Flickering stars give riddles to researchers

So far no explanation for mysterious brightness variations of sun-like stars found

The life of sun-like stars © S. Steinhöfel / ESO
Read out

A comprehensive study with ESO's Very Large Telescope has deepened a long-known puzzle of sun-like stars. It is about unusual brightness fluctuations that repeat over the years and occur in about one third of all sun-like stars towards the end of their star life.

Astronomers have proposed a number of possible explanations for this phenomenon over the last few decades - but the new research is incompatible with any of them. The search for the riddle solution continues accordingly, the researchers write in two articles in the journals "Monthly Notices of the Royal Astronomical Society" and "Astrophysical Journal".

"As much as astronomers appreciate darkness - it's not fun to keep this in the dark, " says Christine Nicholls of Mountstromlo Observatory, Australia, the lead author of the new study. "We have collected the most accurate set of observational data for this class of Sun-like stars, with the result that we can now rule out all the explanations that have been suggested for their unusual behavior."

One third of the sun-like stars are affected

The mystery that the astronomers had adopted dates back to the 1930s and affects about a third of the sun-like stars in our Milky Way and other galaxies. Towards the end of their lives, such stars inflate enormously, cool off and take on a reddish tint: They become in the meantime red giants, before ending their lives as white dwarfs. In red giants, strong fluctuations in brightness are observed, which repeat over periods of several years.

"Such brightness fluctuations are attributed to stellar pulsations, " said Nicholls. "Simplified, the star alternately grows larger and smaller, and its brightness accordingly increases and decreases alternately. However, one-third of the stars show further, previously unexplained variations, which repeat themselves over longer periods of up to five years. "Display

Observed 58 stars in the Large Magellanic Cloud

To find out what these additional variations are, astronomers over a period of two and a half years observed 58 stars in the Magellanic Cloud, the nearest neighbor galaxy. With the help of the high-resolution spectrograph FLAMES / GIRAFFE at ESO's Very Large Telescope, they recorded spectra of the stars, combined this information with images from other telescopes, and thus obtained a considerable amount of data on these perverse stars.

It is often such voluminous data sets that point the way to the proper solution of cosmic riddles - by refuting some of the proposed explanations. In this case, however, it became clear that the new data are incompatible with any of the proposed solutions. Now astronomers know that they need to look for a whole new solution.

Astro-Sherlock Holmes asked

"According to the new data, it is extremely unlikely that the additional brightness fluctuations can be explained by stellar pulsations, " said research team leader Peter Wood. "Another possible explanation that each of these stars is part of a binary star system - that is, orbiting another star - is also incompatible with our data."

However, the researchers found that the unpredictable fluctuations in brightness are associated with the giant stars ejecting matter - either in the form of lumps or expanding slices. "Now we need a Sherlock Holmes to take on this frustrating riddle, " said Nicholls.

(idw - Max Planck Institute for Astronomy, 08.12.2009 - DLO)