It was believed that elliptical galaxies were full of similarly evolved and aged stars.
However, University of Michigan astronomers have discovered that old galaxies are still making new stars, shedding light into how galaxies evolve with time.
Using the Wide Field Camera 3 on the Hubble Space Telescope, U-M research fellow Alyson Ford and astronomy professor Joel Bregman saw individual young stars and star clusters in four galaxies that are about 40 million light-years away. One light-year is about 5.9 trillion miles.
“Scientists thought these were dead galaxies that had finished making stars a long time ago. But we've shown that they are still alive and are forming stars at a fairly low level,” said Ford.
Galaxies generally come in two types - spiral galaxies, like our own Milky Way, and elliptical galaxies. The stars in spiral galaxies lie in a disk that also contains cold, dense gas, from which new stars are regularly formed at a rate of about one Sun per year.
Stars in elliptical galaxies, on the other hand, are nearly all billions of years old. These galaxies contain stars that orbit every which way, like bees around a beehive. Ellipticals have little, if any, cold gas, and no star formation was known.
“Astronomers previously studied star formation by looking at all of the light from an elliptical galaxy at once, because we usually can't see individual stars,” said Ford.
“Our trick is to make sensitive ultraviolet images with the Hubble Space Telescope, which allows us to see individual stars,” he added.
The technique enabled the astronomers to observe star formation, evenif it is as little as one Sun every 100,000 years.
The team''s breakthrough came when they observed Messier 105, a normal elliptical galaxy that is 34 million light-years away, in the constellation Leo. Though there had been no previous indication of star formation in Messier 105, Ford and Bregman saw a few bright, very blue stars, resembling a single star 10 to 20 times the mass of the Sun.
They also saw objects that aren't blue enough to be single stars, but instead are clusters of many stars. When accounting for these clusters, stars are forming in Messier 105 at an average rate of one Sun every 10,000 years, the astronomers concluded.
"This is not just a burst of star formation but a continuous process," said Ford.
The findings will be presented at a meeting of the Canadian Astronomical Society in Ontario.