By Paul Rincon
Science reporter, BBC News
Baby stars have been discovered spawning in the otherwise barren outskirts of a galaxy.
The finding has surprised astronomers because the galactic periphery was assumed to lack high concentrations of ingredients needed to form stars.
The stars can be seen in a new image of the Southern Pinwheel galaxy, or M83, obtained by a Nasa space telescope and a ground-based observatory.
They are forming more than 100,000 light-years from M83's bustling centre.
Nasa's Galaxy Evolution Explorer (Galex) satellite spotted bright features in the long "arms" of the galaxy - coloured red in the image - which astronomers think are large clusters of stars.
"Every little pixel we see probably represents hundreds to thousands of stars. But we view them as a single blob," said Mark Seibert from the Carnegie Observatories in California.
"It would add up to quite a good number of stars out there."
Galex is equipped with a 50cm (19.7-inch) -diameter telescope to sweep the sky in search of ultraviolet light sources. But it cannot see individual stars because the design trades fine resolution for a large field of view.
Dr Seibert told BBC News: "A telescope with finer resolution would wash out a bit in the background. But the lower resolution of Galex actually improves detection of these features."
To better understand how stars could form in such unexpected territory, the astronomers used the Very Large Array (VLA) in New Mexico to carry out radio observations of the galaxy.
Galex sweeps the sky in search of ultraviolet light sources
"It is absolutely stunning that we find such an enormous number of young stars up to 140,000 light-years away from the centre of M83," said lead author Frank Bigiel from the Max Planck Institute for Astronomy in Germany.
Light emitted in the radio portion of the electromagnetic spectrum can be used to locate gaseous hydrogen atoms. These are seen as a good sign that the molecular form of the gas is also present. And it is from this molecular gas that stars are born.
When the astronomers combined the radio and the Galex data, they found that they matched up.
"Clearly, the basic ingredients for star formation are out in those regions," said Dr Seibert.
Co-author Fabian Walter, also of the Max Planck Institute for Astronomy, said: "The degree to which the ultraviolet emission and therefore the distribution of young stars follows the distribution of the atomic hydrogen gas out to the largest distances is absolutely remarkable."
Dr Seibert said that about 20% of the galaxies he had looked at showed ultraviolet emission in their outer regions.
In the case of some galaxies, stars on the outskirts could have been scooped away from another galaxy that came too close. But this seems unlikely for M83. It appears to be too symmetrical - lacking the uneven appearance of a galaxy that has collided with another.
The astronomers speculate that the young stars seen in far-flung regions of M83 could have formed under conditions resembling those of the early Universe, a time when space was not yet enriched with dust and heavier elements.
But this process is not well understood.
M83 is located 15 million light-years away in the southern constellation Hydra.