Declining Birth Rate

Our home galaxy, the Milky Way, is a vast collection of hundreds of billions of stars. And the population is growing. On average, the galaxy adds a few new stars every year.

Across the universe, though, the rate of star formation has slowed down by a good bit since the first era of starbirth. And it will continue to slow in the coming eons as the raw materials for making stars are used up.

Stars are born from giant clouds of hydrogen and other elements. Nuclear reactions in the cores of the stars “fuse” the hydrogen atoms to make helium, which in turn is fused to make even heavier elements. This process uses up the hydrogen. In fact, it’s dropped from about 75 percent of the total mass of the universe just after the Big Bang, to about 73 percent today.

That still leaves a lot of hydrogen. But it’s much harder to gather it up to make new stars. Much of it is already locked up in stars, so there’s not as much available for the big gas clouds that give birth to stars. In fact, some galaxies have already used up almost all of their supplies.

There’s still enough in gas-rich galaxies like the Milky Way to keep making stars for many, many billions of years. Eventually, though, almost all of the hydrogen will be incorporated into stars. And the little free hydrogen that remains will be so widely spread that there won’t be enough to coalesce to make more stars — and the stellar nurseries will go quiet.
 

Script by Damond Benningfield