The team, headed by Dr Steve Phillipps, an astrophysicist at the University of Bristol, and Dr Michael Drinkwater of the University of Queensland, used the famous Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile, amongst others, to study the galaxies, which they have christened ultra-compact dwarfs (UCDs). Their findings are published in Nature this week.

The galaxies were originally found in a large spectroscopic survey of the region of sky around the Fornax Cluster of galaxies, undertaken at the Anglo-Australian Observatory. They are so small that they look just like individual stars on ordinary photographs of the sky.

Only the fact that the team obtained spectra (representations of the distribution of energy emitted) for all the objects visible in this part of the sky enabled them to see that seven of the tiny specks were in fact galaxies in the Cluster, about 60 million light years away, and not merely foreground stars in our own Galaxy.

Dr Steve Phillipps said: "The new data show that although the UCDs contain tens of millions of stars, these are contained in volumes not much more than 100 light years across, making them far smaller, that is, more compact, than any previously known galaxies.

"For comparison, the Milky Way galaxy, in which we live, is over 100,000 light years in diameter. An entire UCD would fit comfortably in the space between the Earth and relatively nearby bright stars like Rigel or Betelgeuse in Orion."

It is suspected that the UCDs may be produced from initially larger but more diffuse galaxies, whose outer stars were lost as the galaxy flew through the central parts of the galaxy cluster, leaving only a dense knot of stars which was originally in the nucleus of the UCDs' ancestor.

The paper A class of compact dwarf galaxies formed by disruptive processes in galaxy clusters appears in Nature on 29 May 2003.