Asterisms and Constellations

On a recent warm and humid summer night, sky transparency was very poor. Only the brighter stars punched through the water-laden atmosphere but three stars were very prominent. They formed a triangular pattern aptly called the Summer Triangle.

The Summer Triangle begins to rise in the Spring.  As seen from mid-northern latitudes in mid-May near midnight.

The Summer Triangle is an example of an asterism: a group of stars that form a recognizable pattern or shape. The Big Dipper, the Little Dipper and the Great Square of Pegasus are other examples of asterisms.

Asterisms are often confused with constellations and indeed, in ancient times, constellations were mythological figures, animals or objects that were seen in groupings of stars.

The Big Dipper asterism as seen from mid-northern latitudes in mid-May at 10:00 p.m.

Almost everyone in North America is familiar with the Big Dipper which is part of the figure of the Big Bear, or the constellation of Ursa Major.

The Big Dipper asterism belongs to the constellation Ursa Major (Great Bear).

The modern constellation of Ursa Major includes all stars within an area defined by the International Astronomical Union in 1930. So the star 24 Ursae Majoris "belongs" to the constellation Ursa Major even though it is not part of the figure of the bear.

Modern constellation boundaries

Some asterisms such as the Big Dipper, the Sickle of Leo, the teapot of Sagittarius and the Great Square of Pegasus have been known for a long time. All are best appreciated when viewed without optical aid because of their large angular size.

But over the years, people using binoculars and telescopes have come across other striking asterisms and some of these have become well known to amateur astronomers.

Here are some examples.

The Diamond Ring

A tight group of 7th and 8th magnitude stars with Polaris as the "solitaire". Best seen with binoculars in a dark sky or a small telescope with a low power eyepiece showing about a 1° field.

The Coathanger
RA = 19h 25m, Dec = 20° 04'

A group of fifth and sixth magnitude stars in Vulpecula appearing like an upside down coathanger to northern hemisphere observers. Use binoculars for best views.

ET Cluster
RA = 1h 19 m, Dec = 58° 17.5'

This open cluster, also known as NGC 457, is located in Cassiopeia. With a bit of imagination you can make out the figure of ET. (Hint: the two bright stars are ET's eyes). Because of its small size, a telescope is needed to make out this asterism.

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