The mysterious objects known as planetary nebulae, of which examples are shewn in Plate II (p. 28), ought perhaps to be regarded as stars of still larger diameter. At the centre of each of these the telescope discloses a comparatively faint star with an extremely high surface-temperature. Surrounding this is the nebulosity from which these objects derive their somewhat unfortunate name. This is in all probability merely an atmosphere of even greater extent than that surrounding the four stars of our table. Van Maanen estimates the diameter of the nebulosity of the Ring Nebula in Lyra (fig. 2 of Plate II) to be 570 times that of the earth’s orbit, or about 106,000,000,000 miles. This nebulosity, however, differs from the atmosphere of an ordinary star in being very nearly transparent; we can see through 106,000,000,000 miles of the Ring Nebula but can only see a few tens or hundreds of miles into an ordinary star. At the other extreme of size, the smallest known star, van Maanen’s star (p. 268) is just about as large as the earth; over a million such stars could be packed inside the sun and still leave room to spare. And yet its weight is in all probability comparable, not with that of the earth, but with that of the sun; at a guess it may have about a fifth of the sun’s weight. To pack a fifth of the sun’s substance inside a globe of the size of the earth, the average ton of matter must be packed into a space of about the size of a small cherry—ten tons or so to the cubic inch. The solidity of the earth suggests that its atoms must be packed pretty closely together, but the atoms in van Maanen’s star must be packed 66,000 times more closely. How is it done? As we shall shortly see, there is only one possible answer. The atom consists mostly of emptiness—we compared the carbon atom to six wasps buzzing about in Waterloo Station. Let us break the atom up into its constituent parts, pack these together as closely as they will go, and we see the way in which matter is packed in van Maanen’s star. Six wasps which can roam throughout the whole of Waterloo Station can nevertheless be packed inside a very small box. GIANTS AND DWARFS. There is a continuous series of stars between the limits of weight we have mentioned, and the same is true of the limits of temperature (and so also of colour) and of size. [Illustration: Fig. 21. Stars of different colours arranged in order of luminosity.]