Stellar Sizes—Then and Now

Stellar Sizes—Then and Now

“I’ll tell you one thing about the universe, though. The universe is a pretty big place. It’s bigger than anything anyone has ever dreamed of before. So if it’s just us… seems like an awful waste of space. Right?” Ellie Arroway, Contact

But is it really? Not according to a recent Astrophysical Journal article, which provides a reason for such a large universe.

Human life requires at least two things: substantial amounts of elements heavier than both hydrogen and helium (particularly carbon, oxygen, iron, and uranium), and a stable, long-burning star with a suitable planetary system. After the big bang creation of the universe, but before the first stars formed, virtually no elements except helium and hydrogen existed.

Without any elements heavier than helium, the gas clouds forming the first stars could not cool sufficiently to form anything but very massive stars. In an astronomically very brief period of time, these massive stars fuse hydrogen and helium to elements as heavy as iron before exhausting their nuclear fuel. The stars then explode in a massive supernova, which produces even heavier elements all the way up to uranium, and ejects these heavy elements back into space.

After a short epoch when these massive stars dominated, the universe contained enough heavy elements such that the size of subsequent stars fell dramatically. The gas clouds forming these future stars, which incorporated the heavy elements, could then cool to lower temperatures as they collapsed. Consequently, the clouds fragmented into smaller pieces and formed smaller stars. Though smaller than the first-generation stars, these second-generation stars were larger on average than those formed around the time our sun was born. More importantly, these stars still lack sufficient heavy elements for habitable planetary systems to form.

The third generation of stars (like the sun) can readily form planetary systems and the stars burn long enough and stably enough to form habitable planetary systems. If the universe were physically larger or smaller, or if it had a different mass density, its expansion rate would not allow a third generation of stars to form. Rather than being a waste of space, the universe is just the right size to permit the formation of a planetary system where human life can thrive.