Star clusters are groups of stars that are ‘connected’
by a significant gravitational force ands move around tougher as the galaxy rotates.
The motion of the Sun through the galaxy does not affect the appearance of a star cluster from Earth over a long period of time.
Examples of star clusters are the Great Cluster (M13) in the constellation of Hercules and some of the stars in the Pleiades.
M13 is a globular cluster and the Pleiades an open cluster.
However a constellation is a group of stars that
appear to be related simply because of the view of them from the Earth.
They may be at very different distances from the Earth and so as time passes the appearance of the constellation will change.
This change will be very slow and differences are only seen over some hundreds of thousands or years.
Examples of constellations are Orion, Ursa Major, Taurus, Perseus, Cassiopeia etc.
The three photographs show M13, the Pleiades and part of the constellation of Orion.
The following diagrams show how the constellation appears from the Earth and then a ‘side view’ from a point far out in space showing how the stars are spread out at different distances.
Betelgeuse lies at only 310 light from the Earth while delta Orionis is another 2000 light years further away and yet still seems to be part of the same constellation when viewed from the Earth. The three stars in the belt (zeta, epsilon and delta) are 1108, 1206 and 2347 light away respectively.
