KNOWLEDGE & SCIENTIFIC NEWS. 



[November, 1905. 



round the sun's limb during- total eclipses ; but these 

 are of comparatively small importance, and not likely 

 at any time to endang^er the earth's safety. An out- 

 burst on a much larger scale would be necessary to 

 produce anything in the way of a catastrophe which 

 would destroy all life on our terrestrial alxxle. 



\ow is tiicre any cause which would produce a great 

 outburst of light and heat in the sun ? I think we have 

 such a cause in the possible collision of the sun with a 

 dark body in space. The distance of the stars is so 

 great, that the collision of the sun with a star is a con- 

 tingency which may be at once dismissed. Such an 

 event, if it ever took place, could not possibly happen 

 for thousands of years to come. To pass over the 

 distance which separates the sun from even the maresl 

 fixed star would take, at the rate of, say, 10 miles a 

 second, about 80,000 years ! 



The existence of dark bodies in space has been sus- 

 pected by astronomers. I say suspected, for really we 

 have no direct evidence that such bodies exist The 

 idea seems to have originated in the so-called "dark 

 companion " of the variable star .Algol. But we have 

 no evidence whatever that .Algol's companion is really 

 a " dark body," that is, a body devoid of all inherent 

 light of its own, like the earth. It is true that in the 

 case of Algol the spectroscope shows no sign of a 

 second spectrum, as in some variables of the .Algol 

 type in which both components are of nearly equal 

 brightness. But it has Ixjen recently found by Profes- 

 sor Hartmann that " a difference of only about one 

 magnitude would be sufHcient to bring the spectrum of 

 the fainter component to almost complete disappear- 

 ance, and a difference of two magnitudes would make it 

 impossible for even a trace of the fainter spectrum to 

 be visible on the plate." * The companion of Algol 

 might, therefore, be of the 4^ or 5th magnitude, and 

 neither telescope nor spectroscope would show anv sign 

 of its existence. But, apart from the above considera- 

 tions, it seems very probable that many dark bodies do 

 exist in space. In the ca.se of large bodies of this 

 kind, they would have their origin in cooled down suns, 

 ."^tars cannot go on shining for ever. They commence 

 I heir course with a limited amount of potential energy, 

 and this energy is being incessantly dissipated in the 

 form of radiant light and heat. This dissipation of 

 energy cannot clearly g'o on continually, and in the 

 course of ages must become exhausted. It is like a 

 man living on his capital. If he receives no interest 

 on it, and goes on spending the money steadily, the 

 day must come, sooner or later, when the capital will 

 disappear, and the man will be reduced to a state of 

 bankruptcy. So it is with a sun. It can receive no 

 energy from without, and it is constantly wasting its 

 capital of energy in the radiation of heat and light. 

 It is true that this waste may be apparently compen- 

 sated for a time by the contraction of the sun's mass 

 due to gravity. But this is only the conversion of 

 potential energy into heat, and eventually the prtxress 

 must cease, as after a time — counted, of course, by 

 ages — the sun's density will lK;cf)me so great that the 

 contraction will cease, owing to the overcrowding of 

 the molecules, no further heat will lie pro<luced, and the 

 fKKly will begin to cf)ol down. When Ibis cor)ling prr>. 

 cess has sufficiently advanced, the sun will lose its 

 light, and " roll through space a cold and dark ball." 

 There is evidence to show that in some of the long- 

 period variable stars, this permanent waning of light 

 has already commenced, and it seems highly probable 



• A iirophytital Journal, May. 1904 



that, in many cases, the "cold and dark" stage has 

 been actually reached. These dark bodies may, indeed, 

 be very numerous, but we have no means of observing 

 them, as they show no light, and would not be visible, 

 even as faint stars, by the largest telescopes which 

 could ever be constructed. 



It is now well known that the sun is moving through 

 sp.ice with a considerable velocity, :ind, of course, 

 carrying with it the earth, and all the planets and satel- 

 lites of the solar system. X'arious estimates have been 

 made of the point towards which the sun is moving, 

 but the most recent and accurate calculations seem to 

 point to a spot near the bright star Vega [a Lyr.-r). 

 In its flight through space it seems quite within the 

 bounds of possibility that the sun may .some day come 

 into collision with a dark body. Should such an event 

 occur, the collision would, of course, produce an enor- 

 mous amount of heat and light, .nul St. IVter's predic- 

 tion would at once be fulfilled. " The heavens " would 

 be " on fire," and the whole surface of the earth, and 

 everything on it, would be reduced to cinders in a few 

 minutes. It would be like the destruction of St. Pierre 

 on a colossal .scale. The world would end 



" In unrcmorseful folds of rolling fire."* 



But such a catastrophe could not occur without our 

 knowing of the coming disaster months, and perhaps 

 years, beforehand. When the approaching dark body 

 came within a certain distance of the sun it would 

 begin to shine by rellected light, like the planets. If 

 a very large body, comparable with the sun itself in 

 size, it would first become visible far beyond the con- 

 fines of the solar .system. l""or some months, or years, 

 its motion would be very slow, owing to its groat 

 distance from the sun. It would probably be 

 first discovered as a telescopic star, not differ- 

 ing in appearance from other stars of the 

 same brightness in its vicinity. It would then, 

 perhaps, shine as a star of about the glh magni- 

 tude, as any much fainter star would probably be over- 

 looked. Doubtless it would at first be mistaken for a 

 "new" or "temporary star," or a variable star at its 

 maximum brightness; but the comparative constancy 

 of its light, and its great parallax, or apparent change 

 of place among the neighbouring stars, would soon 

 reveal its true character, and show that it was really 

 near the earth compared with the distance of the 

 stars. It might, jiowcver, be mistaken for a distant 

 comet, but if coming directly towards the sun, its 

 change of place would be small, and its light examined 

 with the spectroscope would show a solar spectrum, 

 indicating that, like the plancl.s, it was shining by re- 

 flected sunlight. h"urthcr, its distance could be calculated 

 from its par.illax, and the result would show that no 

 comet would be visible at such a distance from the 

 Sim. 



1 have made some c.ilculalions on the motion of this 

 hypothetical Ixxly after it became visible as a star of the 

 9th magnitude, and. therefore, e.isily visible in a tele- 

 scope of three inches aperture. I.et us suppose the 

 approaching dark body tr) have the same mass as the 

 Sim and the same density as the earth. T:d<ing the 

 earth's density as four limes that of the sun, and the 

 sun's di.-imeter as 866,000 miles, I find that the dia- 

 meter of the dark body would be about 546,000 miles. 

 Now, taking the diameter of Uranus as ,^,V<'oo miles, 

 its stellar magnitude as .s-7. and assuming that the dark 



• Tennyson. Th( I Inly Grail. 



