Hidden within ancient, oversized galaxies are enough dwarf stars to triple the estimated number of stars in the universe, astronomers reported Wednesday.
Galaxies are the vast islands of stars filling space, such as our own spiral-shaped Milky Way made of hundreds of billions of stars. The just-released Nature journal estimate suggests that round-shaped "elliptical" galaxies harbor far more "red dwarf" stars than expected.
"A typical star in the universe, we can now say, is in an elliptical galaxy," says study lead author Pieter van Dokkum of Yale. "The surprising thing is only now are we getting a handle on a fundamental thing like the number of stars in the sky." Based on a survey of eight nearby elliptical galaxies, he says the universe likely contains some 300 sextillion stars (that's 300,000,000,000,000,000,000,000 stars for folks who like big numbers), triple previous estimates.
" I don't think it was expected at all, and it has quite far-reaching implications," says astronomer Martin White of the University of California, Berkeley, who was not part of the study. Essentially, the results suggest that astronomers have misunderstood just how stars formed in the earliest galaxies, and have also mistakenly assumed that stars are distributed similarly throughout all galaxies, regardless of their type.
Because red dwarf stars are dim, their true numbers had escaped astronomers, the team suggests, leading them to suppose they originated in elliptical galaxies at the same rate as our own Milky Way. A red dwarf typically weighs less than half as much as the sun.
In the study, von Dokkum and Harvard's Charlie Conroy captured chemical spectra measures of the light from eight elliptical galaxies, which make up perhaps 15% of all galaxies and are among the largest and most ancient ones. The spectra measurements reveal that red dwarfs packed the galaxies in unexpectedly numbers, making up 80% of their stars and about 60% of their total weight.
The finding also answers a "dark matter" mystery in elliptical galaxies, which had seemed to possess too much of this unseen kind of material in their centers. In reality, much of that extra mass is likely made of dim red dwarf stars, von Dokkum says.
"If this result is confirmed, it would certainly have important ramifications," says astrophysicist Matthew Bate of the United Kingdom's University of Exeter by e-mail. Until now, astronomers had largely supposed that elliptical galaxies, thought to have formed 10 billion years ago or more in the collision of large galaxies, were hostile places for the formation of small stars. Dwarf stars instead must have been unexpectedly good at surviving supernova blasts from giant stars that exploded quickly after the formation of elliptical galaxies.
Our own Milky Way may collide with the Andromeda galaxy, another large spiral, within the next five billion years to become an elliptical galaxy.
The number of planets in the universe circling those small stars has likely tripled as well, van Dokkum suggests, pointing to the September news of a nearby red dwarf star, Gliese 581, likely possessing a rocky Earth-like planet in its habitable zone. "We know these stars have planets, and now we know there are far more of them," van Dokkum says.
Because the results rely on a survey of only eight elliptical galaxies, the team suggests that the next step is to survey more of these ancient giants, to confirm their results. A super-massive black hole is thought to lurk at the center of each one, as well, and the signals of the occasional red dwarf disappearing into the maw of one of them, may also help confirm the finding, he says.