The most detailed image of a distant spiral galaxy ever captured has been assembled using the Hubble Space Telescope, revealing new stellar nurseries where stars are formed.

The remarkable portrait of the Pinwheel Galaxy, also known as Messier 101, has given astronomers an unprecedented opportunity to study the workings of a galaxy of similar type to our own Milky Way.

An international team of astronomers pieced together the picture from 51 separate Hubble images gathered over 10 years, which were combined with observations from powerful ground-based telescopes.

The result is so detailed that it is possible to pick out individual stars, of which the galaxy is estimated to hold about a trillion.

The Pinwheel Galaxy is about 25 million light years away, which is close to the limit at which Hubble can resolve individual stars rather than clusters alone, and ten times farther than the Milky Way’s nearest galactic neighbour, Andromeda.

It is so far away that the light we see now was generated long before the human family tree branched away from the other apes, at about the same time that the extinct elephant-like mastodon first evolved on Earth.

At 170,000 light years across, the galaxy is almost twice as large as the Milky Way, and occupies an area of the sky equivalent to a fifth of the full Moon, though it cannot be seen with the naked eye. It lies in the constellation of Ursa Major, the Great Bear.

The galaxy is an astronomers’ favourite as its spiral shape faces Earth, making it easier to study. Up to 100 billion of its stars may be Sun-like in their age and intensity.

The scientists who assembled the new image have already used it to locate almost 3,000 clusters of stars that had not previously been detected.

K.D. Kuntz of Johns Hopkins University in Baltimore, Maryland, who led the research team, has also used the image to identify the sources of about a third of several hundred points of X-ray light that have been picked up in the galaxy. Four particularly bright areas of X-ray emissions are likely to come from black holes of similar size to the Sun.

The spiral arms of the Pinwheel Galaxy, from which it takes its name, are sprinkled with nebulae in which stars are being formed in clouds of hydrogen. The blueish light of the arms comes from clusters of newborn stars that are being created in these stellar nurseries.

More discoveries are expected to follow as scientists study the galactic portrait in more detail.

"Just last week, I was looking at a random corner of the image I hadn’t paid much attention to and found a very faint wisp of blue stars in the middle of nowhere," Dr Kuntz told NewScientist.com.

"I like to call this the data set that keeps on giving — I’m sure there are other interesting things imaged in the data that we haven’t noticed yet."

The 51 Hubble images used to create the portrait were originally taken to support several different research projects, such as a study of the expansion rate of the Universe and the formation of star clusters.

All the original pictures were taken with Hubble’s Advanced Camera for Surveys and the Wide Field and Planetary Camera 2, between March 1994 and January 2003. Ground-based pictures were used from the Canada-France—Hawaii Telescope in Hawaii and the Kitt Peak National Observatory in Arizona.

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