Fewer stars form as galaxies run out of gas

Galaxies are running out of the basic building blocks of stars. The universe is forming fewer stars than it used to, and an Australian research team says the reason is a shortage of molecular hydrogen.

Astronomers have known for over 15 years that the rate of star formation peaked when the universe was only a few billion years old and has declined steeply ever since.

Dr Robert Braun of CSIRO and his team used CSIRO’s Mopra radio telescope in New South Wales to compare far-off, older galaxies with nearby ones. They found that galaxies observed at between three and five billion years old appear to contain considerably more molecular hydrogen gas – from which galaxies are formed – than comparable galaxies in today’s universe.

“Our result helps us understand why the lights are going out,” saysBraun. “Star formation has used up most of the available molecular hydrogen gas.”

After stars form, he says, they shed gas during various stages of their lives, or in dramatic events such as supernovae. But a good 70  percent remains locked up.

Galaxies are also ‘refueled’ from outside, with gas falling into them from the  the intergalactic medium, the space between galaxies. Two-thirds of the gas in the universe is still found here, with only one third consumed by previous star formation.

“The drop-off in both gas availability and star formation seems to have started around the time that Dark Energy took control of the universe,” Braun says.

Until then, gravity dominated the universe, so the gas was naturally pulled in to galaxies. But as the effect of Dark Energy took ove, the universe started expanding faster and faster, making it harder for galaxies to capture more gas, he suggests.