The best detective is not infallible. In this story our astronomical detective made a reasonable but unsuccessful guess near the beginning of the case. He might have seen his error earlier, only there was a false clue dropped by a third party who happened to be present at the crime, which seemed to confirm the guess. This was very unlucky. But it makes all the better detective story of it. _The Story of the Companion of Sirius_ The title of this detective story is ‘The Nonsensical Message’. Sirius is the most conspicuous star in the sky. Naturally it was observed very often in early days, and it was used by astronomers along with other bright stars to determine time and set the clocks by. It was a _clock star_, as we say. But it turned out that it was not at all a good clock; it would gain steadily for some years, and then lose. In 1844 Bessel found out the cause of this irregularity; Sirius was describing an elliptic orbit. Obviously there must be something for it to move around, and so it came to be recognized that there was a dark star there which no one had ever seen. I doubt whether any one expected it would ever be seen. The Companion of Sirius was, I believe, the first invisible star to be regularly recognized. We ought not to call such a star hypothetical. The mechanical properties of matter are much more crucial than the accidental property of being visible; we do not consider a transparent pane of glass ‘hypothetical’. There was near Sirius something which exhibited the most universal mechanical property of matter, namely, exerting force on neighbouring matter according to the law of gravitation. That is better evidence of the existence of a material mass than ocular evidence would be. However, eighteen years later the Companion of Sirius was actually seen by Alvan Clark. This discovery was unique in its way; Clark was not looking at Sirius because he was interested in it, but because Sirius was a nice bright point of light with which to test the optical perfection of a large new object-glass that his firm had made. I dare say that when he saw the little point of light close to Sirius he was disappointed and tried to polish it away. However, it stayed, and proved to be the already known but hitherto unseen Companion. The big modern telescopes easily show the star and rather spoil the romance; but as romance faded, knowledge grew, and we now know that the Companion is a star not much less massive than the sun. It has ⅘ths of the mass of the sun, but gives out only ¹⁄₃₆₀th of the sun’s light. The faintness did not particularly surprise us;[11] presumably there should be white-hot stars glowing very brightly and red-hot stars glowing feebly, with all sorts of intermediate degrees of brightness. It was assumed that the Companion was one of the feeble stars only just red hot.