As many as half of all stars reside outside of galaxies, study finds

As many as half of all stars reside outside of galaxies, study finds
This time-lapse photograph shows NASA's Cosmic Infrared Background Experiment rocket launch last year. Data from the experiment show that half the extra-galactic background light is coming from rogue stars kicked out of their galaxies. (T. Arai / University of Tokyo)

Even stars can get lost in space.

Scientists who shot a rocket up beyond Earth’s atmosphere for a matter of minutes have made a remarkable discovery about the diffuse background light that permeates the universe: As many as half of all stars may have been stripped from their home galaxies and flung into the darkness of the cosmos.

Astronomers were aware that some stars were intergalactic orphans. But the extent of the dim diaspora, reported in Friday's edition of the journal Science, came as something of a shock.

“I did not expect it to be half the stars — I thought that most stars would be in galaxies,” said Harvey Moseley, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved in the research. “It’s almost like they’re hiding.”

The new information could compel scientists to reevaluate their theories of how the universe formed the galaxies we see today.

"If you want to understand what's happening in the formation of galaxies, you can't just look at the galaxies," said study coauthor James Bock, an experimental cosmologist at NASA's Jet Propulsion Laboratory in La Cañada Flintridge. "You're missing about half the light if you do that."

When astronomers study the light coming from Andromeda, our nearest galactic neighbor, stripped stars seem to contribute less than 5% of the galaxy's total light, said Karoline Gilbert, an astronomer at the Space Telescope Science Institute in Baltimore who wasn't involved in the research. As a result, scientists haven't paid much attention to them.

Now it's clear they can't ignore them anymore. "There is still a large number of stars we aren't accounting for," she said. "We can't ignore orphan stars."


Scientists will also need to reevaluate the true boundaries of the fuzzy halos surrounding galaxies, said Michael Zemcov, an astronomer at Caltech in Pasadena and the study's lead author.

Astronomers have long wondered about the origins of the diffuse light permeating the heavens, which they call extragalactic background light. Earlier work with NASA's Spitzer Space Telescope had revealed a strange, splotchy background in infrared light, and scientists struggled to find a source.

One theory held that the faint radiation might be coming from the first primordial galaxies in the early universe during a critical epoch known as reionization, when the cosmos was only a few hundred million years old. Though not as ancient as the cosmic microwave background radiation that originated moments after the universe’s birth, these galaxies are still old enough to elude detection by astronomers’ telescopes.

Another theory proposed that the extragalactic background light might be coming from closer, more contemporary stars that were ripped from their homes when two galaxies smashed together.

To find out whether either theory might be correct, scientists with NASA’s Cosmic Infrared Background Experiment sent a small telescope beyond the edge of the atmosphere to take clear shots of the sky in wavelengths of near-infrared light. From the resulting images, the CIBER team subtracted all the near-infrared light coming from the known stars and galaxies. What remained were the fluctuations in the background — which the researchers confirmed using data from the Spitzer telescope.

But in the long wavelengths of infrared light that Spitzer observes, it's difficult to distinguish primordial light from more recent light. So the CIBER scientists looked at much shorter wavelengths of infrared light, just below the visible range.

If the background light was primordial, they would only find it at longer wavelengths, because the light would have been stretched out over time. But if the background light was coming from more recent sources, it would show up at shorter wavelengths too.

Sure enough, the background lightwas detectable even at the shortest infrared wavelengths they studied,Bocksaid. In fact, it seemed to gleam even more brightly at these shorter wavelengths.

That's a sign that the light was coming from more nearby stars in the universe — stars that hadn't been accounted for among the known galaxies.

These stars are so distant and faint that there's no way to pick them out individually, Bock said. They could only be detected by looking for this collective glow.

In fact, there's just as much background starlight coming from these dim rogue stars as is produced by all of the galaxies put together, the scientists calculated.

Exactly how many rogue stars there are remains unclear, and the answer depends on what types of stars are out there, Bock said. So although they produce half of the background light in the universe, their population could potentially be markedly smaller than that of galactic stars.

But now that astronomers know what to look for, Moseley said, there could be hints of these hidden stars buried in the data already gathered by current telescopes, just waiting to be found.

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