September 6, 2001: 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. The galaxy NGC 3310 is one such starburst galaxy that is forming clusters of new stars at a prodigious rate. Scientists using NASA's Hubble Space Telescope are perfecting a technique to determine the history of starburst activity in NGC 3310 by studying the colors of its star clusters.
Measuring the clusters' color 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. Once formed, the star clusters become redder with age as the most massive and bluest stars exhaust their fuel and burn out.
Measurements of the wide range of cluster colors in starburst galaxies like NGC 3310 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. 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, once triggered, can continue for an extended interval.