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Universe's star count tripled
Posted: 01 December 2010

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A new study reveals that dim red dwarf stars residing in elliptical galaxies are so much more bountiful than previously thought that there may be three times as many stars in the Universe than realised.

Red dwarfs are small and dim compared to stars like our Sun, and as a result are difficult to detect. But using the powerful Keck Observatory a team of astronomers led by Yale researcher Pieter van Dokkum were able to study eight elliptical galaxies in two nearby clusters – the Virgo cluster and Coma cluster, which lie approximately 50 and 320 million light years away respectively.

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. Image: Yale University.

"We measured the abundance of red dwarf stars in the central regions of the galaxies," describes van Dokkum. "The eight galaxies were remarkably homogeneous, but they were selected to have roughly the same mass and age."

The team discovered that there were twenty times more red dwarfs in the elliptical galaxies than in the Milky Way Galaxy, which is a spiral galaxy. "We usually assume other galaxies look like our own," says Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics. "But this suggests other conditions are possible in other galaxies, so this discovery could have a major impact on our understanding of galaxy formation and evolution."

One implication of the results is that galaxies might contain less dark matter – the invisible substance that exerts its gravitational force on galaxies – than previously thought. "The masses of these elliptical galaxies as measured from the motions of stars are very high, and some of this mass was attributed to the stars but most had been attributed to dark matter," van Dokkum tells Astronomy Now. "Our results imply that a lot of this mass was due to stars after all, and that the dark matter fraction is lower than was previously thought and actually more in line with expectations from numerical simulations."

Since more initial mass went into building these galaxies, an efficient mechanism would be needed to turn the primordial gas into a vast number of low mass stars. "This may have to do with the high densities of elliptical galaxies: a lot of gas was squeezed in a small volume, and recent simulations of the behavior of gas under these conditions suggest that it can indeed lead to the formation of many little stars," continues van Dokkum. "Our results inform such simulations and will hopefully lead to refinements in the future."

Van Dokkum adds that the two must urgent aspects of the team's research are to study the distribution of these stars within elliptical galaxies, in order to determine whether these stars are equally abundant throughout these galaxies; and to study other galaxies with different properties to see whether the results apply to all elliptical galaxies or only for the most massive ones.

A further consideration is the influence of merging events, which mostly add stars to the outer parts of galaxies, building up their outer envelopes around an established core. "Mergers probably did not influence the results very much, but this does lead to the interesting question whether the red dwarfs are equally enhanced away from the central regions," comments van Dokkum. "This is something that we hope to measure this weekend actually – we have more time on the Keck telescope for this and a few other measurements."

Since the discovery inflates the number of total stars in the Universe, it also boosts the number of planets that could exist elsewhere in the Universe – at least six planets are already known to exist around red dwarf star Gliese 581.

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