Lone stars: Half of the stars in the Universe may lie outside galaxies

Caltech scientists may have found some of the missing matter in the Universe. Cathal O’Connell explains.

A time-lapse photograph of the Cosmic Infrared Background Experiment rocket launch from NASA’s Wallops Flight Facility in Virginia. Sub-orbital rockets are ideal for making quick astronomy observations above Earth’s atmosphere. – T. Arai/University of Tokyo/NASA

Look up at the night sky in outback Australia and you see the glittering tapestry of the Milky Way. The heavens have always inspired wonder and awe. But it’s not necessarily that way for stargazers on other planets. A new star survey published in Science this month reports a vast new population of lone stars. It says half of all the stars in the Universe could lie outside galaxies.

For people living on planets orbiting these outliers, “it would be very lonely. The night sky would be profoundly black and dark,” says Michael Zemcov, lead author of the study.

Because the light from these lone stars is so faint no one knew they were there until now. Yet cosmologists have long known there was more to the Universe than meets the eye. There isn’t enough mass to explain why the Universe hangs together. Most of what’s missing is mysterious dark matter. But even when it comes to ordinary matter – stars, galaxies, gas and dust – the books don’t balance. They should account for about 5% of the universe, yet we can only see enough to account for 2.5%.

The missing ordinary matter is a separate problem from the mystery of dark matter. Whereas dark matter cannot be seen by telescopes because it never emits light, we should be able to see the ordinary matter once we know where to look. Astronomers have been trying to guess what the missing ordinary matter might be. A swathe of extra black holes? Dead stars which no longer give off light?

Although large surveys of the sky have been made using the Hubble telescope and other instruments, this extra matter had not yet turned up. This prompted Caltech scientists to wonder if pointing a telescope at large sources of light such as galaxies was the best way to find it. What if the extra matter was spread out between the galaxies?

The team devised an experiment, called CIBER (Cosmic Infrared Background Experiment), to approach the problem a new way, measuring all the light coming from a large patch of sky. That way diffuse sources of light which couldn’t be seen individually might still be detected en masse. It would be like gauging the attendance of a football game by listening to the roar of the crowd rather than counting the spectators individually.

The measurements couldn’t be done from Earth because the atmosphere gets in the way. On the other hand, space telescopes are not up to the job either because they have a restricted field of view. “Hubble has this teeny-tiny little postage stamp of a field of view,” says Zemcov. CIBER, by contrast has a field of view “20 times the area of the full Moon”.

To take a long-exposure snapshot of the universe the team loaded their instruments into a rocket and fired it 300 kilometres straight up, well clear of the atmosphere. As the rocket fell back to Earth a camera popped out the side to collect light for seven minutes, feeding the light to two spectrometers that measured its intensity at different infrared colours.

The CIBER measurement represented light from millions of galaxies at once. The team then blacked out all the light coming from known galaxies, leaving the supposedly dark space between. And they found it wasn’t altogether dark. A faint, splotchy background glow remained that they couldn’t account for.

The only explanation was that the extra light must come from stars lying outside of galaxies – too faint to be seen individually but which could be detected by CIBER from their combined signals.

This artist’s conception shows light from stars flung out of the galaxies they surround. The stars are too distant to be seen individually but are seen instead as a diffuse glow around the bright galaxies. – NASA/JPL-Caltech

How did these lonesome stars get there? Stars are always born within galaxies but we’ve known for decades that when galaxies collide, stars can occasionally be knocked free.

Will the newly detected stars help balance the books in terms of the ordinary matter in the universe? Maybe.

The CIBER experiment suggests up to 50% of stars may lie outside galaxies, which would balance things nicely. But Scott Croom, an astronomer at the University of Sydney, points out that this figure was arrived at by using some big assumptions. “It’s an interesting finding which has to be pushed a bit further before we can convincingly say exactly what fraction of stars live outside galaxies,” he says.

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Cathal O'Connell is a science writer based in Melbourne.
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