A globular cluster is made of hundreds of thousands or even millions of stars packed together in a pretty compact ball. The stars in globular clusters are old and theres never any sign of new stars forming in them. Older stars are usually yellow, orange and red, so those colours predominate in globular clusters. Such elderly stars formed when the Universe was young and heavy elements were rare, so rocky and metallic worlds like ours are probably scarce in these star balls.
This stunning view of the globular star cluster Messier 55 in the constellation of Sagittarius was obtained in infrared light with the VISTA 4.1 m survey telescope at ESOs Paranal Observatory in Chile. (Image credit:ESO/J.Emerson/VISTA. Acknowledgment:Cambridge Astronomical Survey Unit)
This new image of the globular cluster Messier 55 (also known as NGC 6809) from the European Southern Observatory’s VISTA infrared survey telescope shows hundreds of thousands of stars crammed together in a sphere a mere fifty or so light years across. The stars in there are practically in shouting distance of each other! M55 lies in the constellation of Sagittarius (The Archer) and is about 17 000 light years from Earth but can be seen in a modest telescope by southerly observers.
Nobodys quite sure about how globular clusters came to be. They formed a long time ago (about 10 billion years ago), but are not quite as old as our galaxy. Astronomers have speculated that these clusters may be stuff that never got incorporated into the galaxy proper, or a newer theory, which is rapidly gaining ground, says they are the cores of small galaxies absorbed by the Milky Way over the aeons. Globular clusters may in fact be our greedy galaxys left-overs! There are at least 150 of them around the Milky Way but some other galaxies have thousands around them.
If we lived in a globular cluster after sunset the sky would be so full of bright stars that there wouldnt be any night as we know it. It would be glorious, but sadly the radiation levels from all the stars inside a globular cluster are likely to be high, and perhaps fatal to beings like us. Probably there no habitable planets in them, so the only people to enjoy their beauty see them from the outside like ourselves. However others are less pessimistic and it has even been claimed that the mysterious “Wow! signal” received in 1977 originated from the direction M55. Sadly, this still unexplained pulse of radio energy originated from a location rather to the northwest of M55 so I doubt if the source was this cluster (if indeed the signal was genuinely of extraterrestrial origin).
Two interesting hard SF tales featuring the worlds that could just possibly exist inside globular clusters are Poul Andersons Starfog and David Brins Startide Rising.
(Article by Colin Johnston, Science Education Director)
I thought this was a great introduction on the topic of clusters.