By Dr David Whitehouse
BBC News Online science editor
Taking the deepest visible-light images ever taken of our Universe, astronomers have detected unexplained stars in the outskirts of the neighbouring Andromeda galaxy.
Destined to become a classic Hubble image
Surprisingly, they find that many of the stars in Andromeda's halo formed only about 7 billion years ago: quite different from the 12 billion-year age of the stars in our Milky Way's halo.
Astronomers believe that the young stars may have formed during a galactic collision involving Andromeda billions of years ago.
They can also see distant galaxies by looking through Andromeda, many bearing the traces of similar collisions.
Wilderness of stars and galaxies
Hubble's latest picture provides a hint of just how many stars and galaxies there are in the Cosmos.
It is just a hint - any real grasp of the numbers of stars out there is beyond any human comprehension. But peering through the outskirts of this companion galaxy to ours shows an uncharted wilderness of stars and an almost infinitely vaster terra incognita of galaxies in the far distance.
With Hubble's image, astronomers get two glimpses of the Cosmos at the same time, and can ponder the evolution of the nearby Andromeda galaxy, as well as the myriads of galaxies beyond it seen interlaced with Andromeda's stars.
To get the image, Tom Brown of the Space Telescope Science Institute used Hubble's Advanced Camera for Surveys to peer into a small section of Andromeda's suburbs for 120 Hubble orbits, allowing very faint stars to be seen.
Researchers estimate that Hubble's picture shows about 300,000 of these never-before-seen so-called halo stars.
These newly discovered younger stars are richer in heavier elements than they were expected to be meaning they are in some way linked to a mighty collision between Andromeda and some long dispersed galaxy long ago.
Looking through and beyond Andromeda, astronomers point out that a large fraction of the background galaxies they see have peculiar shapes due to galactic collisions and mergers. This reinforces the fact that we live in a vibrant and dynamic Universe undergoing constant change.