An extensive study undertaken with ESO's Very Large Telescope (VLT) has deepened, rather than solved, a long standing mystery of unusual brightness variations seen in some Sun-like stars as they near the end of their lives.

The brightness variations last for several years and have been observed in one-third of all Sun-like stars that have entered into the red giant phase, that is, when they become cool, swollen stars before collapsing into white dwarfs. Many possible explanations have been put forward for the periodic and persistent luminosity variations, but the new survey contradicts them all.

“Astronomers are left in the dark, and for once, we do not enjoy it,” says Christine Nicholls from Mount Stromlo Observatory in Australia, and lead author of one of the papers reporting the study. “We have obtained the most comprehensive set of observations to date for this class of Sun-like stars, and they clearly show that all the possible explanations for their unusual behavior just fail.”

Such variations are usually attributed to stellar pulsations that typically occur as the star swells and shrinks, becoming brighter and dimmer in a regular fashion. “However, one third of these stars show an unexplained additional periodic variation, on even longer timescales – up to five years,” says Nicholls.

Using the FLAMES/GIRAFFE spectrograph on ESO's VLT, and combining the data with images from collaborators working with the MACHO and OGLE instruments (running on telescopes in Australia and Chile), allowed astronomers to build up a comprehensive picture of 58 of these curious objects over a period of two and a half years. The intention was to narrow down some of the possible explanations that had been presented, but instead the observations were found to be incompatible with all the theories, re-opening the case for more consideration.

“The newly gathered data show that pulsations are an extremely unlikely explanation for the additional variation,” says team leader Peter Wood. “Another possible mechanism for producing luminosity variations in a star is to have the star itself move in a binary system. However, our observations are strongly incompatible with this hypothesis too.”

The team also found that whatever the cause of these unexplained variations is, it also causes the giant stars to eject mass either in clumps or as an expanding disc, leaving the astronomers with a thoroughly perplexing mystery to solve.

The research is presented in two papers; one features in the November issue of the Monthly Notices of the Royal Astronomical Society and the other in the current issue of the Astrophysical Journal.