The Crescent Nebula is an unusual emission nebula about 4,700 light-years away in the constellation of Cygnus. Like a planetary nebula, the glowing material that makes up the nebula was ejected from an aging star (the bright one visible in the center of the nebula). However, the Crescent Nebula emits light by a different mechanism than in a planetary nebula, which glows when its ejected material is heated up by strong UV light from its central star. Here, the nebula is heated by compression. The central star in the Crescent Nebula has entered a phase where it is shedding its outer layers in a strong stellar wind. As the high-speed wind slams into previously ejected shells of material, it compresses them and heats them until they glow. The Crescent Nebula is cataloged as NGC 6888.
144 minutes L and 144 minutes RGB on 2012-09-16 plus 224 minutes through an Astrodon 3 nm H-a filter on 2012-09-19 using a QSI 583 from northern New Jersey through an Astro-Physics 155mm refractor at f7.1. North is up. ©2012
Details from the above image.
Details from the above image in Hydrogen-alpha light.