Q&A: Supernova Remnants and Neutron Stars

Q:
I still don't understand the fact that all that photos show something expanding in a "circular/planar" situation and not in a "spherical/3D" configuration. Could that be that the circular expansion is a concentration of a spherical explosion seeing sideways - a mere optical effect?

A:
Yes, many of the 2D "rings" are actually 3D expansions that appear as rings because of a phenomenon known as "limb brightening". If the expanding material is in a 3D shell, the telescope will look through more material on the edges of the shell than the center, causing it to appear as a bright ring.