I get a tingly feeling when I see cosmic objects change within our lifetimes. It’s no exception when I learn about a video from the Chandra X-ray observatory that shows how the Cassiopeia A supernova remnant expanded over a seven year period via Bad Astronomy.
The supernova remnant is about 10 light years across, and despite its unimaginable size, it’s growing visibly in a few scant years. It is expanding at about 11 million miles per hour, which is about 1.6% of the speed of light. For comparison, this is about a hundred times faster than the fastest spacecraft, which in turn is about a thousand times faster than an average car. Astronomers are a greedy bunch, however. Despite the staggering speed of 1.6% the speed of light, they still think it’s too slow. Fortunately, they have a good explanation for why it’s slower than expected: a lot of the energy from the supernova is lost in the form of cosmic rays.
I think my math skills have deteriorated to an alarming degree, though: With an image scale of 8.4 arcminutes and a distance of 10,000 light-years as specified on the Chandra page, the supernova remnant should be about 20 to 25 light-years across, and yet Wikipedia and other sources all say it’s ten light-years across. The discrepancy is too large even when taking the margins into account. Maybe there’s some unspecified cropping that has been done …