The message from Algol A and B was confused, not only on account of the missing word, but because a word or two of another message from Algol C had got mixed up with it; so that even when the missing word was found to be ‘five’ and confirmed in two ways, the message was not quite coherent. In another place the message seemed to waver and read ‘two-and-a-half’. The finishing step is the discovery that ‘two-and-a-half’ belongs to a different message from a previously unsuspected star, Algol C. And so it all ends happily.
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.
In 1914 Professor Adams at the Mount Wilson Observatory found that it was not a red star. It was white—white hot. Why, then, was it not shining brilliantly? Apparently the only answer was that it must be a very small star. You see, the nature and colour of the light show that its surface must be glowing more intensely than the sun’s; but the total light is only ¹⁄₃₆₀th of the sun’s; therefore the surface must be less than ¹⁄₃₆₀th of the sun’s. That makes the radius less than ¹⁄₁₉th of the sun’s radius, and brings the globe down to a size which we ordinarily associate with a planet rather than with a star. Working out the sum more accurately we find that the Companion of Sirius is a globe intermediate in size between the earth and the next larger planet Uranus. But if you are going to put a mass not much less than that of the sun into a globe not very much larger than the earth, it will be a tight squeeze. The actual density works out at 60,000 times that of water—just about a ton to the cubic inch.
We learn about the stars by receiving and interpreting the messages which their light brings to us. The message of the Companion of Sirius when it was decoded ran: ‘I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a match-box.’ What reply can one make to such a message? The reply which most of us made in 1914 was—‘Shut up. Don’t talk nonsense.’
But in 1924 the theory described in the last lecture had been developed; and you will remember that at the end it pointed to the possibility that matter in the stars might be compressed to a density much transcending our terrestrial experience. This called back to mind the strange message of the Companion of Sirius. It could no longer be dismissed as obvious nonsense. That does not mean that we could immediately assume it to be true; but it must be weighed and tested with a caution which we should not care to waste over a mere nonsense jingle.