Astounding Science Fiction, British Edition — page 74: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

Te star otherwise moderately famous for being the first to have its parallax, and hence its distance, measured. In solving such a problem, the data normally consist of long series of measurements of the apparent direc- tion and distance of one star from the other ; if the stars are actually moving around each other, and the observa- tions cover a sufficient fraction of a revolution, it is ordinarily possible if not easy to compute the actual rela- tive orbit of the system—that is, the path of one assuming that the other is stationary. Dr. Strand’s work differed from the more usual exercises of this type in that his measures were made from photographs. This eliminated some of the difficulties usually en- countered in visual observation, and supplied a number of others; but there was a net gain in overall ac- curacy, to the extent that he was not only able to publish a more accurate set of orbital elements than had pre- viously been available, but to show that the orbital motion was not regular. The fainter star, it seemed, did not move around the brighter in a smooth ellipse at a rate predictable by the straightforward application of Kepler’s laws. It did, however, move in a Keplerian path about an invisible point which was in turn traveling in normal fashion about the other sun. There was nothing intrinsically sur- prising about this discovery; the implication was plain. One of the two stars—it was not possible to tell which, since measures had been made assuming the brighter to be stationary —was actually accompanied by an- other, invisible object; the invisible point which obeys the normal plane- tary and stellar laws was the center of gravity of the star-unknown object system. Such cases are by no means unusual. To learn which of the two suns is actually attended by this dark body, ASTOUNDING SCIENCE FICTION we would have to have more observa- tions of the system, made in relation to one or more stars not actually part thereof. Some stars exist near enough to the line of sight for such observa- tions to be made, but if they have been reduced and published the fact has not come.to my attention. I chose to assume that the object actually circles the brighter star. That may cost me a point in the game when the facts come out, but I won’t be too disheartened if it does. There was still the question of just what this object was. In other such cases where an invisible object be- trayed its presence by gravity or eclipse, as in the system of Algol, we had little difficulty in showing that the companion was a star of some more or less normal type—in the case of Algol, for example, the “dark” body causing the principal eclipse is a sun larger, hotter, and brighter than our own; we can tell its size, mass, luminosity, and temperature with very considerable precision and reliability. In the case of the 61 Cygni system, the normal methods were put to work; and they came up immediately with a disconcerting fact. The period and size of the orbit, coupled with the fairly well-known mass of the visible stars, indicated that the dark body has a mass only about sixteen thou- sandths that of the sun—many times smaller than any star previously known. It was still about sixteen times the mass .of Jupiter, largest planet we knew. Which was it—star or planet? Before deciding on the classi- fication of an object plainly very close to the borderline, we must obviously decide just where the borderline lies. For general purposes, our old grade- school distinction will serve; a star shines by its own light, while a planet is not hot enough for that and can be seen only by reflected light from some CoOnnicloooks Com