An international team of astronomers has published a new study suggesting that nearly all of the stars that will ever exist in our universe have in fact already been born.
They found that the rate of star formation has decreased to such an extent that only 5% more stars than those that exist today will be produced in the remaining lifespan of the Universe.
David Sobral of Leiden University, the lead author of the study, tells TIME that the astronomers looked for a certain indicator—the H-alpha photons emitted by hydrogen atoms when a star forms—in their hunt for data “from stranger and smaller galaxies.”
They compiled snapshots of star-forming galaxies from different points of time in the Universe’s life span — at 2, 4, 6 and 9 billion years old — which took five years to put together. According to Sobral, “all of the action in the Universe occurred billions of years ago,” with half of all stars that ever existed created more than nine billion years ago and the remaining half created since then.
(MORE: How the Stars Were Born)
The findings have helped to explain the gap between the number of stars that can be observed and the number that scientists believe there should be in current existence. Sobral says that while there are plenty of dying stars which could help make new ones, there is a very narrow set of circumstances that enables star formation:
“You need the gas to become dense and cool enough to form stars. It’s true that when a supernova explodes, it helps, through shockwaves, to make the gas dense to trigger star formation. But if the explosion is too energetic, it can blow the gas out of the galaxy.”
The real conundrum for astronomers is explaining this phenomenon. “There is a big question in terms of really understanding why this is happening,” says Sobral. “It seems that the specifics of gas cooling and becoming dense is much harder now than it was many million years ago.”
Sobral and his colleagues are now looking at the selective samples to study the environment in which stars were formed to see how it has changed, if indeed it has, over time.
He also has comforting words for those saddened by the prospect of a future with fewer stars:
“This could be interpreted as quite depressing, but if you think about it, one of the reasons why we are around is because the rate of star formation is so low. If you were to maintain a steady trend of star production there is almost no chance that a planet like ours could survive.”
Sobral adds that the number of stars in our own galaxy, the Milky Way, is quite high. Given that even the shortest lifespan of stars runs into the billions of years, it will be a very long time indeed before we see the last star being born.