The system of the stars is floating in an ocean -- not merely an ocean of space, not merely an ocean of ether, but an ocean that is so far material that one atom or thereabouts occurs in each cubic inch. It is a placid ocean without much relative motion; currents probably exist, but they are of a minor character and do not attain the high speeds commonly possessed by the stars.

Many points of interest arise, but I will only touch on one or two. Why are the calcium atoms ionized? In the calm of interstellar space we seem to have passed away from the turmoil which smashed the calcium atoms in the interior of a star; so at first it seems difficult to understand why the atoms in the cloud should not be complete. However, even in the depths of space the breaking-up of the atom continues; because there is always starlight passing across space, and some of the light-waves are quite powerful enough to wrench a first or second electron away from the calcium atom. It is one of the most curious discoveries of modern physics that when a light-wave is attenuated by spreading, what it really suffers from is laziness rather than actual loss of power. What is weakened is not the power but the probability that it will display the power. A light-wave capable of bursting an atom still retains the power when it is attenuated a million- fold by spreading; only it is a million times more sparing in the exercise of the power. To put it another way, an atom exposed to the attenuated waves will on the average have to wait a million times longer before a wave chooses to explode it; but the explosion when it does occur will be of precisely the same strength however great the attenuation. This is entirely unlike the behaviour of water-waves; a wave which is at first strong enough to capsize a boat will, after spreading, become too weak. It is more like machine- gun fire which is more likely to miss a given object at greater distance but is equally destructive if it hits. The property here referred to (the quantum property) is the deepest mystery of light.

Thus in interstellar space electrons are still being torn from calcium atoms, only very infrequently. The other side of the question is the rate of repair, and in this connexion the low density of the cosmic cloud is the deciding factor. The atom has so few opportunities for repair.

Roving through space the atom meets an electron only about once a month, and it by no means follows that it will capture the first one it meets. Consequently very infrequent smashing will suffice to keep the majority of the atoms ionized. The smashed state of the atoms inside a star can be compared to the dilapidation of a house visited by a tornado; the smashed state in interstellar space is a dilapidation due to ordinary wear and tear coupled with excessive slackness in making repairs.

A calculation indicates that most of the calcium atoms in interstellar space have lost two electrons; these atoms do not interfere with the light and give no visible spectrum. The 'fixed lines' are produced by atoms temporarily in a better state of repair with only one electron missing; they cannot amount at any moment to more than one-thousandth of the whole number, but even so they will be sufficiently numerous to produce the observed absorption.

We generally think of interstellar space as excessively cold. It is quite true that any thermometer placed there would show a temperature only about 3 degrees above the absolute zero -- if it were capable of registering so low a reading. Compact matter such as a thermometer, or even matter which from the ordinary standpoint is regarded as highly diffuse, falls to this low temperature. But the rule does not apply to matter as rarefied as the interstellar cloud. Its temperature is governed by other considerations, and it will probably be not much below the surface-temperature of the hottest stars, say 15,000 degrees. Interstellar space is at the same time excessively cold and decidedly hot.[Note: As the word temperature is sometimes used with new-fangled meanings, I may add that 15,000 degrees is the temperature corresponding to the individual speeds of the atoms and electrons -- the old-fashioned gas-temperature.]

The Sun's Chromosphere

Fig 10. Solar Prominence. Photograph taken by E. T. Cottingham and the author in Principe at the total eclipse of 29 May 1919.

Once again we shift the scene, and now we are back in the outer parts of the sun. Fig. 10 shows one of the huge prominence flames which from time to time shoot out of the sun. The flame in this picture