Stars tend to form together in one of two types of cluster. Globular clusters typically have a thousand or more old stars that form a symmetrical, sphere-like shape. The stars in open clusters are relatively young and fewer in number.
The stars in any one cluster are related to one another by their common origin – they are all believed to have formed at roughly the same time from the same cloud of gas and dust.
When the stars in an open cluster are less tightly packed, they are called an association.
Image: Thousands of stars inside the giant nebula NGC 3603 (credit: NASA, ESA, and the Hubble Heritage STScI/AURA-ESA/Hubble Collaboration)
Stars often grow up in a group.
Patrick Moore describes some sights in the Milky Way.
Sir Patrick Moore and Chris Lintott describe some of the beautiful sights in our galaxy, the Milky Way.
Patrick Moore and his guest discuss how stars get their start.
Sir Patrick Moore and his guest Heather Couper discuss how stars get their start inside clouds of gas.
Patrick Moore describes the famous Seven Sisters star cluster.
Sir Patrick Moore describes the famous Seven Sisters star cluster, also known as the Pleiades.
Patrick Moore listens to pulsars at Jodrell Bank.
Sir Patrick Moore listens to pulsars at Jodrell Bank Observatory.
Use the constellation Pegasus to find interesting objects in the autumn night sky.
Patrick Moore and his guest Dr John Mason discuss the main constellation northern hemisphere stargazers can see when looking south in autumn - Pegasus. In mythology Pegasus was a flying horse, in the night sky it is a square. Sights in this region of the sky include star clusters and a group of five galaxies called Stephan's Quintet.
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.
Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster.