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Star cluster's clocks corrected

L. Bedin / STScI / NASA / ESA
Click for video: Two background galaxies are visible amid a glittering array of
stars in this detail from a Hubble image showing the open star cluster
NGC 6791. Click on the image to watch a video that zooms in on the cluster.

The time is out of joint in the open star cluster NGC 6791: Three different types of stars show three different ages for the cluster, and that poses a puzzle for the scientists who use stars as celestial timepieces. Fortunately, new observations from the Hubble Space Telescope - combined with some scientific sleuthing - go at least halfway toward setting things right in the cosmic clockwork.

Normal stars follow a well-known course of development as they age, and astronomers can figure out how old the stars are just by correlating their brightness and their color. There's a separate brightness-and-color scale for figuring out the ages of white dwarfs, which are the burnt-out embers of sunlike stars that gradually fade into darkness.

Usually, the two time scales confirm each other. But in NGC 6791's case, the normal, main-sequence stars indicated an age of 8 billion years, while the white dwarfs signaled significantly younger ages. Some bright white dwarfs indicated an age of 4 billion years. Other, dimmer dwarfs looked as if they were 6 billion years old.

So what do you go with? Four billion years? Six billion? Eight billion? All of the above? None?

"The age discrepancy is a problem because stars in an open cluster should be the same age," astronomer Luigi Bedin of the Space Telescope Science Institute said in an image advisory issued today. "They form at the same time within a large cloud of interstellar dust and gas. So we were really puzzled by what was going on."

The off-kilter clocks suggested that astronomers might have been missing something fundamental about the way star clusters work. Or is there something special about NGC 6791? When the Hubble research team analyzed imagery of the cluster in detail, they were intrigued to find that the bright white dwarfs were roughly twice as shiny as the dim white dwarfs.

Could the brighter stars actually be pairs of dim white dwarfs, positioned so close together that they appear to be single stars? That's what the researchers suggest in a report published in the May 20 issue of the Astrophysical Journal Letters.

A. Feild / STScI / NASA / ESA
This chart shows the three scales that
scientists used to calculate NGC 6791's
age. One of the curves for the white
dwarfs is twice as bright as the other.
Click on the image for a larger version.

"It's a surmise, but things fall into place so beautifully," said the leader of the Hubble study, University of Washington astronomer Ivan King.

The binary-star scenario solves half of the problem: If you assume that the doubly bright white dwarfs are actually double stars, that would result in an age estimate of 6 billion years.

"We've still got the problem that the white dwarfs are giving a younger age than the main-sequence stars," King told me. "It casts doubt on the whole white-dwarf method. We'd dearly love to know what's going on."

Theorists have already come up with an idea to explain away the discrepancy. It turns out that the white dwarfs in NGC 6791 are not your typical white dwarfs. "This is a very unusual cluster, in that it has more than twice as much of the heavy elements as the sun does," King said.

Perhaps the heavy-metal white dwarfs evolve somewhat more slowly than the more common, lighter white dwarfs. If that's the case, the star cluster would be 8 billion years old - with white dwarfs that are particularly good at hiding their age.

King said he and his colleagues should know more once they're able to look at another star cluster for comparison. That could happen as early as this year, if NASA's shuttle mission to repair Hubble is successful.

During the mission, scheduled for launch on Oct. 8, spacewalkers are due to repair and replace Hubble's cameras - including the Advanced Camera for Surveys, which sent back the data used by King's team but is now mostly out of commission. If the upgrades go as planned, Hubble should be in good working order until 2013 or so.

Like many other scientists, King can hardly wait to see the Hubble Space Telescope returned to full service. At the age of 18, the greatest of NASA's Great Observatories may be getting a bit long in the tooth for a space telescope - but it's still a youngster in King's eyes. 

"I expect to draw upon it for the rest of my career," he said. "I'm 81, by the way."

In addition to King and Bedin, the research team includes Maurizio Salaris of Liverpool John Moores University in Britain, Giampaolo Piotto and A.P. Milone of the University of Padua in Italy, Santi Cassisi of the Collurania Astronomical Observatory in Italy and Jay Anderson of the Space Telescope Science Institute.

For still more glittering pictures from Hubble, check out our zoomable look at the Coma Cluster and msnbc.com's gallery of greatest hits from the cosmos.