What is a Constellation? - Definition, Facts, Formation & Examples

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you will learn what a constellation is and how it is different to a star cluster. You will also learn how star clusters are formed and go over a few examples of both constellations and star clusters. A short quiz will follow.

What is a Constellation?

Have you ever looked at a fluffy cloud and imagined it looked like a bunny rabbit? Most people have done this from time to time. Our human brains have an amazing ability to see meaning in the abstract, and it is a big part of how we have been successful in the world.

For centuries, people have looked at the stars in the night sky and seen shapes: perhaps a lion here, a centaur there. These shapes or patterns in the stars are called constellations, and they contain stars that appear to be near to each other. Different cultures have different ideas of what's up there. But if there's so much inconsistency, how do we know which stars are really grouped together?

Constellations are shapes in the night sky
Constellations are Shapes in the Night Sky

Constellations vs Star Clusters

Stars that appear to be near to each other from our viewpoint on Earth form constellations, but stars that are truly near to each other are grouped into star clusters. A star cluster is a bundle of stars that are gravitationally bound to each other, rotating around a common center of mass. Most star clusters are too faint to see without a telescope.

17th Century Dutch Constellation Map
17th Century Dutch Constellation Map

Constellations in the sky very rarely contain stars that are truly near to each other. One star in Orion might be 50 light years away, where as the star that appears right next to it could be 120 light years away. Their brightnesses don't even tell us how far away they are, because a dim star that's close might look exactly the same as a bright star that's further away. We only know by analyzing the light and using some mathematics.

Examples of constellations include things like Orion, Leo, Draco or Cancer. Examples of star clusters on the other hand have scientific names like M67, M92 or the Pleiades cluster.

Modern astronomers only use constellations to break the sky into regions.
Modern astronomers only use constellations to break the sky into regions.

Formation of Star Clusters

Star clusters are part of galaxies, like our own Milky Way, and so they formed at the same time as the galaxies themselves. But how did they form?

All material with mass in the universe is gravitationally attracted to other material with mass. Over time, the mass present after the big bang pulled together under gravity and started to form clumps. Those clumps became galaxies, and the spinning of those galaxies caused them to flatten into disks. But even the galaxies themselves have clumps, and the exact way all of these clumps were positioned was just a result of the random energy fluctuations present right after the big bang.

M2 Globular Star Cluster
M2 Globular Star Cluster

Cultural Constellations

While star clusters were formed by the big bang and gravitational pull, constellations were 'created' -- or at least named -- by humankind. In the western world we have sets of constellations that we know and love that are based on Greco-Roman astronomy, but other cultures have completely different ones.

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