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.

Writing in Nature, researchers say they have seen evidence that the enormous Phoenix cluster makes 740 stars a 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, BBC reports.

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 have managed to get a picture of, on more than one occasion.