Q&A: Supernova Remnants and Neutron Stars

Q:
The Cassiopeia A pictures are very interesting. Is there an estimate of the mass and density of the stuff in the 10 light year diameter surrounding the supernova event? And is that stuff mostly silicon or a mix of elements such as is found in clouds?

A:
We hope you have a chance to view the Cas A images and information on the "Celebrating 5 Years with Chandra" web site:
http://chandra.harvard.edu/fifth/casa/

The 1999 observations showed the intensities of various elements (http://chandra.harvard.edu/photo/1999/casajph/) but it was really the 2000 data set that gave clear detailed maps of silicon, calcium, and iron in this supernova remnant (http://chandra.harvard.edu/photo/2000/cas_a062700/).

The total mass of the remnant (according to a 2002 paper, here:
http://xxx.lanl.gov/abs/astro-ph/0207273) is estimated to be 10 times the mass of the Sun. The density could be roughly estimated from that, assuming a sphere of diameter 10 light years. When we did the calculation we found roughly:
3 atoms per cubic centimeter,
assuming the atom is hydrogen (i.e. a proton).

If you'd like to check the calculation (always a smart thing to do), we suggest using Martin Zombeck's "Handbook of Space Astronomy and Astrophysics," which is something of a bible to astronomers. You will find the mass of the Sun, mass of a proton, light year to centimeter conversion, and much more:
http://ads.harvard.edu/books/hsaa/