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This Entry is about the star-shapes which aren't constellations1 in their own right, but are instantly recognisable to amateur astronomers who invariably find their way around the night sky by referencing them. An asterism is a familiar (to night-sky watchers) group of stars which forms a pattern and the stars aren't part of a related cluster but just a chance alignment. Asterisms are also a good way to start children off with astronomy.
Since humans first looked up at the night sky and wondered about the twinkling lights, the star patterns have remained constant (except for the odd supernova2, nova and comet3). Asterisms have therefore been useful to ancient mariners since before the days of satellite navigation. For example, the poet Callimachus wrote in 240 BC: Tempt not the winds forewarned of dangers nigh, when the Kids glitter in the western sky. The Kids – the asterism Haedi (meaning the goat kids) – are a pair of stars, zeta and eta Aurigae, which make an appearance in October. Roman naturalist Gaius Plinius Secundus (23 - 79 AD), better known as Pliny the Elder, reinforced this view, declaring that voyages were hazardous from this time. Once the Kids' dire influence had waned, a feast4 called the Natalis Navigationis marked the recommencement of sea voyages.
Different cultures have their own nicknames for such star groupings, and some asterisms have several names, eg the pan-shape (probably the best-known asterism) contained within Ursa Major 'the Great Bear' is known as 'The Plough', 'The Big Dipper' or even just 'The Pan'.
The Plough/Big Dipper
Ursa Major is one of the more famous of the northerly constellations. The seven brightest stars form the most well-known asterism; it has many different names depending upon which part of the world you hail from:
- The Plough (formerly King Charles' Wain) [UK]
- The Big Dipper [USA]
- Großer Wagen (Great Cart) [Germany]
- Karlavagnen (Charles' Wain or Wagon) [Sweden]
- Saptarshi (Seven rishis – sages or wise men) [India]
- Hokutoshichisei (Northern Saucepan) [Japan]
- Leg and haunch (of a bull) [ancient Egypt]
- Hunting Party and Bear [Native American]
- Buruj Biduk (the Ladle) [Malaysia]
- Bukduchilseong (Northern Saucepan) [Korea]
- Grote Beer (Great Bear) or Steelpannetje (saucepan) [The Netherlands]
- Göncölszekér (medicine man's cart) [Hungary]
The stars which make up the asterism are labelled alpha, beta, gamma, delta, epsilon, zeta and eta in that order, ending at the end of the 'handle'. Johann Bayer made an exception to his rule in their case. The usual way to classify the stars was to give the brightest of the constellation the 'alpha' status, followed by the next brightest getting 'beta' and so on. Bayer designated these stars in order that they appeared in the asterism, so their magnitudes don't match their order. Of the entire constellation, the brightest star is epsilon (Alioth) at +1.7 mag. The seven stars of the asterism all have proper names; some have several because this shape of stars was important to so many cultures all over the world. The asterism appears on the Cherokee Peace Flag: seven red seven-point stars on a white background. The Alaska state flag is blue with the seven stars of the Big Dipper and the northern pole star5 Polaris (alpha Ursae Minoris) depicted in yellow.
|alpha||Dubhe||Dubb||+1.8||124||Official State Star of Utah|
|beta||Merak||Meraqq||+2.3||79||Has a dust disk|
|gamma||Phad||Phecda||+2.4||84||Part of Collinder 285|
|delta||Megrez||Kaffa||+3.3||81||Triple star system|
|zeta||Mizar||Arundhati||+2.2||78||Multiple star system|
|eta||Alkaid||Benetnash||+1.8||101||Leader of 'the mourning maidens'|
The Great Square of Pegasus
Some of the familiar asterisms cover two or more separate constellations, eg the Great Square of Pegasus. The enormous box is an easily recognisable feature, with four bright corners and very few visible stars inside the square. Three of the four corners are alpha Pegasi (Markab) in the south-west, gamma Pegasi (Algenib) in the south-east, and beta Pegasi, the red6 giant Scheat, at the northwestern point. The fourth corner, however, presents one of those anomalies that occurs occasionally in the nomenclature of astronomy. The northeastern star of the square is alpha Andromedae but used to be delta Pegasi, a border star between Pegasus and Andromeda.
