Science & Environment

Background light suggests many stars 'outside galaxies'

colliding galaxies Image copyright SPL
Image caption Collisions and mergers can strip stars out of galaxies

A new study of the universe's background light has suggested that as many as half its stars might be hidden in the space between galaxies.

Measurements were made by two cameras sent beyond the atmosphere on a rocket.

After subtracting all the interference from dust and galaxies, the leftover light has ripples in it, which the study's authors ascribe to lone stars, flung out during galactic collisions.

Other scientists believe it comes from whole galaxies that are very distant.

The new results are published in the journal Science.

Prof Jamie Bock from Nasa's Jet Propulsion Laboratory, one of the report's authors, described the extragalactic background light (EBL) as "kind of a cosmic glow".

"It's very faint - but basically the spaces between the stars and galaxies aren't dark. And this is the total light made by stars and galaxies during cosmic history," Prof Bock told the BBC.

Earlier measurements from rockets and satellites had shown that there was more fluctuation in this background than the sum total of known galaxies could explain.

At least two proposals were made to account for the extra light: it might come from very early, distant galaxies that formed when the universe was much younger, or it might come from stray stars outside galactic boundaries.

Prof Bock's team set out to study the EBL in detail, in terms of its colour and its distribution, to try and settle the debate.

Image copyright CIBER/JPL/CalTech
Image caption The cameras were taken to sub-orbital space on a sounding rocket

Checking it twice

Two rocket flights were used to collect the data, in 2010 and 2012, as part of an experiment dubbed CIBER: the cosmic infrared background experiment.

On each 10-minute flight, a 10m (30ft) sounding rocket travelled briefly beyond the Earth's atmosphere and two infra-red cameras took wide-angle images of the sky.

Doing the measurements twice allowed the researchers to rule out fluctuations coming from the dust within our own solar system.

"[On the second flight] we're looking through a completely different patch of the solar system, and we see the same signal," Prof Bock explained.

"It's been really nice to have multiple flights, so we can do all these checks."

Once all the non-background light, such as galaxies themselves, was discarded - "you kind of surgically remove them" from the images, Prof Bock said - the team was left with a clean picture of the EBL.

The brightness and the blueness of the light in that picture, they claim, support the idea that it comes from stars stripped of their galaxies.

"It's inconsistent with [the light coming from] the very first galaxies, because it would look a lot redder," Prof Bock said.

The report also says there is so much light in the recordings that there might be just as many stars outside galaxies as inside them.

"Astronomers know this stripping happens, but we're saying it's much more prevalent."

'Perfectly possible'

Other researchers are less certain of the data's implications.

Jo Dunkley, a professor of astrophysics at the University of Oxford, said she did not think the evidence for vast numbers of lonely stars was compelling - "though it would obviously be really interesting if it were".

"This is a big question in astronomers' minds, because we can't assign all the light we see to galaxies we know," Prof Dunkley told BBC News. "So where is all the rest of it coming from?"

She said that the work represented "a really nice measurement" and agreed that the reported background light probably could not have come from the very earliest galaxies - but argued that there is much still to know about those galaxies, and the ones that followed soon after.

"There's so many galaxies out there, very faint galaxies, very far away, and I don't we've got a full census of those yet."

Image copyright SPL
Image caption To measure EBL, things like zodiacal light (caused by dust in our solar system) must first be substracted

Prof Steve Eales, an astronomer at Cardiff University, was also cautious about the findings.

"When you see a result like this, you never say 'well yes that's obviously right'," he said.

Prof Eales said the JPL team's interpretation - and their conclusion about extragalactic stars - was "perfectly possible" but that corroborating evidence would be needed.

If so many stars exist outside galaxies, he explained, you might expect to find more examples in between the galaxies closer to our own.

"But the problem is, once single stars move away, they're very hard to see.

"There's nothing to rule out the possibility but there's probably no other evidence yet that they exist."

Prof Bock acknowledged that his findings would probably not meet with universal acclaim.

"We have to hear how other people react to it," he said.

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