The bright star near the center of this image is a rare
variety: a Wolf-Rayet star, a giant (this one about 40x the
mass of the sun) that burns hot, sheds a lot of material into
space, and will soon end in a supernova. The material
cast off by this star plows into the thin interstellar gas, and
piles it up in a denser ridge (think about a snow plow piling up
the snow). The radiation from the star ionizes this denser gas,
which then glows faintly red. In this case, the material has formed
a spherical bubble around the star. The bubble is 7 light years in
diameter, and is expanding at 4 million miles per hour. The Bubble nebula
is 7100 light years away from earth, in the constellation Cassiopeia. For
other examples of nebulae produced by Wolf-Rayet stars, see the images in
this gallery of the Crescent Nebula and Thor's Helmet.

Image data:
Camera: Canon 350 XT (modified)
Exposure: ISO 800; 5 minutes x 20, plus 8 minutes x 28 through 7nm hydrogen alpha filter
Telescope: 10" Schmidt-Newtonian, Baader MPCC