Multi-band observations of NGC 3766, an open star cluster located about 5,500 light-years away in the constellation Centaurus, have yielded the discovery of a new type of variable stars.

Many stars are known as variable or pulsating stars, because their apparent brightness changes over time. How the brightness of these stars changes depends in complex ways on the properties of their interiors. This phenomenon has allowed the development of a whole branch of astrophysics called asteroseismology, where astronomers can ‘listen’ to these stellar vibrations, in order to probe the physical properties of the stars and get to know more about their inner workings.

The new results, published in the journal Astronomy & Astrophysics, are based on regular measurements of the brightness of more than three thousand stars in NGC 3766 over a period of 7 years. They reveal a large population (36 stars) of variable stars in the cluster that followed an unexpected pattern – they had tiny regular variations in their brightness at the level of 0.1% of the stars’ normal brightness. These variations had periods between 2 and 20 hrs. The stars are somewhat hotter and brighter than the Sun, but otherwise apparently unremarkable. The new class of variable stars is yet to be given a name.

“The very existence of this new class of variable stars is a challenge to astrophysicists. Current theoretical models predict that their light is not supposed to vary periodically at all, so our current efforts are focused on finding out more about the behavior of this strange new type of star,” said study co-author Dr Sophie Saesen from the Geneva Observatory.

Although the cause of the variability remains unknown, there is a tantalizing clue: some of the stars seem to be fast rotators. They spin at speeds that are more than half of their critical velocity, which is the threshold where stars become unstable and throw off material into space.

“In those conditions, the fast spin will have an important impact on their internal properties, but we are not able yet to adequately model their light variations,” said lead author Dr Nami Mowlavi, also from the Geneva Observatory.

“We hope our discovery will encourage specialists to address the issue in the hope of understanding the origin of these mysterious variations,” he concluded.

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Bibliographic information: N. Mowlavi et al. 2013. Stellar variability in open clusters I. A new class of variable stars in NGC 3766. A&A, vol. 554, article no.: A108; doi: 10.1051/0004-6361/201321065