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.
It should be understood that it was very difficult to explain away the original message as a mistake. As to the mass being ⅘ths of the sun’s mass there can be no serious doubt at all. It is one of the very best determinations of stellar mass. Moreover, it is obvious that the mass must be large if it is to sway Sirius out of its course and upset its punctuality as a clock. The determination of the radius is less direct, but it is made by a method which has had conspicuous success when applied to other stars. For example, the radius of the huge star Betelgeuse was first calculated in this way; afterwards it was found possible to measure directly the radius of Betelgeuse by means of an interferometer devised by Michelson, and the direct measurement confirmed the calculated value. Again the Companion of Sirius does not stand alone in its peculiarity. At least two other stars have sent us messages proclaiming incredibly high density; and considering our very limited opportunities for detecting this condition, there can be little doubt that these ‘white dwarfs’, as they are called, are comparatively abundant in the stellar universe.
But we do not want to trust entirely to one clue lest it prove false in some unsuspected way. Therefore in 1924 Professor Adams set to work again to apply to the message a test which ought to be crucial. Einstein’s theory of gravitation indicates that all the lines of the spectrum of a star will be slightly displaced towards the red end of the spectrum as compared with the corresponding terrestrial lines. On the sun the effect is almost too small to be detected having regard to the many causes of slight shift which have to be disentangled. To me personally Einstein’s theory gives much stronger assurance of the real existence of the effect than does the observational evidence available. Still it is a striking fact that those who have made the investigation are now unanimous in their judgement that the effect really occurs on the sun, although some of them at first thought that they had evidence against it. Hitherto Einstein’s theory has been chiefly regarded by the practical astronomer as something he is asked to test; but now the theory has a chance to show its mettle by helping us to test something much more doubtful than itself. The Einstein effect is proportional to the mass divided by the radius of the star; and since the radius of the Companion of Sirius is very small (if the message is right) the effect will be very large. It should in fact be thirty times as large as on the sun. That lifts it much above all the secondary causes of shift of the lines which made the test on the sun so uncertain.
The observation is very difficult because the Companion of Sirius is faint for work of this kind, and scattered light from its overpoweringly brilliant neighbour causes much trouble. However, after a year’s effort Professor Adams made satisfactory measurements, and he found a large shift as predicted. Expressing the results in the usual unit of kilometres per second, the mean of his measurements came to 19, whilst the predicted shift was 20.
Professor Adams has thus killed two birds with one stone. He has carried out a new test of Einstein’s general theory of relativity, and he has shown that matter at least 2,000 times denser than platinum is not only possible but actually exists in the stellar universe.13 This is the best confirmation we could have for our view that the sun with a density 1½ times that of water is still very far indeed from the maximum density of stellar matter; and it is therefore entirely reasonable that we should find it behaving like a perfect gas.
I have said that the observation was exceedingly difficult. However experienced the observer, I do not think we ought to put implicit trust in a result which strains his skill to the utmost until it has been verified by others working independently. Therefore you should for the present make the usual reservations in accepting these conclusions. But science is not just a catalogue of ascertained facts about the universe; it is a mode of progress, sometimes tortuous, sometimes uncertain. And our interest in science is not merely a desire to hear the latest facts added to the collection; we like to discuss our hopes and fears, probabilities and expectations. I have told the detective story so far as it has yet unrolled itself. I do not know whether we have reached the last chapter.