Asterism (astronomy)

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

In observational astronomy, an asterism is a pattern or group of stars that can be seen in the night sky. 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. 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.

The stars within an asterism may be physically associated. For example, the stars of Collinder 399, which resembles a coathanger, are members of an open cluster, the stars of Orion's Belt are all members of the Orion OB1 association, and five of the seven stars of The Plough are members of the Ursa Major Moving Group. In other cases, the stars are unrelated, such as in the Summer Triangle.

The 88 constellations into which the sky is divided are based on asterisms considered to represent an object, person, or animal, often mythological. However, they are formally defined regions of sky, and contain all the celestial objects within their boundaries. Asterisms do not have officially determined boundaries and are a more general concept which may refer to any identified pattern of stars.[1][2]

Background of asterisms and constellations[edit]

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]

Large or bright asterisms[edit]

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

Some major asterisms on a celestial map

Constellation-based asterisms[edit]

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.[9] 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.[6] 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]
  • 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.[10]
  • Boötes is sometimes known as the Ice Cream Cone.[11] It is also known as the Kite.[12]
  • The stars of Cassiopeia form a W which is often used as a nickname.[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").[9]

Commonly recognised asterisms[edit]

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.

Cross-border asterisms[edit]

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]

Telescopic asterisms[edit]

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[edit]

References[edit]

  1. ^ "An Etymological Dictionary of Astronomy and Astrophysics: constellation". Dictionary.obspm.fr. January 2018.
  2. ^ "An Etymological Dictionary of Astronomy and Astrophysics: asterism". Dictionary.obspm.fr. January 2018.
  3. ^ a b Allen, Richard H. (1899). Star Names: Their Lore and Meaning. Dover Publication* 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."[citation needed]. p. 11, p. 184–185. ISBN 978-0-486-21079-7.
  4. ^ AstronomyOnline: Image of Big Dipper, Diamond of Virgo, The Sail, Sickle, and Asses and the Manger, Astronomyonline.org
  5. ^ Spring triangle at Space.com, Accessed March 2011
  6. ^ a b c d e "Warren Rupp Observatory: Table of Asterisms". Wro.org. Retrieved 10 March 2019.
  7. ^ AstronomyOnline: Image of Cassiopeia, Square of Pegasus, The Circlet, and Y of Aquarius, Astronomyonline.org
  8. ^ 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.
  9. ^ a b c d e f "Asterisms". 14 February 2010. Archived from the original on 2010-02-14. Retrieved 10 March 2019.
  10. ^ Space.com: Hercules: See the Celestial Strongman Archived May 23, 2009, at the Wayback Machine
  11. ^ History of the Constellations: Bootes Archived May 12, 2011, at the Wayback Machine
  12. ^ "Astronomy Online - View Images Template". Astronomyonline.org. Retrieved 10 March 2019.
  13. ^ "Astronomy Online - View Images Template". Astronomyonline.org. Retrieved 10 March 2019.
  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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