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.
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.
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.
These stellar bulbs may be dim individually, but if they exist there in sufficient numbers, and the spectrometer gathered the still-dim light long enough to yield a decent exposure, the unique spectral "fingerprints" for red dwarfs should appear, they reasoned.
Indeed, the fingerprints they sought appeared and were strong enough to suggest that there are about 1,000 red dwarfs in these galaxies for each sun-like star, and that they account for about 60 percent of the mass of all the stars in these galaxies.
Elliptical galaxies, essentially the oldest galaxies in the cosmos, make up only about a third of all the galaxies out there. But they hold so many more stars than other types of galaxies that their newly identified red-dwarf populations triple the number of stars in the cosmos.
In recent years, astrophysicists have put forward some plausible theoretical reasons for expecting the universe to hold more stars than previously estimated, notes Betsy Barton, an astronomer at the University of California at Irvine who has focused her research on galaxy evolution.
Still, "this is a big deal," she says of the results.
A clue in dark matter mystery
For some, it may come as welcome news. If observations of additional elliptical galaxies confirm the new results, it would solve one puzzle involving these galaxies and so-called dark matter – an as-yet unidentified form of matter that gives off no light, hence the term "dark," but whose gravitational influence is pervasive.
Astrophysicists say that without dark matter's additional mass, galaxies would fly apart as they spin because the visible matter present lacks the mass, hence the gravity, needed to hold the galaxies together.
Elliptical galaxies posed a problem: The motions of the stars they contained implied that they had more mass than one would get by adding the mass of the normal matter astronomers observed to the expected amount of dark matter in the neighborhood. Some suggested that ellipticals somehow had extra dark matter associated with them.
Instead, the newly detected red dwarfs could account for the difference, van Dokkum says.
What you see is not what you get
But the find also adds an element of complexity to galaxy studies, he adds.
Scientists gauge the mass of a distant galaxy by its intrinsic brightness. But only the brightest, most massive stars, which tend to be the fewest in number, provide enough light to reach Earth-bound telescopes. Astronomers have to extrapolate to estimate the relative abundance of stars in other mass ranges, based what they've gleaned about the distribution of stellar masses in our own galaxy.
"Now, it turns out that two galaxies can have the same light coming off of them, but they might have very different masses because they have a different number of these very low-mass stars," van Dokkum explains.
Still, the results need to be taken with caution, he acknowledges. When it comes to red dwarfs, astronomers "have made the mistake of assuming that the Milky Way was typical of all galaxies in the universe," he says. "We shouldn't make the mistake of assuming that these eight elliptical galaxies are representatives for all elliptical galaxies in the universe."
To that end, he says he's off to Keck this weekend to expand his hunt for the elusive red dwarfs in additional elliptical galaxies.