Monday, December 12, 2005

Star time

Here you can see a stunning new Hubble Space Telescope mosaic of the Crab Nebula in the constellation Taurus. As many of you will know, this is the remnant of a star that blew up in the year 1054 A.D., Earth time, an event that was observed and recorded by Chinese and, maybe, Native American skywatchers. (It's also possible that the Irish took note.) The progenitor star was 6500 light-years away, which means it actually blew up 6500 years before it was observed on Earth.

Most nebulas photographed by astronomers appear to hang motionless in the sky. What motions they have are made imperceptible by size and distance. These things exist in a space and time that dwarfs human existence.

The Crab was first photographed a century ago. If old photographs are carefully compared to contemporary photographs, it is possible to detect the outward motion of the gases against the background stars. I used to do this exercise with my students. One can work backwards and confirm (more or less) the date the star went supernova. I always liked this exercise because it let us directly observe an event unfolding in cosmic time.