The Crab Nebula was a supernova that exploded in 1054. It is, in a manner of speaking, still exploding today. Don’t believe me? Voila!
The animated .gif above shows two composite images – one from the Palomar Optical Sky Survey I, the other from the imaginatively named Palomar Optical Sky Survey II, which have both been digitized and made available online. In the time between when those images were taken, the Crab Nebula has expanded even more! Yes, even 900 years (plus light travel time) after it went off, it’s still an expanding ball of gasses.
In a way, this makes perfect sense. In space, there isn’t much to make you stop. On Earth, an explosion is limited in size by gravity and air resistance. In space, there’s the tenuous clouds of dust between the stars; otherwise, the bubble just expands until it’s too thin to see. That takes millions of years, so after 900… it’s still going.
The Palomar Optical Sky Surveys captured that wonderful evidence of our changing sky entirely incidentally – their purpose was simply to scan the entire sky so for the purposes of being able to conduct surveys, match up targets, and… just to see what was up there.
You can see the improvement in technology between the 1950s and 1990s in the fine detial that’s only visible in the second image. Since then, we’ve had the Sloan Digital Sky Survey filling in more of the sky with even better resolution and even more colors (though they don’t cover the Crab Nebula, otherwise I’d have thrown it in as well), but that original 1950s survey is still important. You can throw all the best technology of today at the problem of making a sky map, but you’ll never be able to see what the sky looked like back in 1950… unless you use those old plates.
There’s an effort underway called DASCH to digitize the Harvard Plate collection. Harvard has been collecting images of the sky since the 1880s, and they’ve covered the entire sky a few times. Sure, the original images from 130 years ago will look pretty bad today, but they’re all we have to see what the sky was doing back then. It’s going to be monumentally useful once it’s finished. I bet we’ll have an even better view of the Crab expanding then, and numerous other things we haven’t ever noticed before.