Red dwarfs are relatively small and dim compared to stars like our Sun, so astronomers haven't been able to detect them in galaxies other than our own Milky Way and its nearest neighbors - so there were only conjectures about how much of the total stellar population of the universe is made up of red dwarfs.
Now astronomers have used powerful instruments on the Keck Observatory in Hawaii to detect the faint signature of red dwarfs in eight massive, relatively nearby galaxies called elliptical galaxies, which are located between about 50 million and 300 million light years away. They discovered that the red dwarfs, which are only between 10 and 20 percent as massive as the Sun, were much more plentiful than expected.
Filtering out the light from brighter stars, astronomers detected the faint signature of small, dim red dwarf stars in nearby elliptical galaxies (right), and found these are much more numerous than in our own Milky Way (left). This finding suggests that the total number of stars in the universe could be up to three times higher than previously thought. Credit: Illustration by Yale University
The team discovered that there are about 20 times more red dwarfs in elliptical galaxies than in the Milky Way, said Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics, who was involved in the research.
"We usually assume other galaxies look like our own. But this suggests other conditions are possible in other galaxies," Conroy said. "So this discovery could have a major impact on our understanding of galaxy formation and evolution."
For instance, galaxies might contain less dark matter, a theoretical substance that has mass but cannot be directly observed, than previous measurements of their masses might have indicated. Instead, the abundant red dwarfs could contribute more mass than realized.
In addition to boosting the total number of stars in the universe, the discovery also increases the number of planets orbiting those stars, which in turn elevates the number of planets that might harbor life - recently discovered exoplanet Gliese 581g, which one of its discovers believes could potentially support life, orbits a red dwarf star, Gliese 581.
"There are possibly trillions of Earths orbiting these stars," said Pieter van Dokkum, a Yale University astronomer who led the research described in Nature, adding that the red dwarfs they discovered, which are typically more than 10 billion years old, have been around long enough for complex life to evolve. "It's one reason why people are interested in this type of star."