Motions and Distances of Planetary Nebulae
Planetary nebulae are symmetrical clouds of gas surrounding very hot stars. They probably represent a late stage in the evolution of many stars like the sun. The distances of these objects from us, and hence their intrinsic luminosities, are rather poorly estimated. Cudworth measured the angular motions of these objects across the sky and adopted other data from Perek and Kohoutek (1967). Application of the technique known in astronomy as statistical parallax, essentially the method of least squares, yielded distances for the nebulae. Table 23.1 presents the angular motions, μα and μδ, together with their mean errors and the sources for the proper motions of each nebula. Table 23.1 also lists whether the nebula is optically thick, TK, or thin, TN, the extinction parameter C, a measure of distance d and the class obtained from Greig (1971,1972).
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