Lastly, it will be seen that there is always a possibility that our own or any other sun may undergo precisely such a change as the stars in Cygnus and the Northern Crown. Some indeed, even among men of science (as the Abbé Moigno, for example) believe that it was an event of this sort which St. Peter predicted when he wrote, that as the old world, being overflowed with water, perished, so "the heavens and the earth which are now, by the same word are kept in store, reserved unto fire." According to that view, the day of destruction will come "as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also, and the works that are therein shall be burned up."
Let us consider how the sudden brightness of a new star may be explained.
I must confess that for my own part I do not attach much weight to the suggestion once made by Mr. Huggins, that an actual conflagration had taken place in the case of the new star in the Northern Crown. It does not seem to me that any process of mere burning could account for the enormous accession of light and heat which that sun underwent.
Consider the case of our own sun. His heat is very far beyond that which would be given out by any matter known to us undergoing any known process of true combustion. That is to say, if a mass as large as the sun of any known substance were caused to burn, under any conditions we can imagine, the momentary emission of heat by that mass would be very much less than the momentary emission of heat by the sun.
Now it is quite conceivable that by some great accession of combustible matter, some supply of fuel exceeding many times his entire mass, the sun's entire emission of heat might be very largely increased. But though such an idea is conceivable, it seems altogether far-fetched. The conception is, in fact, inadmissible as an explanation of the increase of heat of a temporary star, not because of the improbability of the sudden accession of so enormous a quantity of matter (though that improbability is very great), but because if so enormous a quantity of matter fell upon the sun, many times as much heat would be generated by the mechanical effect of the impact as by the combustion of the freshly received matter. So that even with the daring assumption here made, combustion would account for only a small portion of the increase of light and heat.
Huggins' idea was indeed somewhat different. He supposed that in consequence of some great internal convulsion of the sun in the Northern Crown a large volume of hydrogen and other gases was evolved from the interior, the hydrogen then by burning giving out the light corresponding to the bright lines. At the same time, the mass of the sun would be intensely heated by the surrounding mass of glowing hydrogen. When the liberation of gas from the interior ceased the flame would die out, and the sun's surface would gradually cool. But if we judge by the case of our own sun, the heat of the burning hydrogen would be nothing near so great as the heat of the glowing hydrogen already outside and within the visible globe of a sun.
On the whole it seems altogether more probable that the accession of splendour observed in the case of temporary stars is due to the downfall of enormous masses of matter upon the surface of these suns. It is, no doubt, well known to most of my readers that the downfall of meteoric matter upon the surface of our own sun has been considered a sufficient explanation of the sun's entire emission of light and heat. The theory that the sun's heat and light are thus excited has long since been abandoned; but not because the cause would be insufficient. It has been abundantly proved that a downfall of meteors, not sufficient in quantity to add appreciably to the sun's size in many thousands of years, would generate more heat and light than he emits in that time. The meteoric theory has been abandoned simply because it has been shown that no such downfall is taking place.
The reason why meteoric impact would suffice to warm the sun to his present temperature if the meteoric showers were heavy, and to warm him far beyond his present temperature if for a few days very heavy meteoric showers fell upon him, is simply that his attraction upon matter approaching him from without is capable of generating a tremendous velocity. We know that when a cannon-ball strikes a metal target, with a velocity perhaps of some 400 yards per second, great heat is excited, and there is a momentary flash of light. If the velocity were doubled, the quantity of heat would be doubled also. Conceive, then, the tremendous heat which would be excited if a cannon-ball could be caused to strike a target with a velocity exceeding that just named some 1500 times! The ball and target would both be vaporised by the shock, if—which, however, could never happen—the target resisted the blow and brought the ball to rest. Now matter which reaches the sun from without, under the influence of his tremendous attraction, strikes his globe with a velocity 1500 times greater than that of a cannon ball striking a target at a distance of two or three hundred yards. The heat excited is, therefore, very intense; and if meteors were showering at all times and in dense flights upon the sun's surface, we should require no other explanation of the sun's heat.
But it appears that meteoric systems are neither so numerous nor so rich as to account for the sun's uniform emission of heat, though occasional meteoric showers upon the sun may be heavy enough to increase appreciably the amount of heat he emits. It would seem, from experiments which have been made by Professor Piazzi Smyth, of the Edinburgh Observatory, and later by the Astronomer Royal at Greenwich, that from time to time the sun's emission of heat really is greater than usual. It seems not at all improbable that the increase is due to the occasional fall of large masses of meteors in great numbers upon the sun.