The biggest galaxies in the universe are elliptical galaxies. The largest of these hold over one trillion stars according to astronomical census takers, compared to 400 billion in our Milky Way. However, new research shows that elliptical galaxies actually hold five to ten times as many stars as previously believed. This means that the total number of stars in the universe is likely three times bigger than realized.
The hidden stars are known as red dwarfs for their color and small size. Because red dwarfs are small and dim compared to stars like the Sun, astronomers hadn't been able to detect them in galaxies beyond the Milky Way before now. As such, they didn't know how many stars in the universe were red dwarfs.
Scientists used powerful instruments on the Keck Observatory in Hawaii to detect the faint signature of red dwarfs in the cores of eight 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 30 percent as massive as the Sun, were much more bountiful than expected.
"As it turns out, the universe thinks small, at least when it comes to star size," said Harvard astronomer Charlie Conroy. "Our stellar inventory has changed dramatically."
"No one knew how many of these stars there were," said Pieter van Dokkum, a Yale University astronomer who led the research. "Different theoretical models predicted a wide range of possibilities, so this answers a long-standing question about just how abundant these stars are."
Their results imply that stellar population counts depend on what type of galaxy astronomers examine, just as a census of the city of New York and the town of Derby, Kansas will find very different population numbers.
"We usually assume other galaxies look like our own. But this suggests other conditions are possible in other galaxies," Conroy stated. "This discovery could have a major impact on our understanding of galaxy formation and evolution."
In particular, galaxies might contain less dark matter - a mysterious substance only detectable due to its gravitational effects - than previous measurements of their masses indicated. Instead, the abundant red dwarfs might contribute more mass than previously calculated.
Their findings appear in the Dec. 1st online issue of the journal Nature.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.