Out of Gas

Soon after the universe was created in the unique event dubbed the Big Bang, over 99% of the early universe's material was in the form of ephemeral clouds of the lightest and simplest of all elements: mostly hydrogen with a dash of helium. So where did all those blazing stars come from, and even more curious, how were all the heavier elements created, so that rocks and humans could later arrive?

The very first stars formed when those clouds of hydrogen gas began to lump together here and there, in the early universe. As gravity drew a huge glob of hydrogen tighter and tighter, the pressure became great enough that the hydrogen atoms began to fuse and ignited, the same way a hydrogen bomb does. The process emits enormous amounts of light and heat. We call it a star.

As a star's nuclear furnace cooks away, it's turning its hydrogen into helium. At the end of the star's useful life it will most likely blow up, blasting and spewing the remaining hydrogen, along with the created helium into space. We call it a nova or super nova. Of the original hydrogen that started the furnace, about 30% is left and is exploded away; the other 70% was transmuted into heavier elements—mostly helium, which is also blown away. And this process goes on in subsequent stars, forging heavier and heavier elements.

This nuclear process has continued for some 13 billion years now; stars igniting, burning for a few billion years, and blasting their remnants into space for the next generation of stars to be formed from the remains of the earlier ones. It's the universe's great recycling system.

But isn't there a limit to the number of times this recycling can occur? If every star consumes some 70% of the hydrogen that formed it, isn't the universe going to run out of hydrogen some day? Won't our great universe run out of gas? Well, yes, it will, and astronomers have recently detected that gradual depletion process. The universe isn't yet quite “running on empty,” but we certainly are slowing down. The rate of star formation has dropped off; it peaked when the universe was only a few billion years old.

The universe has only so much time left, but it needn't worry us humans too much as yet. It'll be many more billions of years before the last star is born, and billions (if not trillions) of years before they all wink out. Let our grandkids worry about it.

(On a more serious note, the fact that the universe will someday end—albeit may be trillions of years away—has significant implications to those who once thought we inhabit an eternal cosmos. That's the topic of another blog.)