Stellar birth rate falling fast

Posted by Emma Woollacott

The rate of star formation in the universe is now only one-thirtieth of its peak, says an international team of astronomers - and the decline is set to continue.

"You might say that the universe has been suffering from a long, serious "crisis": cosmic GDP output is now only three percent of what it used to be at the peak in star production!" says Dr David Sobral of the University of Leiden in the Netherlands.

The accepted model for the evolution of the universe suggests that stars began to form about 13.4 billion years ago, or around three hundred million years after the Big Bang. Many of these first stars are thought to have been monsters by today's standards, hundreds of times more massive than our sun.

These stars aged very quickly, exhausted their fuel, and exploded as supernovae within a million years or so - much faster than lower-mass stars, which last for billions of years.

Much of the dust and gas from stellar explosions was - and is still - recycled to form newer generations of stars. Our sun, for example, is thought to be a third generation star.

In their study, the team used the UK Infrared Telescope (UKIRT), the Very Large Telescope (VLT) and the Subaru telescope to carry out the most complete survey ever made of star-forming galaxies at different distances, with around ten times the data of any previous attempt.

With the range of distances, the time taken for the light to reach us means that we see identically selected galaxies at different periods in the history of the universe, showing 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 assessed the rate at which stars are being born - and found that the production of stars in the universe as a whole has been continuously declining over the last 11 billion years. Indeed, it's now 30 times lower than at its likely peak, 11 billion years ago.

If the measured decline continues, then no more than five percent more stars will form over the remaining history of the cosmos, says Sobral.
We live in a universe dominated by old stars, half of which were born in the 'boom' that took place between 11 and nine billion years ago.

"The future may seem rather dark, but we're 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," says Sobral.

"Moreover, while these measurements provide a sharp picture of the decline of star-formation in the universe, they also provide ideal samples to unveil an even more fundamental mystery which is yet to be solved: why?"