Asterism (astronomy)

An asterism is a pattern or group of stars that we see in the sky. These patterns can be simple shapes made of just a few stars or more complex groups of many stars. Some of these patterns are very bright and easy to see, which helps people learn about the night sky.

Unlike constellations, which have official boundaries set by astronomers, asterisms are just any noticed pattern of stars. This means asterisms can be any group of stars that people see together, even if they are not actually close to each other in space.

Some well-known asterisms include the Summer Triangle, Orion's Belt, and the Big Dipper. These patterns help us find our way around the night sky and are fun to spot when we look up at the stars.

Background of asterisms and constellations

Many ancient cultures, like India and Babylon, saw patterns in the stars. Some of these patterns, such as Orion and Scorpius, appeared in many cultures. Back then, there was no real difference between a group of stars called a constellation and one called an asterism.

Later, astronomers like Hipparchus began listing groups of stars. When Europeans explored the southern sky, astronomers such as Johann Bayer and Nicolas Louis de Lacaille added many new star groups. In 1928, the International Astronomical Union officially divided the sky into 88 official constellations. Any other special groups of stars we see are usually called asterisms.

Asterisms consisting of first-magnitude stars

Some star patterns, called asterisms, are made of very bright stars that form simple shapes in the sky.

One example is the Summer Triangle, made of three bright stars: Deneb, Altair, and Vega. These stars are easy to see in the summer skies of the northern hemisphere. Another pattern is the Winter Triangle, which includes the bright stars Betelgeuse, Sirius, and Procyon. There is also a larger pattern called the Winter Hexagon, which includes several of these bright stars and others, forming a big shape in the winter sky.

The Southern Cross is another well-known pattern, featuring bright stars like Acrux and Mimosa, and is often used to find other stars in the southern sky. Each of these patterns helps us learn and remember where stars are located in the night sky.

Constellation-based asterisms

• The Big Dipper, also called The Plough or Charles's Wain, is made of the seven brightest stars in Ursa Major. These stars look like the Bear's hind legs and tail, or like a handle for the bear's head and neck. Ursa Minor, with its longer tail, is often called the Little Dipper.

• The Northern Cross is found in Cygnus. It runs from Deneb in the Swan's tail to Albireo in the beak.

• The Southern Cross is known as an asterism, but it is now the constellation Crux. Its main stars are Alpha, Beta, Gamma, Delta, and possibly Epsilon Crucis.

• The Fish Hook is the Hawaiian name for Scorpius. If you draw lines from Antares to Beta Scorpii and Pi Scorpii, it looks like a large capped "J."

• Boötes is sometimes called the Ice Cream Cone or the Kite.

• The stars of Cassiopeia form a W shape.

• The Great Square of Pegasus is a quadrilateral made by the stars Markab, Scheat, Algenib, and Alpheratz. Alpheratz is now part of Andromeda, but it used to belong to both constellations.

• The Bowl of Virgo is formed by the stars Beta, Gamma, Delta, Epsilon and Eta Virginis. With Spica, they make a Y shape.

• The Three Leaps of the Gazelle are three pairs of stars in Ursa Major in a row about 30 degrees long. The pairs are Xi and Nu, Upsilon and Lambda, and Kappa and Iota Ursa Majoris.

• The Water Jar or Urn of Aquarius is a Y-shaped figure with Zeta Aquarii and includes Gamma, Eta and Pi. It "pours" water through more than 20 stars ending at Fomalhaut.

• The Crab Breast of Cancer is a quadrilateral made by the stars Gamma, Delta, Eta and Theta Cancri, forming the inner shell of the Crab. Inside this is the Beehive Cluster (Messier 44), which includes Epsilon Cancri.

• The Snake Head is the western part of Hydra with the stars Delta, Epsilon, Zeta, Eta, Rho and Sigma Hydrae.

• Orion's Belt is three bright stars — Zeta (Alnitak), Epsilon (Alnilam) and Delta Orionis (Mintaka) — forming Orion's belt.

• Orion's Sword, hanging from Orion's Belt, includes the stars 42 Orionis, Theta Orionis, Iota Orionis, and the Orion Nebula (M42) in the middle.

• The Bull's Face of Taurus is a V-shaped figure from stars in the Hyades cluster, including Gamma, Delta¹, Delta², Delta³, Epsilon, Theta Tauri, and the bright star Alpha Tauri (Aldebaran), which is the red eye of the Bull.

Other particular asterisms

An asterism is a pattern of stars that people see in the night sky. These patterns can be made from stars in a group called a constellation. Some famous examples include:

• Four stars in Carinae make a shape called the Diamond Cross.
• In Orion, stars form a shape known as the Saucepan or Pot.
• In Hercules, four stars create a shape called the Keystone.
• Stars in Leo form a curve known as the Sickle.
• The brighter stars in Sagittarius make a shape called the Teapot.
• Near the Teapot, fainter stars form the Teaspoon.
• In Delphinus, four stars create a shape called Job's Coffin.
• Four faint stars in Sagittarius form the Terebellum.
• In Pisces, stars create a circle called the Circlet.
• Two stars in the Big Dipper, called the Pointers, point toward Polaris, the North Star.
• In the southern sky, stars called the Southern Pointers lead to the Southern Cross.

Asterisms across multiple constellations

Some star patterns, called asterisms, are made from stars in more than one area of the sky we call constellations.

One example is the Egyptian X, made from bright stars like Sirius, Procyon, and Betelgeuse. These stars form two triangles that meet at a point. The Lozenge is a small diamond shape made from stars in the head of the dragon Draco and the foot of Hercules. Another pattern, the False Cross, looks like the famous Southern Cross but is made from different stars and can be mistaken for it. The Northern Y is a shape formed by stars including Arcturus and is easy to see even when lights from cities make other stars harder to spot. The Lightning Bolt is a line of stars helpful for finding directions between three constellations, and the Serpent Bowl is a large curved shape of stars. The Eagle Tail Corona is a curved pattern in the tail of the eagle Aquila and nearby areas.

Telescopic asterisms

Asterisms can be small and need a telescope to see them. Some examples are:

• The 37 or LE of NGC 2169, in Orion.
• The Engagement Ring in Ursa Minor has the north star Polaris as the center, with a ring of fainter stars around it.
• The Broken Engagement Ring in Ursa Major.
• The Christmas Tree shape of the Christmas Tree Cluster, in Monoceros. It has about 40 stars.
• The Coathanger, in Vulpecula, also called Brocchi's Cluster.
• Kemble's Cascade, a chain of stars ending in open cluster NGC 1502, in Camelopardalis.
• Napoleon's Hat (Picot 1), in Bootes (south of α Bootis, Arcturus).
• The Ring of the Nibelungen (Ferrero 27) in Draco, named after an old German epic drama.
• The V-shaped Messier 73 in Aquarius, which was found to be an asterism in 2002.
• Ally's Braid, a line of six stars leading away from Alcyone, the brightest star of the Pleiades open cluster.