Asterism (astronomy)

An asterism is an observed pattern or group of stars in the sky. Asterisms can be any identified pattern or group of stars, and therefore are a more general concept than the formally defined 88 constellations. Constellations are based on asterisms, but unlike asterisms, constellations outline and today completely divide the sky and all its celestial objects into regions around their central asterisms.[1][2] For example, the asterism known as the Big Dipper comprises the seven brightest stars in the constellation Ursa Major. Another is the asterism of the Southern Cross, within the constellation of Crux.

A picture of stars, with a group of appearingly bright blue and white stars. The bright stars together are identified as the asterism Coathanger resembling a coathanger, in the constellation Vulpecula.

Asterisms range from simple shapes of just a few stars to more complex collections of many stars covering large portions of the sky. The stars themselves may be bright naked-eye objects or fainter, even telescopic, but they are generally all of a similar brightness to each other. The larger brighter asterisms are useful for people who are familiarizing themselves with the night sky.

The patterns of stars seen in asterisms are not necessarily a product of any physical association between the stars, but are rather the result of the particular perspectives of their observations. For example the Summer Triangle is a purely observational physically unrelated group of stars, but the stars of Orion's Belt are all members of the Orion OB1 association and five of the seven stars of the Big Dipper are members of the Ursa Major Moving Group. Physical associations, such as the Hyades or Pleiades, can be asterisms in their own right and part of other asterism at the same time.

Background of asterisms and constellationsEdit

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.[citation needed]

Asterisms consisting of first-magnitude starsEdit

Some asterims consist completely of bright first-magnitude stars, which mark out simple geometric shapes.

Other asterisms consist partially of multiple first-magnitude stars.

All other first-magnitude stars are the only such stars in their asterisms or constellations, with Canopus in the Argo Navis asterism south of Sirius, visually east of the Carina Nebula and near the Large Magellanic Cloud (both being first-magnitude deep-sky objects), Achernar in the Eridanus constellation east of Canopus, Fomalhaut in the Southern Fish constellation east of Achernar and Antares in the Scorpius constellation visually near the Galactic Center.

 
Some major asterisms on a celestial map (the projection exaggerates the stretching)

Constellation-based asterismsEdit

 
The Big Dipper asterism
  • The Big Dipper, also known as The Plough or Charles's Wain, is composed of the seven brightest stars in Ursa Major.[5] These stars 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.
  • The Northern Cross in Cygnus.[4] The upright runs from Deneb (α Cyg) in the Swan's tail to Albireo (β Cyg) in the beak. The transverse runs from ε Cygni in one wing to δ Cygni in the other.
  • The Southern Cross is an asterism by name, but the whole area is now recognised as the constellation Crux. The main stars are Alpha, Beta, Gamma, Delta, and arguably also Epsilon Crucis. Earlier, Crux was deemed an asterism when Bayer created it in Uranometria (1603) from the stars in the hind legs of Centaurus, decreasing the size of Centaur. These same stars were probably identified by Pliny the Elder in his Naturalis Historia as the asterism 'Thronos Caesaris.'[3]
  • 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 Beta Scorpii (β Sco) and Pi Scorpii (π Sco) are replaced with a line from Beta through Delta Scorpii (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]
  • Boötes is sometimes known as the Ice Cream Cone.[9] It is also known as the Kite.[10]
  • The stars of Cassiopeia form a W which is often used as a nickname.[11]
  • The Great Square of Pegasus is the quadrilateral formed by the stars Markab, Scheat, Algenib, and Alpheratz, representing the body of the winged horse.[12] The asterism was recognized as the constellation ASH.IKU "The Field" on the MUL.APIN cuneiform tablets from about 1100 to 700 BC.[13]

Some asterisms may also be part of a constellation referring to the traditional figuring of the whole outline, for example Orion's Belt, and the Y in Aquarius (historically called "the Urn").[5]

Other particular asterismsEdit

 
The "Teapot" asterism in Sagittarius. The Milky Way appears as "steam" coming from the spout.

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

Asterisms across multiple constellationsEdit

Other asterisms 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.[citation needed]
  • 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.[citation needed]
  • The diamond-shaped False Cross is composed of the four stars Alsephina (δ Velorum), Markeb (κ Velorum), Avior (ε Carinae), and Aspidiske (ι Carinae).[14] 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 is sometimes mistaken, causing errors in astronavigation. Like the Southern Cross, three of its main four stars are whitish and one orange.[19]
  • From latitudes above 40 degrees north especially, a prominent upper-case Y is formed by Arcturus (α Boötis), Seginus (γ Boötis) and Izar (ε Boötis), 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.[citation needed]
  • The Lightning Bolt, visible in the northern sky's summer and autumn, consists of the stars Epsilon Pegasi, Alpha Aquarii, Beta Aquarii and Delta Capricorni.

