An extensive study made with the European Southern Observatory's (ESO) Very Large Telescope (VLT) deepens a long-standing mystery in the study of stars similar to the Sun. Unusual year-long variations in the brightnesses of about one third of all Sun-like stars during the latter stages of their lives still remain unexplained. During the past few decades, astronomers have offered many possible explanations, but the new observations contradict them all and deepen the mystery. The search for a suitable interpretation is on.

"Astronomers are left in the dark, and for once, we do not enjoy it," said Christine Nicholls from Mount Stromlo Observatory, Australia. "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."

The mystery investigated by the team dates back to the 1930s and affects about a third of Sun-like stars in our Milky Way and other galaxies. All stars with masses similar to our Sun become, towards the end of their lives, red, cool, and extremely large just before retiring as white dwarfs. Also known as red giants, these elderly stars exhibit strong periodic variations in their luminosity over timescales up to a couple of years.

"Such variations are thought to be caused by what we call 'stellar pulsations'," said Nicholls. "Roughly speaking, the giant star swells and shrinks, becoming brighter and dimmer in a regular pattern. However, one third of these stars show an unexplained additional periodic variation on even longer timescales — up to five years."

To find the origin of this secondary feature, astronomers monitored 58 stars in our galactic neighbor, the Large Magellanic Cloud, over two and a half years. They acquired spectra using the high resolution FLAMES/GIRAFFE spectrograph on ESO's VLT and combined them with images from other telescopes, achieving a collection of the properties of these variable stars.

Sets of data like the one collected by Nicholls and her colleagues often offer guidance on how to solve a cosmic puzzle by narrowing down the possible explanations proposed by theoreticians. In this case, however, the observations are incompatible with all the previously conceived models and re-open an issue that has been thoroughly debated. Thanks to this study, astronomers are now aware of their own "ignorance" — a genuine driver of the knowledge-seeking process.

"The newly gathered data show that pulsations are an extremely unlikely explanation for the additional variation," said 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 found from further analysis 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. "A Sherlock Holmes is needed to solve this very frustrating mystery," said Nicholls.