Berlin, October 1 : A thorough survey using the NASA/ESA Hubble Space Telescope has observed around 14 million stars in 69 galaxies, which has given astronomers clues about how stars form.
The detailed study, called the ACS Nearby Galaxy Survey Treasury (ANGST) program, explored a region called the Local Volume, where galaxy distances range from 6.5 million light-years to 13 million light-years from Earth.
Some galaxies were found to be full of ancient stars, while others are like sun-making factories.
A typical galaxy contains billions of stars but looks smooth when viewed through a conventional telescope because the stars appear blurred together.
In contrast, the galaxies observed in this new survey are close enough to Earth that the sharp view provided by Hubble's Advanced Camera for Surveys and Wide Field Planetary Camera 2 can resolve the brightness and colour of some individual stars.
This allows scientists to determine the history of star formation within a galaxy and tease out subtle features in a galaxy's shape.
"Past Hubble observations of the local neighborhood have provided dramatic insights into the star-formation histories of individual galaxies, but the number of galaxies studied in detail has been rather small", said Julianne Dalcanton of the University of Washington in Seattle (USA) and leader of the ANGST survey.
"Instead of picking and choosing particular galaxies to study, our survey will be complete by virtue of looking at 'all' the galaxies in the region. This gives us a multi-colour picture of when and where all the stars in the local Universe formed," she added.
Many stars in nearby galaxies are the fossil equivalents of new stars forming in the far Universe.
"When we look back in time at distant, young galaxies, we see lots of vigorous star formation. However, we can only guess as to what those galaxies might eventually turn into", Dalcanton explained.
"Using the galaxies in the nearby Universe as a 'fossil record', we can compare them with young galaxies far away. This comparison gives us a history of star formation and provides a better understanding of the masses, structures, and environments of the galaxies," she added.
Early results of the ANGST survey show the rich diversity of galaxies.
Some are made up entirely of ancient stars, while others have been forming stars nearly continuously during their whole lives. There are even a few examples of galaxies that have only started forming stars in the recent past.
"With these images, we can see what makes each galaxy unique", said team member Benjamin Williams of the University of Washington.
The ANGST survey also includes maps of many large galaxies, including M81.
According to Evan Skillman of the University of Minnesota, "With these maps, we can track when the different parts of the galaxy formed."