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Universe might hold three times more stars than previously thought

A new study suggests that a specific kind of galaxy might hold 10 times more red dwarf stars than estimated. That would triple projections for the number of stars in the observable universe, with implications for explanations of how stars and galaxies form and evolve.

By Staff writer / December 1, 2010

NGC 4472 is one of the galaxies from the Virgo cluster observed by astronomers studying the number of red dwarf stars in elliptical galaxies.



It's a cosmic embarrassment of riches – the universe appears to hold three times the number of stars many astronomers might have estimated only a year ago.

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That's the implication a pair of scientists has drawn after measuring eight huge elliptical galaxies that they selected from two vast galaxy clusters located between 53 million to 321 million light-years from Earth.

With as many as 200 billion galaxies in the observable universe, each with hundreds of billions of stars, the result – if it holds up – implies an enormous number of additional burning gas balls out there, with intriguing implications for explanations of how stars and galaxies form and evolve, researchers say.

IN PICTURES: Where stars form

The cause of this huge revision of the stellar census are stars known as red dwarfs, literally the dimmest stellar bulbs on the shelf. These stars weigh in at no more than about 30 percent of the sun's mass.

Surveys of our own galaxy, the Milky Way, have found that these dwarfs outnumber sun-like stars by about 100 to 1, explains Pieter van Dokkum, an astronomer at Yale University in New Haven, Conn. But the dwarfs are so dim and other galaxies so distant that red dwarfs fail to appear when astronomers try to account for the stars other galaxies contain.

As a consequence, when estimating how much of a galaxy's mass stars account for – important to understanding a galaxy's life history – astronomers basically had to assume that the relative abundance of red-dwarf stars found in the Milky Way held true throughout the universe for every galaxy type and at every epoch of the universe's evolution, Dr. van Dokkum says.

"We always knew that was sort of a stretch, but it was the only thing we had. Until you see evidence to the contrary you kind of go with that assumption," he says.

Initial evidence that other galaxies might hold larger populations of red dwarfs appeared nearly 20 years ago, when a team of astronomers claimed to have spotted evidence for these dim, low-mass stars in other galaxies. The results were fascinating, but many scientists considered the data ambiguous.

How they did it

In 2009, however, the Keck Observatory atop Hawaii's Mauna Kea modified a spectrometer used by one of its twin 10-meter telescopes, improving the likelihood of detecting these dim stars beyond the Milky Way.

Van Dokkum and colleague Charlie Conroy with the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass., used the spectrometer to hunt for evidence of red dwarfs in eight bright elliptical galaxies, four in the Coma cluster and four in the nearer Virgo cluster.


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