The formation of one type of these objects is characterized by a free expansion phase, a Sedov-Taylor phase, and finally a cooling phase. 84 of these objects are listed in the Gum catalogue, and "dark" objects of this type are so dense that light from surrounding objects cannot pass through them. A well-studied one of these objects is the first entry in the Messier catalogue. Very old red giant stars can eject material that forms the "planetary" type of these objects. The Pillars of Creation are a part of one of these objects named "Eagle." For 10 points, name these pretty astronomical clouds of star-forming gas and dust that include ones named "Orion" and "Crab."
The formation of one type of these objects is characterized by a free expansion phase, a Sedov-Taylor phase, and finally a cooling phase. 84 of these objects are listed in the Gum catalogue, and "dark" objects of this type are so dense that light from surrounding objects cannot pass through them. A well-studied one of these objects is the first entry in the Messier catalogue. Very old red giant stars can eject material that forms the "planetary" type of these objects. The Pillars of Creation are a part of one of these objects named "Eagle." For 10 points, name these pretty astronomical clouds of star-forming gas and dust that include ones named "Orion" and "Crab."
The formation of one type of these objects is characterized by a free expansion phase, a Sedov-Taylor phase, and finally a cooling phase.
nebulae [or emission nebulae; or planetary nebulae; or dark nebulae]