Most galaxies form new stars at a fairly slow rate, but members of a rare class known as "starburst" galaxies blaze with extremely active star formation. Scientists using NASA's Hubble Space Telescope are perfecting a technique to determine the history of starburst activity in galaxies by using the colors of star clusters. Measuring the clusters' colors yields information about stellar temperatures. Since young stars are blue, and older stars redder, the colors can be related to the ages, somewhat similar to counting the rings in a fallen tree trunk in order to determine the tree's age.
The galaxy NGC 3310 is forming clusters of new stars at a prodigious rate. Astronomer Gerhardt Meurer of The Johns Hopkins University leads a team of collaborators who are studying several starburst galaxies, including NGC 3310, which is showcased in this month's Hubble Heritage image.
There are several hundred star clusters in NGC 3310, visible in the Heritage image as the bright blue diffuse objects that trace the galaxy's spiral arms. Each of these star clusters represents the formation of up to about a million stars, a process that takes less than 100,000 years. In addition, hundreds of individual young, luminous stars can be seen throughout the galaxy.
Once formed, the star clusters become redder with age as the most massive and bluest stars exhaust their fuel and burn out. Measurements in this image of the wide range of cluster colors show that they have ages ranging from about one million up to more than one hundred million years. This suggests that the starburst "turned on" over 100 million years ago. It may have been triggered when a companion galaxy collided with NGC 3310.
These observations may change astronomers' view of starbursts. Starbursts were once thought to be brief episodes, resulting from catastrophic events like a galactic collision. However, the wide range of cluster ages in NGC 3310 suggests that the starbursting can continue for an extended interval, once triggered.
Located in the direction of the constellation Ursa Major, NGC 3310 has a distance of about 59 million light-years. Hubble's Wide Field Planetary Camera 2 was used to make observations of NGC 3310 in March 1997 and again in September 2000. The color rendition of the combined images was created by the Hubble Heritage Team.
Acknowledgment: G.R. Meurer and T.M. Heckman (JHU), C. Leitherer, J. Harris and D. Calzetti (STScI), and M. Sirianni (JHU)