Asterism (astronomy)

This picture of Brocchi's Cluster or "the Coathanger" asterism in the constellation Vulpecula was taken through binoculars.

In astronomy, an asterism is a pattern of stars seen in Earth's sky which is not an official constellation. Like constellations, they are composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth. An asterism may be composed of stars from one or more constellations. Their mostly simple shapes and few stars make these patterns easy to identify, and thus particularly useful to those just learning to orient themselves when viewing the night sky.

Background

Even before the dawn of civilization, it became common to clump various stars together in connect-the-dots stick-figure patterns. The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius. Historically, without an "official" list, there was really no difference between a constellation and an asterism. Anyone could arrange and name a grouping which might or might not be generally accepted. Still, some of our own constellations go back at least as far as the Babylonians.

Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 85–c 165). His list of 48 constellations was accepted as the standard for 1800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use the left over, non-figure ("amorphic") stars to create and squeeze in a new grouping among the established constellations. Two astronomers particularly known for attempting to expand Ptolemy's catalogue 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, unobserved by the ancients. Many of their proposed constellations have been accepted, the rest remaining asterisms, mostly obsolete. Clarification was necessary to determine which groupings are constellations and which stars belonged to them. The situation was finally regularized in 1930 when the International Astronomical Union (IAU) divided the sky into 88 official constellations with precise boundaries. Any other grouping is an asterism.

Points to bear in mind —

• 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

By happenstance, in each of the four seasons there is a large asterism visible near midnight, when observing from the northern hemisphere^[1]. Their component stars are bright and mark out simple geometric shapes.

• Spring is marked by the Diamond of Virgo consisting of Arcturus, Spica, Denebola, and Cor Caroli^[2]. An East-West line from Arcturus to Denebola forms an equilateral triangle with Cor Caroli to the North (Spring Triangle), and another with Spica to the South. Together these two triangles form the Diamond. Stretching from 38°N to 11°S, the Diamond is too large to be seen all at once. Formally, the stars of the Diamond are located in the constellations Boötes, Virgo, Leo, and Canes Venatici.

• 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^[3]. The stars of the Triangle are located in the band of the Milky Way which marks the galactic equator.

• 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^[4]. 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 700—1100 BC^[5].

• Fully one-third of the 1st magnitude stars visible in the sky (seven of twenty-one) are in the Winter Hexagon with Sirius, Procyon, Pollux - toss in 2nd magnitude Castor - Capella, Aldebaran, and Rigel on the periphery, and Betelgeuse located off-center^[3]. Although somewhat flattened, and thus more elliptical than circular, the figure is so huge that it is impossible to take it all in at a single glance, 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'^[6].

Other examples

The Big Dipper asterism

One of the best-known asterisms is the Big Dipper or Plough. It is composed of the seven brightest stars in Ursa Major^[6], where they delineate the Bear's hindquarters and exaggerated tail. With its longer tail, Ursa Minor hardly appears bearlike at all, and is widely known by its pseudonym, the Little Dipper^[7].

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^[3]. 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^[8]
• Although hardly an ancient notion, it is easy to see why the Ice Cream Cone is sometimes applied to Boötes^[9]. It is even better-known as the Kite^[7].
• The stars of Cassiopeia form a W which is often used as a nickname^[10].
• 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:

• the Belt of Orion
• The Y in Aquarius (historically called "the Urn"),
• Hercules' Club.

There are many others^[6].

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^[3].
• 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^[6].
• The bow and arrow of the Archer also make a well-formed Teapot^[11]. (There is even a bit of nebulosity near the "spout" to serve as steam).

• Alpha, Beta, Gamma, and Delta Delphini form Job's Coffin^[6].
• The Terebellum is a small quadrilateral of four faint stars (Omega, 59, 60, 62) in Sagittarius' hindquarters^[12].
• Four other stars (Beta — Miaplacidus, Upsilon, Theta, and Omega Carinae) form a well-shaped diamond — the Diamond Cross^[13].
• The Saucepan 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^[3][6].
• 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^[14]: 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^[15] 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 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)^[13]. 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 by ships' navigators.

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 recognized by the eye^[16].
• Kemble's Cascade in Camelopardalis is a chain of stars (the waterfall) that ends in open cluster NGC 1502^[16].
• Approximately 40 stars in Monoceros, including a portion of NGC 2264, fill out the triangular Christmas Tree Cluster.^[17]

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 200s BCE, 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.^[18] 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."^[19] 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.

See also

• Former constellations
• Nakshatra
• Chinese constellation
• Australian Aboriginal astronomy

Notes

References

• Allen, Richard Hinckley (1969). Star Names: Their Lore and Meaning. Dover Publications Inc. (Reprint of 1899 original). ISBN 0-486-21079-0.
• Burnham, Robert (1978). Burnham's Celestial Handbook (3 vols). Dover Publications Inc. ISBN 0-486-23567-X, ISBN 0-486-23568-8, ISBN 0-486-23673-0.
• Michanowsky, George (1979). The Once and Future Star. Barnes and Noble Books. ISBN 0-06-464027-2.
• Pasachoff, Jay M. (2000). A Field Guide to the Stars and Planets (4th ed.). Houghton Mifflin Co. ISBN 0-395-93431-1