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Astronomers have seen a huge galaxy cluster doing what until now was only theorised to happen: making new stars.

Most galaxy clusters - the largest structures in the Universe - are "red and dead", having long since produced all the stars they can make.

But cluster formation should, according to theory, include a cooling phase, resulting in blue light from new stars.

, researchers say they have seen evidence that the enormous Phoenix cluster makes 740 stars a year.

In our own Milky Way, only one or two new stars are made each year.

The cluster, some seven billion light-years away, is formally called SPT-CLJ2344-4243 but the researchers have renamed it for the constellation in which it lies.

It contains the mass equivalent to about two and a half million billion Suns.

While astronomers are getting better at spotting these huge conglomerates of galaxies - the Planck space telescope has reported notable hauls of them - there is still much to learn about how they form and what goes on within them.

Just as planets are thought to form by the eventual coalescence of matter that is around after star formation, galaxy clusters are believed to form through colossal galactic mergers - another event that sharp-eyed astronomers .

Theory has it that in addition to a central black hole that is usually present, clusters have a great deal of gas at their cores, sprayed from nearby galaxies and supernovae, that should eventually cool down enough to draw together and start the process of star formation anew.

Yet astronomers have only ever seen red, dead regions at clusters' cores; the lack of evidence for the idea was called the "cooling flow problem".

But a find by the South Pole Telescope in Antarctica, with follow-up observations by the space-based Chandra X-ray Observatory, found one bright cluster among several new finds.

Researchers at the Massachusetts Institute of Technology led an international team that went on to use a total of 10 different telescopes worldwide, each looking at different colours of light, to characterise the surprise Phoenix finding.

The Phoenix cluster showed particularly bright emission in the ultraviolet range of the spectrum, corresponding to hundreds of young stars and suggesting that 740 were being born each year.

"Not only is it the most X-ray luminous cluster in the Universe, but the central, most massive galaxy is forming stars at an unmatched rate," said Michael McDonald of the Massachusetts Institute of Technology (MIT) in the US.

The findings shed light on the interplay between the supermassive black holes that appear to be at the centres of all big galaxies, and the vast tracts of gas that surround them.

These black holes are the source of a kind of tug-of-war - their gravity draws material inward, but they also release huge amounts of energy in the form of jets that tend to heat the gas and keep it at a distance.

The researchers believe that the black hole at the core of the Phoenix cluster's central galaxy must not be putting out much energy at the time we now see it.

Commenting on what he called the "tussle" between the central black hole and the surrounding material, the UK's Astronomer Royal Sir Martin Rees, from the University of Cambridge, said: "It's not able to hold that infall of gas at bay, and this gas is falling in and forming stars.

"That's a very extreme phenomenon, that's what's so special about this system. This is a fascinating step toward putting this picture together of the tussle."