Our Night Sky: Constellations
Historically, constellations were groupings of stars
that were thought to outline the shape of something, usually with mythological
significance. There are 88 recognized constellations,
with their names tracing as far back as Mesopotamia, 5000 years ago.
To the right is the well known Northern Constellation, Pegasus, the horse.
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Star Groupings and Asterisms
Some familiar "constellations" are
technically not constellations at all. The
grouping of stars known as the Big Dipper
is not actually a constellation.
The Big Dipper is part of a larger grouping of stars
called the Big Bear (Ursa Major) which is a constellation.
A grouping of stars, like the Big Dipper, that is not officially a constellation is called an asterism.
Constellations Are Not Physical Groupings
The apparent groupings of stars into constellations that we
see on the celestial sphere are not physical groupings.
In most cases the
stars are each at very different distances from us, and only
appear to be grouped because they lie in the same direction.
This is illustrated in the figure below, where the physical
distance from the Earth for each star in the Big Dipper
is drawn to scale. Numbers beside each star give the distance
from Earth in light years.
