Cometary Knot
Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406). They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System (i.e. the orbit of Pluto), with masses of around ''10−5'' times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula. At optical wavelengths, the knots are seen as "the ionized skin of a dense, dusty molecular globule" forming a crescent-shaped head that is ionized and illuminated by the central star, with a trailing spoke or tail. In molecular hydrogen and carbon monoxide data, the tails of cometary knots are observed to be highly molecular. The ce ...
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