Big numbers
We look at answers to an obvious question.
How many stars are there?
To most people nowadays, this isn’t an obvious or pressing question. The stars that can compete with city lights are few and not impressive by comparison. They aren’t a big part of our daily lives, and prompt questions only when they appear in a YouTube video or TV documentary.
But for those lucky or persistent enough to get out to a dark-sky location and look up, they can be truly awesome (in the old-fashioned sense of “inspiring awe”). The heavens seem drowned in innumerable points of light. Otherwise level-headed people have been prompted to write poetry. We suggest the experience is worth quite a bit of trouble and inconvenience. (It was common for our great-grandparents, of course, and remains so for the rural poor of the world.)
But, as scientists exercise their talent to transform the sublime into the mundane, the stars of the night sky have been numbered. Ptolemy’s catalog of some nineteen centuries ago contains about a thousand. That can sound like a lot, but it means that (roughly speaking, on the average) if you spread your hand at arm’s length you’d blot out a single star. Well, it did not pretend to list all you could see. The venerable Norton’s (which has probably started more professional and amateur astronomers on their careers than any other book) tries to match the sensitivity of the eye at a dark site, and contains about six thousand. There are more, of course. The smallest telescope will add thousands more faint stars, and big photographic surveys more still. It certainly looks like more and more will appear as one goes fainter, without end or limit.
But there is a limit. At some point we reach the smallest objects that fuse hydrogen to produce energy, the faintest true stars. And at some point we reach the boundaries of the Milky Way: the stars don’t go on forever in a uniform density. In practice we haven’t gotten there observationally; we can’t image the faintest stars from the far side of our galaxy. But we can confidently say, within reasonable limits, how many there are.
It’s mostly a matter of measuring the total light output of our galaxy. (It’s much easier for other galaxies, since we’re outside them, but the basic principle is the same.) If each star produces light-energy, it must go somewhere. Shrouded in a cloud of opaque dust or gas, the energy is transformed, but still shows up eventually as infrared or submillimeter radiation or whatnot. By being careful and clever, astronomers have worked out that there are something like 1011 stars in the Milky Way. That’s a lot, but not infinity. (In particular, it’s not enough to account for the mass of the galaxy by a large factor; but we’ll go into Dark Matter another time.)
There are other galaxies, of course. But counting the stars in them adds additional complications that we’ll leave for next week.
