The formation of new stars in galaxies like the Milky Way has declined five-fold in the last three billion years, according to astronomers.
While astronomers already knew that star formation was more prolific billions of years ago, the European Space Agency's Herschel telescope has for the first time been able to start measuring the rate of decline, says project scientist Steve Eales.
Three billion years ago "galaxies were forming stars at ... five times the rate we know today," he says.
Eales says the Herschel telescope's infrared technology allowed scientists to see galaxies, mainly spiral ones like the Milky Way, that were previously hidden from scientists' view by cosmic dust clouds.
The telescope, launched a year ago to study star formation, is the biggest ever sent into space, orbiting at a distance of 1.5 million kilometres from the Earth.
No new stars?
Scientists already knew that 10 billion years ago "there were these galaxies that were forming stars really fast," says Eales, but previous telescopes were unable to see up to a distance of 10 billion light years.
"We haven't been able to fill that gap until today," he says.
"What Herschel has been able to do because of the wave length it is observing at, it can suddenly see lots of galaxies in the nearby universe, up to about the last three or four billion light years. It can fill the gap in cosmic history.".
Eales says the findings suggest that "at some point stars will stop forming" altogether, unless solar conditions changed.
Scientists do not know the reasons for the decline.
Star in the making
Herschel has also found an embryonic "massive star" - a celestial object more than eight times the mass of our Sun.
"Massive stars are rare and short-lived," the researchers say. "To catch one during formation presents a golden opportunity to solve a long-standing paradox in astronomy."
Annie Zavagno says radiation emitted by massive stars should destroy them at some point - instead they continue to grow.
According to accepted scientific principles, stars should not be able to become more than eight times massive than our Sun.
Understanding how these "impossible" stars were formed was critical because they "control the dynamical and chemical evolution of galaxies," says Zavagno.
The ESA's director of science and robotic exploration, David Southwood, says "a new universe" was emerging from Herschel's findings.
"We can look at the complex chemistry that goes on in space that ultimately has created the things that we are made of."