We Live in a Big, Big, Big, Big, Big Universe
This is just one very small piece of sky, much smaller than the diameter of the full moon as seen from earth. It looks completely empty and black to the unaided eye, but the Hubble Space Telescope stared at it for several days and finally collected enough light to photograph all these galaxies in it. (You can’t see individual stars here. Every dimmest, tiniest, all-but-invisible speck of light is a whole galaxy. The original, very large photograph shows many times more galaxies than you can see even here.)
When I studied science in high school almost 60 years ago, we learned that astronomers estimated there were between 100 billion and a trillion (1,000 billion) stars in our Milky Way Galaxy. They also estimated there were between 100 billion and a trillion galaxies in the known universe. (This doesn’t imply the Milky Way is necessarily an average galaxy in all ways, of course. It is not.)
It always struck me as a little bit strange that the range of estimated stars per galaxy and the range of estimated galaxies in the known universe were the same, but it’s just a coincidence with no meaning except inside my head, and maybe inside the heads of the astronomers doing the estimating. (Yes, coincidences are sometimes really just coincidences.)
More 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 between for each number. These particular ranges of estimates have remained the same for more than half a century. The most common actual estimate seemed, until recently, to be about 200 billion each: 200 billion stars, on average, in each of 200 billion galaxies. In the past couple of years, though, I seem to have read estimates of a trillion galaxies in the known universe and a trillion stars in an average galaxy more and more often. (This doesn’t mean the universe is getting bigger. It really is, but that’s a different story. What this means is that the estimates are finally getting better. And bigger.) These are the figures we’ll play with here: a trillion galaxies and an average of a trillion stars per galaxy.
That’s a lot of stars.
How many stars does that make? It’s 1,000,000,000,000 times itself; 1,000,000,000,000 squared. That’s 1,000,000,000,000,000,000,000.000, or a septillion stars in the known universe. It’s an unimaginably huge number. The human mind simply didn’t evolve to comprehend any number that large, so we have to break it down and illustrate it some way.
This is why the stars of the sky have been described as “innumerable.” They literally are. Not the ones you can see on a clear night with your unaided eyes. There are only a few thousand of those, and even that assumes you have really good eyes and a really dark place from which to observe. But nobody — nor even everybody all put together — could count all the stars in space, else we wouldn’t have to estimate.
But there are a lot of us, too.
There are more than 7.4 billion (7,400,000,000) people on earth right now. If we were all lined up and jammed together so that we each had only one foot of space (about 30 cm) in which to stand, we’d make a line 1,401,515 miles (2,340,530 km) long. Long enough to stretch all the way around the planet 55 times with enough over to tie a nice bow.
But suppose we were all given extremely powerful telescopes and asked to count all the stars in the “known universe.” Then suppose every human now alive could count a star every second of every day, 24 hours a day and seven days a week, and nobody ever counted the same star twice. It would take us just over 4,284,174 years to finish the job.
Even that vast number of stars, along with the accompanying planets, asteroids, comets, nebulae, black holes and all the other inhabitants of the cosmological zoo may not comprise the whole universe; it’s the part we can see and study, at least in principle. The part we call “the known universe.” If there’s more — and there almost certainly is — it’s so far away that even the light from it could not have reached us in all the 13.8 billion years the universe has existed. There doesn’t seem to be any possible way humans will ever be able to see it or study it directly.
The moral of this story is, the universe is big. Really big. Fantastically big. It’s far, far bigger than you or I can possibly imagine.