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Understanding the Differences Between Asterisms and Constellations

Asterism Vs. Constellation: What's the Difference?
Out of the uncountable yet identifiable grouping of stars in the sky, only a few are referred to as constellations. This is due to the fact that only these have been recognized as constellations by the International Astronomical Union. UniverSavvy focuses on the asterism vs. constellation debate, to bring out the differences between the two.
Smita Pandit
Last Updated: Mar 19, 2018
Did You Know?
Constellations have long been defined on the basis of the pattern or the arrangement of stars, but it was through a series of resolutions from 1922 to 1930, that the International Astronomical Union (IAU) divided the celestial sphere into 88 constellations, thereby defining constellation boundaries.
Constellations that have been identified by the IAU divide the celestial hemispheres into regions, which in turn helps to locate various celestial objects. To a layman, constellations are groups of stars that form patterns. In fact, there are myths associated with them, and they are represented by mythical figures or objects. For instance, the Orion constellation is named after Orion, a giant Boeotian hunter who pursued Pleiades, the seven daughters of Atlas. According to Greek mythology, he was killed by Artemis, and placed in the sky as a constellation. The constellation comprises seven main stars, with two stars each representing the shoulders and feet, and three stars making up the Orion's belt. Basically, dividing the groups of stars into identifiable patterns makes the task of locating stars and other celestial objects easier.

Besides the 88 constellations that are recognized by the IAU, there are several groups of stars that form patterns in the sky. These are called asterisms. It must be noted that asterisms could be formed by stars belonging to one or more constellations. An asterism is a part of one or more constellations, but is not recognized as a constellation by the IAU. It must be noted that the stars in an asterism could be located at varied distances from the solar system.
Asterism Vs. Constellation
Asterisms are defined as a pattern of bright stars that can be identified on the Earth's night sky. Basically, asterisms are groups of stars that form a recognizable pattern. These stellar groupings are known by historical names, but don't figure in the list of 88 constellations.
Big Dipper
Big Dipper
✦ One of the common examples that clearly differentiate asterisms from constellations is that of the Big Dipper. Known by different names, Big Dipper was referred to as 'Seven Macaw' by Mayans, and 'Saptarshi Mandal or Seven Sages' by Hindus. In Southern France, its shape is likened to a 'saucepan', whereas it is represented as a 'plow' in Britain. More often than not, the Big Dipper is mistakenly referred to as a constellation.
Ursa Major Constellation
Ursa Major constellation represented as bear
Basically, the Big Dipper is an asterism that is located within the Ursa Major constellation, which in turn is a region of the sky. While Ursa Major is a region in the sky that contains several stars, the Big Dipper refers to the bowl-and-handle pattern formed by seven bright stars in the Ursa Major constellation.
Big Dipper and Little Dipper in Ursa Major and Ursa Minor
Ursa major and Ursa minor
Besides the symbolic interpretation of Big Dipper, with four stars making up the bowl, and three making up the handle, this pattern also helps point towards other stars. For instance, the outer two stars on the bowl point towards the Pole Star, while the handle points to the direction of Arcturus, which is the fourth brightest star, as well as the brightest star in the constellation, Bootes. Even the Little Dipper in Ursa Minor constellation is an asterism.
Bright Vega, Deneb, and Altair stars in Summer Triangle Asterism
bright stars
✦ Asterisms can also extend over two or more constellations. The Summer Triangle is an example of one such asterism. It is formed by three bright stars that are a part of three constellations. As the name suggests, it forms a triangular pattern. The three stars that form this asterism are Vega, Deneb, and Altair. Vega is a brightest star of the Lyra constellation, whereas Deneb and Altair are the brightest stars in Cygnus constellation and Aquila constellation, respectively.
Stars within Summer Triangle Asterism
Milky way and summer triangle
Summer Triangle Asterism over Monte Bravo
Summer triangle and Milky way above Monte Bravo
Northern Cross Asterism in Cygnus Constellation
Northern cross asterism
✦ Easy to locate during summer, the Northern Cross is an asterism that is formed by the stars of the Cygnus constellation. It is made by Deneb, Sadr, Albireo, Rukh, and Gienah. Deneb is a part of the Summer Triangle asterism, and the Northern Cross can be found within the latter.
Orion's Belt Asterism
Orion constellation in night sky
✦ There are several groups of stars that lie within constellations. For instance, Orion's belt, which comprises three stars called Alnitak, Alnilam, and Mintaka, located at the center of the Orion constellation, are a group of stars that often helps in locating this constellation.
Y-shaped Asterism in Aquarius Constellation
Aquarius constellation
✦ The Aquarius constellation also comprises an asterism. Its Y-shaped asterism is referred to as the 'Water Jar or Urn' of Aquarius. It can be observed at the top left in the image given above. At the top left side of the image, the Y shape is formed by Zeta Aquarii at the center, Seat (Pi) at the upper right of the 'Y', and Eta to the left. The asterism is at the left of the alpha or Sadalmelik Aquarii. To the right of Sadalmelik Aquarii lies the brightest star of the constellation, which is called Sadalsuud Aquarii.

✦ In the Northern Celestial Hemisphere, stars Cor Caroli (in Canes Venatici), Denebola (the tail of Leo), Spica (the wheat of Virgo), and Arcturus (in Bootes) form an asterism, which can be seen during spring. This asterism is called the Great Diamond. The triangular pattern formed by Denebola, Spica, and Arcturus makes up another asterism called the Spring Triangle.

✦ The Great Square of Pegasus is another asterism that can be seen in the night sky in the Northern Celestial Hemisphere during autumn. It is a quadrilateral formed by four stars: α Pegasi, β Pegasi, γ Pegasi, and α Andromedae.

✦ Winter Circle, which is sometimes called Winter Hexagon, is an asterism that can be seen during the winter months in the Northern Celestial Hemisphere. Betelgeuse, which is the bright red star of the Orion constellation, lies at the center. Other stars include Rigel, Aldebaran, Capella, Procyon, Sirius, Castor, and Pollux. Winter Triangle is another asterism that is formed by Betelgeuse, Sirius, and Procyon.

Keystone (formed by four central stars in the Hercules constellation), Sickle (six stars forming a backward question mark in Leo constellation), Job's Coffin (box-like pattern formed by four brightest stars in Delphinus constellation), Milk Dipper (spoon-like shape formed by five bright stars in the Sagittarius constellation), Teapot (eight stars forming a teapot-like shape in the Sagittarius constellation), Orion's Sword (three stars forming a pattern right under the Orion's Belt), etc., are some of the many popular groups of stars that are termed as asterisms.

Mostly named after mythological figures or the pattern that forms when lines are drawn to connect the stars, constellations are specific regions of the celestial sphere that have been recognized by the IAU. On the other hand, an asterism can be defined as a group of stars forming a recognizable pattern. It might be a part of one or more constellations, but is not recognized as a constellation by the IAU.