Both Pegasus and Andromeda featured in the Greek astronomer Ptolemy's Almagest, and the star at the top left corner of the square was listed as being in both constellations. Johann Bayer, when he came up with his system of naming the stars, continued the tradition of this star belonging to both Pegasus and Andromeda, cataloguing it alpha Andromedae and delta Pegasi. It also bears the names Alpheratz and Sirrah. In 1930 the International Astronomical Union (IAU) decided to clear up the ambiguous borders of the ancient star patterns and set clearly defined limits for all of the 88 constellations. They would not allow a star to be in two different constellations; the star became a member of Andromeda only, and the name delta Pegasi is no longer used. So the Great Square of Pegasus now has only three corners in the winged horse constellation, the other corner belongs to Andromeda.
Orion is visible in both hemispheres, so most of the world's population has seen Orion's Belt. It is composed of three blue-white supergiant stars – easily identifiable, although their individual names are not so well-known. The closest one of the trio at 800 light years is Alnitak, which means 'girdle'. From the Northern Hemisphere this star is the lowest and appears on the left of the other two. The centre star and most distant at over 1,000 light years away is Alnilam, its name means 'a belt of pearls'. Mintaka 'the belt' on the right is 900 light years distant. From our perspective we see a line of three stars marking the waist of the giant hunter – his spear arm raised, poised to strike – chasing down the celestial bull every night.
Neolithic people of what is now North Yorkshire in England built the Thornborough Henges over 5,000 years ago. After setting up a 3D model of the complex in 2006, researchers from Newcastle University confirmed that the Thornborough Henges were deliberately aligned to mirror the stars of Orion's Belt.
Thornborough was a sacred landscape, a place of religious worship, and we should try to interpret these astronomical orientations within that context. This astronomical association was emphasised by the banks of the henges being coated in brilliant white gypsum. Neolithic people surely felt they were at the centre of the very cosmos as they worshipped the heavens above.
– Senior Lecturer Dr Jan Harding of Newcastle University
Some people believe that the three pyramids at Giza in Egypt were deliberately sized and placed in order to represent the stars which form Orion's Belt. While this may ultimately remain speculative, it is known that the ancient Egyptians definitely believed in an afterlife, and that the souls of their dead pharaohs journeyed to join their gods via the stars. They knew the constellation of Orion as Osiris who was, among other things, their god of life and death.
The ancient Mayans, unconnected with any other cultures outside of their homeland, were equally fascinated by the asterism of Orion's Belt. They also built structures which are almost identical to the alignment of the three bright stars at Orion's waist. Such fascinating stories sell lots of books, so several novels have been written about the 'Orion complex'.
The Summer Triangle
The Summer Triangle was dubbed by author and TV presenter Sir Patrick Moore, but the actual stars have been around a lot longer than he has! This geometric group of three stars is tri-constellational, created by:
- Vega (alpha Lyrae) – the first star ever to be photographed, on the night of 16 July, 1850, by JA Whipple
- Deneb (alpha Cygni) – the tail of 'the swan'
- Altair (alpha Aquilae) spins so fast, once every 6.5 hours, that it loses its shape!
This three-sided figure only looks like a flat triangle from our perspective because we see it in two dimensions. The actual stars are some distance from each other, and if we could travel towards them the 'triangle' would disappear. Altair is a close (in astronomy terms) neighbour at a mere 17 light years7 distance. Vega is a sometime 'pole' star due to the precession of the Earth's axis. It is also relatively close at only 25 light years away. Deneb is a blue supergiant star much, much further away from either of its two fellow sparkling points. At approximately 3,200 light years distant we can only see it because it is one of the most luminous stars known.
The area the Summer Triangle covers is so large that it completely encloses one of the smaller constellations, Sagitta 'the arrow', which itself is 80 square degrees.
The Northern Cross
Five of the stars of Cygnus 'the swan' form the Northern Cross, or what German cartographer and astronomer Julius Schiller8 called the Cross of Calvary (which Jesus was crucified upon). The stars which make up the cross are: Deneb (top), Gienah (left), Sadr (centre), delta (right) and Albireo (base). The asterism can clearly be seen in the northwest in December, looking like a cross mapped in stars, hovering just above the ground, and rising into the sky. Towards the end of December on the opposite side of the sky, Praesepe (also known as the Beehive Cluster and M44) rises as the Northern Cross sets. Praesepe is Latin for 'manger' or 'crib'.
