What are Asterisms?

At a recent Denver Astronomical Society (DAS) Astronomical Observing 101 class, some students were surprised when the instructor, former DAS Vice President, Dr. Mehmet Unsoy, told them that the Big Dipper is not actually a constellation. Instead, it’s something called an asterism in the larger constellation Ursa Minor! But what does that mean?

A constellation is often thought of as a pattern of stars in the night sky. While these patterns after which most constellations are named date back to ancient times; as of 1928, the International Astronomical Union has defined constellations as specific areas of the celestial sphere with boundaries defined by arcs of right ascension and declination. The IAU recognizes 88 constellations that cover both hemispheres, encompassing the entire night sky. These constellations help astronomers map the stars and assign locations to celestial objects. In star maps and charts, line patterns are often used to outline the shapes that correspond to constellation names, which are helpful in memorizing the patterns of the night sky. However, the IAU does not officially define these shapes or any artistic representations.

An asterism, on the other hand, is an unofficial pattern of stars – typically connecting several of the brightest stars in an area of the sky. Asterisms can be part of a single constellation or stretch across multiple constellations. But we shouldn’t diminish the value of these common patterns. Even though they are not “official constellations”, like the stick figures and art used to help us memorize constellations, asterisms are very useful for learning the night sky. They provide easy-to-spot patterns that amateur astronomers can use as guides to locate stars and other celestial objects.

What are the Common Asterisms Seen from Denver?

Denver’s night skies offer many asterisms to observe throughout the year. Here are 15 of the most well-known asterisms shown with the IAU boundaries and Stellarium’s artistic representations of the constellations that they span:

The Big Dipper

• Constellation: Ursa Major
• Best Time to See: All year (circumpolar), but summer for right-side up (as pictured)
• Details: The Big Dipper is perhaps the most famous asterism. Its seven bright stars form a shape resembling a ladle or scoop. The two stars at the end of the “bowl” point to Polaris, the North Star.
• Historical Note: Native American cultures saw the Big Dipper as a bear being chased by hunters, while ancient Greeks associated it with Callisto, a nymph transformed into a bear by Zeus.

• Deep Sky Objects: The Big Dipper is a guide to Mizar and Alcor, a famous double star system, and M81 and M82, two galaxies located nearby.

The Little Dipper

• Constellation: Ursa Minor
• Best Time to See: All year (circumpolar), but winter for right-side up (as pictured)
• Details: The Little Dipper is fainter than the Big Dipper and includes Polaris at the tip of its handle. It’s more challenging to see from light-polluted areas but can be spotted on a clear night.
• Historical Note: Polaris has been used as a navigation aid for centuries, guiding sailors and explorers due north.

• Deep Sky Objects: The Little Dipper helps locate NGC 3172 (Polarissima Borealis), one of the closest deep sky objects to the North Celestial Pole.

Cassiopeia’s W

• Constellation: Cassiopeia
• Best Time to See: All year (circumpolar), but summer for right-side up (as pictured)
• Details: Cassiopeia’s W is an easy-to-identify zigzag pattern that resembles the letter ‘W’ or ‘M’ depending on the season.
• Historical Note: Cassiopeia was a vain queen in Greek mythology who was punished by being placed in the sky.
• Deep Sky Objects: Cassiopeia’s W points toward M52, an open cluster, and the Double Cluster (NGC 869/884) in Perseus.

The Spring Triangle

• Constellations: Boötes, Virgo, Coma Berenices, and Leo
• Best Time to See: Spring
• Details: The Spring Triangle connects the stars Arcturus, Spica, and Regulus. It helps stargazers locate spring constellations.
• Historical Note: These stars marked the arrival of spring and the planting season for ancient civilizations.
• Deep Sky Objects: The Spring Triangle is near M104, the Sombrero Galaxy, and the Virgo Cluster of galaxies.

The Kite

• Constellation: Boötes
• Best Time to See: Spring
• Details: The Kite resembles a diamond shape, with the bright star Arcturus marking the tail.
• Historical Note: Boötes is often identified as a herdsman driving the Great Bear (Ursa Major) around the pole.
• Deep Sky Objects: The Kite is near M3, a bright globular cluster visible with binoculars.

The Sickle

• Constellation: Leo
• Best Time to See: Spring
• Details: The Sickle looks like a backward question mark and represents the head and mane of the lion in the constellation Leo.
• Historical Note: Ancient Egyptians linked Leo with the annual flooding of the Nile, which coincided with the rising of this constellation.

• Deep Sky Objects: The Sickle points to the Leo Triplet, a group of three interacting galaxies: M65, M66, and NGC 3628.

