Starstruck: half of universe's stars are orphans with no galaxy

(Reuters) - You can call them celestial orphans, stars flung out of their galaxies in colossal collisions that have occurred in space for billions of years. These forsaken stars may be far more common than anyone ever realized.

New observations from suborbital rocket launches and an orbiting observatory show that as many as half the stars in the universe may be this kind, scientists said on Thursday. They found that the dim light these stars produce from the far reaches of the cosmos equals the amount coming from all the galaxies.

The data was collected during 2010 and 2012 flights of a NASA suborbital rocket with the Cosmic Infrared Background Experiment (CIBER) measuring background fluctuations in a compilation of all the light emitted by stars and galaxies in the universe's history.

The researchers then checked the findings using NASA's Spitzer Space Telescope, an orbiting infrared observatory. The data indicated huge numbers of orphan stars, previously undetected, populate what had been thought to be the dark spaces between galaxies.

So how did billions of stars become orphans? Violently, to say the least.

Galaxies like our Milky Way are made up of stars, dust and dark matter bound together by gravity. As galaxies drift through space, they periodically crash into each other.

Stars and the other galactic stuff can merge together in these mash-ups, but some stars that were born and resided in these galaxies are stripped away and cast as debris into the cosmic wilderness.

The phenomenon of the orphan star has been well known. Astronomers have witnessed tidal streams of stars being stripped away from colliding pairs of galaxies.

"There are many beautiful pictures of this happening. Of course, since we are viewing this from such a great distance we don't see the stars individually, we see a glowing haze made by billions of stars," said experimental cosmologist Jamie Bock of NASA's Jet Propulsion Laboratory and the California Institute of Technology.

The data suggests orphan stars are probably relatively small, less massive and cooler than our Sun, but typical of most stars in the universe, said Caltech experimental astrophysicist Michael Zemcov.

The night sky as seen from Earth is brimming with starlight. But these orphans would be so distant from other stars that a view from one would offer almost complete nothingness.

"The night sky on a planet around such a star would be profoundly boring and black to human eyes - no other stars, or at least very few, no Milky Way band, only distant galaxies. You might be lucky and see your parent galaxy off in the distance like we see Andromeda," Zemcov said.

Zemcov said scientists have traced the origin of galaxies to about 13.2 billion years ago, 500 million years after the Big Bang that created the universe.

"Galaxies have been forming and interacting continuously since then, with a peak in the star formation rate about 2 billion years after the Big Bang," Zemcov said. "You have enough interactions over enough time, and you end up stripping out a lot of stars."

The research appears in the journal Science.

(Reporting by Will Dunham; Editing by David Gregorio)