Colour composite of one of the red supergiant star clusters. Blue represents hot interstellar gas, stars show up as green and hot dust shows as red. The red supergiants are the bright stars.

Two star clusters of gigantic red supergiant stars are ticking timebombs in our Galaxy, it was revealed yesterday at the National Astronomy Meeting at Queens University, Belfast. "The next supernova could go off in one of these clusters at an time," says Dr Ben Davies of the Rochester Institute of Technology, USA. "We estimate that it is about 5,000 years between explosions for these clusters.

The two star clusters are extraordinary, containing fifteen percent of all known red supergiants in the Galaxy. One cluster contains 26 red supergiants and is 17 million years old. The other contains 14 red supergiants ad is a little younger, at 12 million years old. The age difference allows astronomers to compare how the clusters are evolving.

Red supergiants are extraordinary stars. "They are the final stage in the life of a giant star between eight and thirty solar masses," says Davies. Giant stars have relatively short lives, as they exhaust their enormous supplies of hydrogen fuel rapidly to remain stable. After a few dozen million years they use up their hydrogen in their centres and for a short time initiate helium burning in their cores while continuing hydrogen burning in their shells, which causes them to swell up into red supergiants. Eventually they run out of fuel all together and collapse, resulting in a Type IIP supernova. The most famous red supergiant is Betelgeuse in the constellation Orion.

Intriguingly, both clusters are located in a unique region of the Galaxy. Our Galaxy is a spiral galaxy that contains a small central 'bar' that runs through the central region of the Milky Way. Gas races around the bar before being funnelled into a bottleneck at each tip. Conditions at the tip of the bar are therefore high pressure and high density, which is ideal for the formation of giant stars. "The stars in the bar move with the bar (as it swings around the centre of the Galaxy)," says Davies. "The stars at the edge of the bar, we don't know."

Davies found the two clusters in archive images from the Galactic Plane Survey (GLIMPSE) made with NASA's infrared Spitzer Space Telescope. He has also searched for equivalent star clusters at the opposite end of the bar, but has so far failed to find any. Only 200 are known to exist in the Galaxy, but Davies does expect that there will be similar clusters somewhere on the other side of the Milky Way, only that their light is blocked by intervening clouds of dust.