This is just a very small section of sky, smaller than the diameter of the full moon as seen from earth. It looks empty and black to the unaided eye, but the Hubble Space Telescope photographed all these galaxies in it.
When I studied science in high school more than 50 years ago, we learned that astronomers estimated there were between 100 billion and 1,000 billion (one trillion) stars in our Milky Way Galaxy. They also estimated there were between 100 billion and one trillion galaxies in the known universe.
It always struck me as a little bit strange that the range of estimated stars per galaxy and the range of estimated galaxies were the same, but it’s just a coincidence with no meaning except inside my head. (Yes, coincidences are sometimes real.)
Recently, I’ve read and heard exactly the same range of numbers: 100 billion from one source, a trillion from another source, and several estimates in between for both numbers. These particular ranges of estimates have remained the same for more than half a century. Let’s be conservative and use the number 200 billion for this essay. 200 billion stars in each of 200 billion galaxies. This seems to be the single estimate I have heard most often during all these years.
How many stars total is that? It’s 200,000,000,000 times itself; or 200,000,000,000 squared. That’s 40,000,000,000,000,000,000.000, or forty billion trillion, or forty sextillion stars in the known universe. It’s an unimaginably huge number. The human mind simply cannot understand a number that large unless we illustrate it some way.
The stars of the sky have been described as “innumerable,” and they are. Not the ones you can see on a clear night. There are only a couple of thousand stars you can see with your unaided eyes at night, and that assumes you have very good eyes and a really dark place from which to look. But nobody can count all the stars in space, else we wouldn’t have to estimate.
There are about seven billion (7,000,000,000) people on earth. The United Nations actually estimates we’ll pass that number sometime this year. If we were all lined up and crowded together so that we only had one foot of space to stand in, we’d make a line 1,325,758 miles long. Long enough to stretch to the moon and back more than 2 ½ times.
But suppose we were all given extremely powerful telescopes and asked to count all the stars in the universe. If each human now alive could count a star every second, 24 hours a day with no breaks, and nobody ever counted the same star twice, it would take us 181,000 years to finish the job. That’s almost as long as our species, H. sapiens, has existed on earth.
Even that may not be the whole universe; it’s the part we can study, at least in principle. The part we call “the known universe.” If there’s more — and there probably is — it’s so far away that even the light from it could not have reached us in all the 13.7 billion years the universe has existed. There doesn’t seem to be any possible way humans will ever be able to study it directly.
The moral of this story is, the universe is huge. Really huge. Fantastically huge. It’s far, far larger than you or I can possibly imagine.