... Triggered by the collapsing core of a massive star or the nuclear demise of a white dwarf, supernovae occur in average spiral galaxies only about once every century. Thor s Emerald Helmet . ... The Cocoon Nebula from CFHT . ... Polaris Dust Nebula . ... Hidden Galaxy IC 342 from Kitt Peak . ... A sprawling island universe, IC 342 would be a prominent galaxy in our night sky, but it is almost hidden from view behind the veil of stars, gas and dust clouds in the plane of our Milky Way galaxy. ...