The upper half of the constellation Hercules, formed by the stars pi, eta, zeta and epsilon, is commonly known as the Keystone. The shape of the four stars when joined up in the imagination looks like the central stone in an door's archway, hence the asterism's moniker. If you know your Messier objects, then the Keystone is easy to locate because M13, the most well-known globular cluster of all, is between eta and zeta Herculis from our line of sight.
The Winter Triangle
The Winter Triangle asterism has a separate, more detailed Entry. It contains three prominent winter (in the Northern Hemisphere) stars, each from a different constellation. The triangle is made up of:
The Winter Hexagon
Somewhat larger than the Winter Triangle is the Winter Hexagon asterism. Find Betelgeuse then look at the bright stars encircling it; make sure you look early enough to include bright Sirius. The six stars that make up the Winter Hexagon are Capella (alpha Aurigae), Castor or Pollux of Gemini, Procyon, Sirius, Rigel and Aldebaran. The arm of our Milky Way galaxy runs through the middle of the asterism (between Capella and Sirius), so the Winter Hexagon is a prominent 'shape' to distinguish from a dark-sky viewing area.
The Coathanger, an asterism contained within Vulpecula 'the fox', has the official designation Collinder 399 and two common names, Brocchi's Cluster and Al Sufi's Cluster. Persian astronomer Abd-al-Rahman Al Sufi (903 - 986) described the 'nebulous object' in his Book of Fixed Stars published in 964. We now know that the stars aren't related so the group is not a real cluster, but the common names remain to honour the original discoverer and US amateur astronomer DF Brocchi who mapped it in the 1920s. The asterism is too faint to be seen with the naked eye but it can be viewed through binoculars or a telescope.
The Great Diamond of Spring
Cor Caroli (which means 'heart of Charles') is the brightest star of Canes Venatici, a third magnitude double star and one of the four partners of a little-known asterism, the Great Diamond of Spring. The other three stars completing the diamond shape are: Arcturus (alpha Boötis), Spica (alpha Virginis) and Denebola (beta Leonis).
The Great Diamond of Spring entirely encompasses the constellation of Coma Berenices, which itself contains eight objects featured in the famous Messier catalogue: three globular clusters and the nearest massive cluster of galaxies (known as the 'Coma Cluster'). Therefore this asterism is worth learning for new telescope owners as there is a whole treasure chest of glorious sights to be sought out.
Just south of the distinctive Great Square of Pegasus is the much-fainter constellation of Pisces. It straddles the ecliptic (the apparent path of the Sun through the sky), so if you're in the Northern Hemisphere you'd need a high stance (top floor of a building, or a hill) to view the asterism known as 'the Circlet' which is supposed to represent the head of the lower (western) fish. The group of seven stars which make up the Circlet are: gamma, 7 Psc, theta, iota, 19 Psc (also known as TX Piscium), lambda and kappa. The brightest of the group is gamma at +3.7 mag, then the stars dim as low as +5.5 mag. The stars' distances from us range from 45 light years (yellow dwarf iota) to the irregular variable carbon star TX Piscium which is 760 light years away. TX Piscium is a deep red colour and a jewel to behold in binoculars if the viewer has the opportunity of a clear horizon to detect it.
The constellation Leo 'the lion' boasts a quite striking asterism. The stars which form the head of the lion (eta, gamma, zeta, mu and epsilon) create a shape like a backwards question mark, with the striking royal star Regulus (alpha Leonis) marking the point. In olden times it was considered by some cultures to be a constellation in its own right, and bore the name The Sickle9.
A stunningly beautiful cascade of unrelated stars lies within the neck of Camelopardalis 'the giraffe', one of the most northerly constellations. This asterism is named 'Kemble's Cascade' after Fr Lucian Kemble, a Franciscan monk and amateur astronomer who died in 1999.
The Teapot and Teaspoon
Someone once joined up the dots on a map of Sagittarius and decided they had made a teapot complete with teaspoon. This asterism has been snapped up by enterprising salespeople on the World Wide Web in the hopes of cornering a specific market: what to buy the tea-loving astronomer friend who is so hard to buy for? The only answer has to be the Teapot Asterism mug. If they already own a such a mug, you could always get them the t-shirt instead!
