Moreover, it is far from absolutely certain that such stars have not been found. Very massive white dwarfs ought to have higher surface-temperatures than either massive main-sequence stars or than the known white dwarfs, all of which are of small weight. A whole group of stars is known—the _O_-type stars—whose spectra indicate very high temperatures indeed. These are usually interpreted as stars of enormous luminosity at enormous distances, but it is possible that some at least of them may be stars of feeble luminosity at moderate distances. In particular the central stars of the planetary nebulae are of types _O_ and _B_. But whereas the normal main-sequence star of these spectral types is generally about a thousand times as luminous as the sun, the central stars of the planetary nebulae are found to be considerably less luminous than the sun. Dr Gerasimovič has recently shewn that 52 of these stars have an average luminosity only about two-thirds of that of the sun. Other observers have found even lower luminosities. As the surface-temperatures of these stars must be of the order of 30,000 degrees, they must be minute in size; indeed a simple calculation shews that they can hardly have more than about a tenth of the sun’s radius. Not only are they small in size, but there are indications that they are of great weight. Spectroscopic evidence shews that in many cases the surrounding nebulae are in rotation, and, just as with the galaxy (p. 69) and the huge extra-galactic nebulae (p. 71), their weights can be calculated from their observed speeds of rotation. These seem to indicate average weights of something like 40 or 50 times the weight of the sun. Campbell and Moore, of Lick Observatory, have calculated that the planetary nebula N.G.C. 7009 must have 162 times the weight of the sun, but many factors combine to make this estimate rather uncertain.