Fewer stars now

BY SPECIAL ARRANGEMENT

IN the largest ever study of its kind, a team of Portuguese, British, Japanese, Italian and Dutch astronomers has found that the rate of formation of new stars in the universe is now only 1/30th of its peak 11 billion years ago and that this decline is set to continue. The team, led by David Sobral of the University of Leiden, the Netherlands, published its results in the journal Monthly Notices of the Royal Astronomical Society.

The accepted model for the evolution of the universe predicts that stars began to form about 13.4 billion years ago, or around 300 million years after the Big Bang. Many of these first stars are thought to have been monsters by todays standards, and were probably hundreds of times more massive than the sun. Such beasts aged very quickly, exhausted their fuel, and exploded as supernovae within a million years or so. Lower mass stars, in contrast, have much longer lives and last for billions of years. Much of the dust and gas from stellar explosions was (and still is) recycled to form newer and newer generations of stars (see graphic). Stars are key ingredients of galaxies like our own Milky Way. Unveiling the history of star formation across cosmic time is fundamental to understanding how galaxies form and evolve.

In this study, scientists used the U.K. Infrared Telescope, the Very Large Telescope in Chile and the Subaru Telescope in Hawaii to carry out the most complete survey ever made of star-forming galaxies at different distances, with around 10 times the data of any previous effort. With the range of distances, the time taken for the light to reach the earth means that one sees identically selected galaxies at different periods in the history of the universe, so one can really understand how conditions change over time. By looking at the light from clouds of gas and dust in these galaxies where stars are forming, the team has been able to assess the rate at which stars are being born.

If the measured decline continues, then no more than 5 per cent more stars will form over the remaining history of the cosmos.... The research suggests that we live in a universe dominated by old stars. Half of these were born in the boom that took place between 11 and nine billion years ago and it took more than five times as long to produce the rest. The future may seem rather dark, but were actually quite lucky to be living in a healthy, star-forming galaxy which is going to be a strong contributor to the new stars that will form, Sobral said.

R. Ramachandran