The Ben Ben
The stars alpha, gamma and beta Hydri form a triangle shape when connected in the imagination. This was viewed as auspicious by cultures such as the ancient Egyptians, who designed and built pyramids from that basic geometric shape. They knew the triangle shape as a Ben Ben, therefore the corresponding star alignment in the night sky earned the asterism the name 'Ben Ben'. It was believed by the ancient Egyptians that this was the resting place of the Bennu (phoenix) before it burst into flames and rose to its own place in the heavens.
M73: from Cluster to Asterism
The Y-shaped group of four stars situated near the boundary of Aquarius was discovered in October 1780 by Charles Messier, who described it as 'a cluster with very little nebulosity' and catalogued it M73. Since then it has been studied to find out whether it is a related cluster or just a chance alignment. By 2001 it had been classified a 'possible Open Cluster remnant'. However in 2002, after an intense study of spectroscopic data at the Observatory in Haute Provence, France, that description was changed to 'four independent stars at different distances' – so M73 is now officially an asterism. The following data (epoch 2000.0) shows just how close the stars appear to each other, and their various distances from us:
The False Cross
Just as asterisms and constellations could aid ancient mariners, so too could there be disaster if one were mistaken. A few degrees to the west of Crux in the southern hemisphere there is a pattern of stars almost identical to that striking celestial marker. A pair from Carina, epsilon and tau Carinae, and two from Vela, delta and chi Velorum, form such an uncanny resemblance to the 'southern cross' constellation that they have been dubbed the 'false cross' asterism, because of their ability to lead inexperienced sailors astray.
The Argo Navis
Asterisms can also be a defunct constellation. In the Southern Hemisphere there is an enormous star grouping which used to represent the legendary ship Argo which transported Jason and his Argonauts to their fabled destinations. Argo Navis was one of the original 48 constellations of Ptolemy's time. In the 18th Century the massive group of stars which was Argo Navis was broken up into three separate 'modern' constellations by French astronomer Nicolas Louis de Lacaille10 (1713 - 62). Carina 'the Keel', Vela 'the Sail' and Puppis 'the Stern' collectively commemorate the dismemberment of the constellation which represented the Argonauts' vessel.
The Argo Navis is the only one of Ptolemy's constellations which is no longer used. In 1930 the IAU set the sky into area rather than pattern and defined the borders of 88 official constellations. They decided to keep Lacaille's three smaller constellations, demoting the Argo Navis to asterism and the majestic constellation sailed into the halls of history.
Constellations are even mentioned in The Bible:
The waters are hid as with a stone, and the face of the deep is frozen. Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion? Canst thou bring forth Mazzaroth11 in his season? Or canst thou guide Arcturus with his sons? Knowest thou the ordinances of Heaven? Canst thou set the dominion thereof in the Earth? Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee?
– Book of Job 38:30-34
In Delphinus 'the dolphin' there is an asterism of four stars: alpha, beta, gamma and delta Delphini. It has been known as 'Job's Coffin' since time immemorial. Job (pronounced Jobe) was a biblical character of some repute, he was a favourite of God, was tested by the Devil, earned a fountain of youth and lived for 140 years.
Friedrich's Ehre (Friedrich's Honour) was a short-lived creation of German astronomer Johann Elert Bode (1747 - 1826) in an attempt to immortalise Frederick the Great of Prussia in the night sky. Bode plucked four stars from Andromeda which formed a 'Y' shape and declared them a royal staff and crown surrounded by a laurel wreath. The Friedrich's Ehre asterism didn't appear in any later astronomy catalogues and it is all but forgotten today. The component stars are still there though, nestled in their original constellation Andromeda.
Below a nimbus the sign of royal dignity hangs, wreathed with the imperishable laurel of fame, a sword, a pen, and an olive branch, to distinguish this ever to be remembered monarch as hero, sage and peacemaker.
– Bode's description of his liege lord Frederick the Great of Prussia
Seeing and Making Shapes
As an interesting aside, an asterism can be created by connecting the four neighbourly first magnitude stars Sirius, Rigel (beta Orionis), Aldebaran12 and Betelgeuse. If you were to sketch this freehand on a star map you'd end up with the shape of an eye or flower petal, although US astronomers would probably compare it to an American football.