By Ian Steadman, Wired UK

Most of the stars that will ever exist have already been born, according to the most comprehensive survey of the age of the night sky.

[partner id="wireduk" align="right"]An international team of astronomers used three telescopes – the UK Infrared Telescope and the Subaru Telescope, both in Hawaii, and Chile's Very Large Telescope – to study trends in star formation, from the earliest days of the universe. Extrapolating their findings has revealed that half of all the stars that have ever existed were created between 9 and 11 billion years ago, with the other half created in the years since. That means that rate at which new stars are born has dropped off massively, to the extent that (if this trend continues) 95 percent of all the stars that this universe will ever see have already been born.

Several studies have looked at specific time "epochs", but the different methods used by each study has restricted the ability to compare their findings and discern a fuller model of how stars have evolved over the course of the entire universe's lifespan.

We do know that many stars around today – including our own – likely formed out of the dust left over from earlier, bigger stars going supernova in the early years of the universe. The problem was figuring out exactly how many stars the universe used to give birth to relative to how many are born in later years, as it seemed that at some point there was a steep drop off in the creation of new stars.

The telescopes searched for alpha particles emitted by Hydrogen atoms (commonly found in star formation, appearing as a bright red light) throughout huge patches of sky. Snapshots were taken of the look of the universe at defined different points in time, when it was 2, 4, 6 and 9 billion years old – a sample that's 10 times as large as any previous similar study.

The results showed clearly that half of all the stars that have ever existed in the universe were created more than 9 billion years ago, with the remaining half coming into existence since then. On the Subaru Telescope's site, the study's lead author, Leiden University's David Sobral, writes: "The production of stars in the Universe as a whole has been continuously declining over the last 11 billion years; it is 30 times lower today than at its likely peak 11 billion years ago. If this trend continues, no more than five percent more stars will exist in the Universe. We are clearly living in a Universe dominated by old stars. All of the action in the Universe occurred billions of years ago!"

Importantly, it also provides a way to reconcile the previously confusing disparity between the number of stars we can observe and the number of stars we know should have been created by the universe. The first generations of stars would have been extremely large – many hundreds of times larger than our Sun – and would have burned through their fuel quickly, undergoing supernovae death and providing the scattered discs of dust that later stars and planetary systems will have formed out of.

The findings map onto this, showing a huge rate of star formation which slowed rapidly after the first generation 9 billion years ago. It then took almost five times as much time for the same number of stars to be born again, accounting for the second half of all observed star formation. Findings from other studies into star formation – which used smaller sample sizes, or different methods – also fit onto the graph that the team derived, reinforcing the "huge early peak then rapid decline" theory. The study has been published in the Monthly Notices of the Royal Astronomical Society, and is available to read here.

Unfortunately, then, it looks like our universe is running out of steam – in only a few more billion years, the study predicts, we may well be seeing the very last star that will ever be born. That's if humans manage to survive that long, of course.

Source: Wired.co.uk