Phil Plait writes about Video shows gas cloud expanding over time. "A thousand years ago—in July 1054, to be somewhat more precise—the light from a cosmic catastrophe reached Earth. A massive star, probably 20 or more times the heft of the Sun, exploded. This titanic event was vast almost beyond human grasp: It released as much energy in a few weeks as the Sun will over its entire 10-billion-year lifetime."
"Amazing as the image is, there’s another, subtler aspect of it that will cook your brain. That debris you see is still expanding, and quite rapidly. Because the Crab is tremendously far away—6,500 light-years or so—any motion is shrunk down to near invisibility. But we’ve been observing it for decades, which is a pretty long baseline. That means that if you compare an earlier image to a later one, you can actually see the physical expansion of the supernova explosion."
M1: The Crab Nebula Supernova Remnant (animation) from Adam Block on Vimeo."This animation shows the expansion of the Crab Nebula between the years of 1999 and 2012. The 1999 picture was taken by ESO using the VLT. The more recent picture was taken at the Mount Lemmon SkyCenter using the 0.8m Schulman Telescope. "