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.

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. ✦ 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. 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. 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, Boötes. Even the Little Dipper in Ursa Minor constellation is an asterism. ✦ 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. ✦ 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. ✦ 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. ✦ 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.