Mystery of Changing Star Brightness Deepens
Born from clouds of gas and dust, stars like our Sun spend most of their lifetime slowly burning their primary nuclear fuel, hydrogen, into the heavier element helium. After several billion years, their fuel is almost exhausted and they start swelling, becoming cool and red -- red giants. These stars do not end in dramatic explosions, but die peacefully as planetary nebulae, blowing out everything but a tiny remnant, known as a white dwarf.
Credit: ESO/S. Steinhöfel

Unusual fluctuations in the brightness of older sun-like stars have long mystified astronomers, and new, detailed observations of the phenomenon have only deepened the mystery.

The new data, taken with the European Southern Observatory's Very Large Telescope, contradict all of the explanations that astronomers have previously put forward to account for years-long variations seen in the brightness of one-third of sun-like stars that are in the later stages of their lives.

"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," said study team member Christine Nicholls of Mount Stromlo Observatory, Australia.

The mystery investigated by Nicholls and her 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 very 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,'" Nicholls explained. "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."

In order to find out the origin of this secondary feature, the astronomers monitored 58 stars in our galactic neighbor, the Large Magellanic Cloud, over two and a half years.

But instead of backing up any of the existing ideas for why these stars see additional fluctuations, the observations contradicted all of them.

"The newly gathered data show that pulsations are an extremely unlikely explanation for the additional variation," said team leader Peter Wood of Australia National University. "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.

So with this new data, astronomers will have to go back to the drawing board to come up with an explanation.

"A Sherlock Holmes is needed to solve this very frustrating mystery," Nicholls said.

The new findings are detailed in two papers appearing in the November issue of the journal Monthly Notices of the Royal Astronomical Society and the Astrophysical Journal.

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