvious if the chart's lines from Antares (α Sco) to Graffias (β Sco) and Pi Scorpii (π Sco) are replaced with a line from Graffias through Dschubba (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 Hercules 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 Boötes. It is even better known as the Kite.

The stars of Cassiopeia form a W which is often used as a nickname.

In Australia, "Frying Pan" for Chamaeleon, 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:

Orion's Belt/The Belt Of Orion

The Y in Aquarius (historically called "the Urn")

Hercules's Club

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 Hercules, Epsilon (ε Her), Zeta (ζ Her), Eta (η Her), and Pi (π Her), form the well-known Keystone.

The curve of stars at the front end of the Lion from Al Ashfar (ε Leo/Epsilon Leonis) to Regulus (α Leo/Alpha Leonis), looking much like a mirror-image question mark, has long been known as the Sickle.

The bow and arrow of the Archer also make a well-formed Teapot. (There is even a bit of nebulosity near the "spout" to serve as steam).

Alpha, Beta, Gamma, and Delta Delphini form Job's Coffin.

The Terebellum is a small quadrilateral of four faint stars (Omega, 59, 60, 62) in Sagittarius' hindquarters.

Four other stars (Beta — Miaplacidus, Upsilon, Theta, and Omega Carinae) form a well-shaped diamond — the Diamond Cross.

The Saucepan or Pot 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 Pavo.

Just south of Pegasus, the western fish of Pisces is home to the Circlet formed from Gamma, Kappa, Lambda, TX, Iota, and Theta Piscium.

Dubhe and Merak (Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of the Big Dipper 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 Polaris (α UMi/Alpha Ursae Minoris), the North Star.

Alpha and Beta Centauri are the Southern Pointers leading to the Southern Cross and thus helping to distinguish Crux 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. Sirius (α CMa), Procyon (α CMi), and Betelgeuse (α Ori) form one to the North (Winter Triangle) while Sirius, Naos (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 Draco and one - Iota Herculis - in the foot of Hercules.

The diamond-shaped False Cross is composed of the four stars Delta and Kappa Velorum (δ and κ Vel) and Epsilon and Iota Carinae (ε and ι Car). Although its component stars are not quite as bright as those of the Southern Cross, it is somewhat larger and better shaped than the Southern Cross, for which it was often mistaken, causing errors in astronavigation. Like the Southern Cross, three of its main four stars are whitish and one orange.

From latitudes above 40 degrees north especially, a prominent upper-case Y is formed by Arcturus, Gamma and Epsilon 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 Kingdom in particular, where there is serious light pollution in many areas and also twilight 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 Coathanger, formally open cluster Collinder 399, in Vulpecula may be seen with the eye.

Kemble's Cascade in Camelopardalis is a chain of stars (the waterfall) that ends in open cluster NGC 1502.

Approximately 40 stars in Monoceros, including a portion of NGC 2264, fill out the triangular Christmas Tree Cluster.

Former asterisms

Argo is a special case. Argo Navis, ("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, Puppis, and Vela), 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 was an asterism when Bayer created it in Uranometria (1603) from stars in the hind legs of Centaurus. It was given constellation status in 1930, thereby mutilating the Centaur.

Centaurus had been reduced in size once before. Lupus was originally considered to be merely a sectional asterism (as an unspecified Wild Beast in the Centaur's grasp). Hipparchus split it off in the 3rd century BC, and Ptolemy's list confirmed its independent status.

In its original figuration, Leo 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's acceptance of Coma, Bayer recorded it and refigured the Lion. The IAU confirmed Coma's status as a constellation.

Even so, venerable a constellation as Libra was once merely an asterism. Until the middle of the first millennium BC, the Zodiac consisted of only eleven constellations. The biblical reference to "the eleven stars" (Genesis 37:9) is more accurately "the eleven asterisms/constellations (of the Zodiac)." At the time, Scorpius' claws were pictured as extending to Zubenelgenubi, "the southern claw" and Zubeneschamali, "the northern claw" (Alpha and Beta Librae). Later, when Virgo 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.

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 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 early telescopic discoveries. Similarly, the Magellanic Clouds are not asterisms, but galaxies in their own right.

Nebulae, clouds of gas and dust that dot the galaxy, whether emission, such as the Pelican, or dark, such as the Horsehead, 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 (M45) and the Hyades in Taurus and the Beehive (M44) in Cancer. (It may be noted that, with the addition of Aldebaran, 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 (θ Ori) is embedded in, and illuminates, the Orion Nebula (M42). Looked at telescopically, it resolved into four stars arranged in a trapezoid, and they were nicknamed the Trapezium. 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 in Aquarius, 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.

References

Asterism (astronomy) Wikipedia

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