Telescopic asterismsEdit

 
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 alsoEdit

ReferencesEdit

  1. ^ "An Etymological Dictionary of Astronomy and Astrophysics: asterism". Dictionary.obspm.fr. January 2018.
  2. ^ "An Etymological Dictionary of Astronomy and Astrophysics: constellation". Dictionary.obspm.fr. January 2018.
  3. ^ a b Allen, Richard H. (1899). Star Names: Their Lore and Meaning. Dover Publication. pp. 11, 184–185. ISBN 978-0-486-21079-7.
  4. ^ a b c d e Grilley, Michael (31 August 2018). "Table of Asterisms". wro.org. Archived from the original on 31 August 2018. Retrieved 11 May 2022.
  5. ^ a b c d e f "Asterisms". 14 February 2010. Archived from the original on 2010-02-14. Retrieved 10 March 2019.
  6. ^ Spring triangle at Space.com, Accessed March 2011
  7. ^ AstronomyOnline: Image of Big Dipper, Diamond of Virgo, The Sail, Sickle, and Asses and the Manger, Astronomyonline.org
  8. ^ Space.com: Hercules: See the Celestial Strongman Archived May 23, 2009, at the Wayback Machine
  9. ^ History of the Constellations: Bootes Archived May 12, 2011, at the Wayback Machine
  10. ^ "Astronomy Online – View Images Template". Astronomyonline.org. Retrieved 10 March 2019.
  11. ^ "Astronomy Online – View Images Template". Astronomyonline.org. Retrieved 10 March 2019.
  12. ^ AstronomyOnline: Image of Cassiopeia, Square of Pegasus, The Circlet, and Y of Aquarius, Astronomyonline.org
  13. ^ Rogers, J. H. (1 February 1998). "Origins of the ancient constellations: I. The Mesopotamian traditions". Journal of the British Astronomical Association. 108: 9–28. Bibcode:1998JBAA..108....9R.
  14. ^ a b "Starry Night Photography – Southern Cross, False Cross & Diamond Cross". Southernskyphoto.com. Retrieved 10 March 2019.
  15. ^ "Astronomy Online – View Images Template". Astronomyonline.org. Retrieved 10 March 2019.
  16. ^ "LacusCurtius • Allen's Star Names – Sagittarius". Penelope.uchicago.edu. Retrieved 10 March 2019.
  17. ^ Darling, David. "Ursa Major". Daviddarling.info. Retrieved 10 March 2019.
  18. ^ Darling, David. "Centaurus". Daviddarling.info. Retrieved 10 March 2019.
  19. ^ Moore, Patrick (2010). Patrick Moore's Astronomy: Teach Yourself. Hachette. ISBN 978-1444129779.
  20. ^ Constellation Guide, Orion Constellation (accessed 2014-03-03)
  21. ^ Chaple, Glenn (May 2019). "Spot the ring that hides in the Little Dipper". Astronomy.
  22. ^ "Asterisms – Broken Engagement Ring". Retrieved 2020-09-11.
  23. ^ "A star hop through Monoceros including M 50, The Christmas Tree Cluster (NGC 2264), Hubble's Variable Nebula (NGC 2261), NGC 2244, NGC 2301, The Rosette Nebula, 11 Beta Monocerotis, Harrington's Star 17 and Harrington's Star 18". Backyard-astro.com. Retrieved 10 March 2019.
  24. ^ a b "Asterisms". Deep-sky.co.uk. Retrieved 10 March 2019.
  25. ^ French, Sue (June 2017). "Deep Sky Wonders: Doodles in the Sky". Sky & Telescope: 56.
  26. ^ "Mon catalogue d'amas d'étoiles". Splendeurs du ciel profond. Retrieved 2020-05-27.
  27. ^ M. Odenkirchen & C. Soubiran (2002). "NGC 6994: Clearly not a physical stellar ensemble". Astronomy & Astrophysics. 383 (1): 163–170. arXiv:astro-ph/0111601. Bibcode:2002A&A...383..163O. doi:10.1051/0004-6361:20011730. S2CID 15545816.

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