An intriguing stellar cluster located 7,000 light years from Earth has prompted astronomers to create a new class of stars with "pulsating" brightness, the European Southern Observatory (ESO) said on Wednesday.

Sky-watchers made the discovery over seven years of patient measurements of 3,000 stars in a cluster called NGC 3766, found in the constellation of Centaurus.

Unexpectedly, 36 of these stars had tiny variations in their normal brightness, they found.

The luminosity varied by just 0.1 percent, with the changes occurring at periods ranging from two to 20 hours.

Astronomers have long known about variable, or "pulsating," stars, whose brightness changes in line with energy fluxes from within the star itself.

But the new stars do not fit into any known category of variability. They are somewhat hotter and brighter than the Sun, but otherwise seem run-of-the-mill. They were created around 20 million years ago.

"The very existence of this new class of variable stars is a challenge to astrophysicists," said Sophie Saesen of the Geneva Observatory, which carried out the painstaking work using a 1.2-metre (3.9-feet) telescope at ESO's site at La Silla, Chile.

"Current theoretical models predict that their light is not supposed to vary periodically at all, so our current efforts are focussed on finding out more about the behaviour of this strange new type of star.