The Summer Triangle

• Constellations: Cygnus, Lyra, and Aquila
• Best Time to See: Summer
• Details: The Summer Triangle connects three of the brightest stars in the summer sky: Vega, Deneb, and Altair. It dominates the summer night sky.
• Historical Note: In Chinese folklore, Vega and Altair are star-crossed lovers separated by the Milky Way, reunited only once a year.
• Deep Sky Objects: The Summer Triangle points to the Ring Nebula (M57), the Dumbbell Nebula (M27), and the Coathanger Cluster.

The Keystone

• Constellation: Hercules
• Best Time to See: Summer
• Details: The Keystone forms the body of Hercules and is a key landmark when searching for the Hercules Globular Cluster (M13).
• Historical Note: Hercules, the mythological hero, was honored in the stars after completing his twelve labors.
• Deep Sky Objects: The Keystone points to M13, the Hercules Globular Cluster, and M92, another globular cluster in Hercules.

The Fish Hook

• Constellation: Scorpius
• Best Time to See: Summer
• Details: The Fish Hook curves along the body of Scorpius and points to the bright red star Antares.
• Historical Note: In Hawaiian mythology, the Fish Hook is associated with the demigod Maui, who used it to pull islands from the sea.
• Deep Sky Objects: The Fish Hook leads to M4, a nearby globular cluster, and the Butterfly Cluster (M6).

The Teapot

• Constellation: Sagittarius
• Best Time to See: Summer
• Details: The Teapot’s stars form the shape of a teapot with steam rising from its spout. The steam points toward the Milky Way’s dense core.
• Historical Note: Ancient Mesopotamian cultures saw this region of the sky as a god pouring water from a jar.
• Deep Sky Objects: The Teapot points toward the Lagoon Nebula (M8) and the Trifid Nebula (M20), both prominent targets for observers.

The Northern Cross

• Constellation: Cygnus
• Best Time to See: Summer and Fall
• Details: The Northern Cross is formed by the brightest stars in Cygnus, with Deneb at the top. It is easy to spot in the summer sky.
• Historical Note: Cygnus is associated with the myth of Zeus transforming into a swan to seduce Leda.
• Deep Sky Objects: The Northern Cross helps locate the North America Nebula (NGC 7000) and the Veil Nebula.

The Great Square

• Constellation: Pegasus
• Best Time to See: Fall
• Details: The Great Square is made up of four bright stars that form a large square in the sky, marking the body of Pegasus.
• Historical Note: Pegasus, the winged horse, was said to have sprung from the blood of Medusa when Perseus beheaded her.
• Deep Sky Objects: The Great Square helps locate M15, a globular cluster, and the Andromeda Galaxy (M31) just outside the square.

The Winter Triangle

• Constellations: Orion, Canis Major, Monoceros, and Canis Minor
• Best Time to See: Winter
• Details: The Winter Triangle connects the bright stars Betelgeuse, Sirius, and Procyon to form a large triangle in the winter sky.
• Historical Note: These stars were important markers for ancient navigators and timekeepers.
• Deep Sky Objects: The Winter Triangle points toward the Rosette Nebula (NGC 2244) and the Christmas Tree Cluster (NGC 2264).

The Winter Hexagon

• Constellations: Canis Major, Canis Minor, Gemini, Auriga, Taurus, Monoceros, Lepus, and Orion
• Best Time to See: Winter
• Details: The Winter Hexagon is an enormous asterism made up of six bright stars: Sirius, Procyon, Pollux, Capella, Aldebaran, and Rigel. It covers a vast portion of the winter night sky.
• Historical Note: The Winter Hexagon stars have been used across cultures to mark seasonal changes.
• Deep Sky Objects: The Winter Hexagon surrounds M35, an open cluster, and the Crab Nebula (M1).

Orion’s Belt

• Constellation: Orion
• Best Time to See: Winter
• Details: Orion’s Belt consists of three bright stars lined up in a row, forming the “belt” of Orion the Hunter.
• Historical Note: Ancient Egyptians believed the belt stars were aligned with the pyramids of Giza.
• Deep Sky Objects: Use Orion’s Belt to find M42, the Orion Nebula, the Horsehead Nebula, and the Flame Nebula.

Keep Looking Up!

Asterisms are wonderful tools for learning and exploring the night sky. While they may not be official constellations, they help amateur astronomers locate constellations and other celestial objects. Next time you’re outside on a clear night in Denver, see how many of these asterisms you can spot. Keep looking up, and you’ll discover new wonders in the night sky!

Clear skies and happy stargazing!