The formation of one type of them involves the Sedov-Taylor phase, while another type includes one with features like the Spire and the Pillars of Creation. Another type, exemplified by the Cat's Eye and the famously bipolar 
The formation of one type of them involves the Sedov-Taylor phase, while another type includes one with features like the Spire and the Pillars of Creation. Another type, exemplified by the Cat's Eye and the famously bipolar Butterfly, is denser than a type commonly called H II regions, like the Horsehead one. In addition to emission and reflection varieties, these include supernova remnants and dark and planetary varieties. Famously exemplified by the Crab and Great Orion ones, these are, FTP, what clouds of interstellar gas and dust that give rise to new stars?
The formation of one type of them involves the Sedov-Taylor phase, while another type includes one with features like the Spire and the Pillars of Creation.
nebulae