(It might sound odd to use massive stellar explosions as distance benchmarks, because, of course, we can’t predict exactly when or where one will go off. But it turns out that the stellar explosion rate is high enough—a good rule of thumb is one supernova per galaxy per century—and there are so many galaxies, that if we just take pictures of lots of galaxies every night, we’re likely pretty often to see a blip in one that wasn’t there the night before, and then we can follow it up with more detailed observations.)
