What is a constellation?

What defines these clusters of stars in the sky as constellations?

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What is a constellation?

Asked by Neil Garrett

Constellations are patterns in the night sky often formed by the most prominent stars to the naked eye. Technically a constellation defines not just the group of stars that form their patterns but also the region of sky in which it rests.

There are 88 constellations across the sky between the northern and southern hemispheres and, in both these parts of the celestial sphere, these patterns of stars differ. The current list, which includes constellations such as Orion, Cassiopeia, Taurus and the Plough, has been recognised by the International Astronomical Union (IAU) since around 1922 and are based on the 48 which were previously identified by Greek astronomer Claudius Ptolemy.

Constellations often carry names and take the shape of gods, hunters, princesses, objects and mythical beasts associated with Greek mythology – however, at times, it requires quite an imagination to draw out what some constellations are supposed to represent! Some of the most obvious stars in a constellation are often given names and in general, the most visible stars of each constellation are assigned Greek letters with the brightest taking on the first letter of the greek alphabet (alpha), the second brightest taking beta and so on. As an example, the brightest star in Lyra is Vega which is also called alpha Lyrae.

Constellations make excellent signposts when it comes to making your way around the night sky – make sure you check out our beginner’s guide to astronomy in issue 8 of All About Space!

Answered by science journalist Gemma Lavender

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