High-mass stars (stars much more massive than our Sun) can find
additional sources of fuel (beyond the "normal" Hydrogen and
Helium) such as Carbon, Oxygen, Nitrogen, Silicon, and other elements
up to Iron. Once the star reaches Iron, it reaches the end of the
line, a star cannot use Iron or any elements heavier then Iron for fuel.
After these high-mass stars run out of fuel to "burn" in their
core, the star must die! The most massive stars explode in a violent
Supernova, blasting their outer parts off into space. They
leave behind their core which becomes a dense Neutron Star (or
maybe even a Black Hole in the most massive cases). The
material that was blasted off into space, is enriched with all sorts
of heavy elements (elements heavier then Iron) that were created in the
explosion. This ejected material is so hot that it glows forming a
nebula around the central Neutron Star.Over time this
glowing remnant of a Supernova explosion cools and dissipates,
mixing into the surrounding gas clouds (i.e., Giant Molecular
Clouds), where it is incorporated into
the next generation of stars.
The following are a few examples of Supernova Remnants. They
are arranged from youngest to oldest:
