word. Moreover, the spectral type of Algol is one that is not usually associated with low mass, and this cast some suspicion on the accepted results.

If we are willing to trust the theory given in the last lecture we can do without the missing word. Or, to put it another way, we can try in succession various guesses instead of 'two' until we reach one that gives the bright component a mass and luminosity agreeing with the curve in Fig. 7. The guess 'two' gives, as we have seen, a point which falls a long way from the curve. Alter the guess to 'three' and recalculate the mass and brightness on this assumption; the corresponding point is now somewhat nearer to the curve. Continue with 'four', 'five' , &c.; if the point crosses the curve we know that we have gone too far and must take an intermediate value in order to reach the desired agreement. This was done in November 1925, and it appeared that the missing word must be 'five', not 'two' -- a rather startling change. And now the message ran --

1. Radius of bright component = 2,140,000 kilometres.
2. Mass of bright component = 4.3 x sun's mass.

If you compare these with the original figures you will see that there is a great alteration. The star is now assigned a large mass much more appropriate to a B-type star. It also turns out that Algol is more than a hundred times as bright as the sun; and its parallax is 0.028" twice the distance previously supposed.

At the time there seemed little likelihood that these conclusions could be tested. Possibly the prediction as to the parallax might be proved or disproved by a trigonometrical determination; but it is so small as to be almost out of range of reasonably accurate measurement. We could only adopt a 'take it or leave it' attitude 'If you accept the theory, this is what Algol is like; if you distrust the theory, these results are of no interest to you.'

But meanwhile two astronomers at Ann Arbor Observatory had been making a search for the missing word by a remarkable new method. They had in fact found the word and published it a year before, but it had not become widely known. If a star is rotating, one edge or 'limb' is coming towards us and the other going away from us. We can measure speeds towards us or away from us by means of the Doppler effect on the spectrum, obtaining a definite result in miles per second. Thus we can and do measure the equatorial speed of rotation of the sun by observing first the east limb then the west limb and taking the difference of velocity shown. That is all very well on the sun, where you can cover up the disk except the special part that you want to observe; but how can you cover up part of a star when a star is a mere point of light? You cannot; but in Algol the covering up is done for you. The faint component is your screen. As it passes in front of the bright star there is a moment when it leaves a thin crescent showing on the east and another moment when a thin crescent on the west is uncovered. Of course, the star is too far away for you actually to see the crescent shape, but at these moments you receive light from the crescents only, the rest of the disk being hidden. By seizing these moments you can make the measurements just as though you had manipulated the screen yourself. Fortunately the speed of rotation of Algol is large and so can be measured with relatively small error. Now multiply the equatorial velocity by the period of rotation; [Note:The observed period of Algol is the period of revolution, not of rotation. But the two components are very close together, and there can be no doubt that owing to the large tidal forces they keep the same faces turned towards each other; that is to say, the periods of rotation and of revolution are equal.] that will give you the circumference of Algol. Divide by 6.28, and you have the radius.

That was the method developed by Rossiter and McLaughlin. The latter who applied it to Algol found the radius of the bright component to be

2,180,000 kilometres.

So far as can be judged his result has considerable accuracy; indeed it is probable that the radius is now better known than that of any other star except the sun. If you will now turn back to p. 44 and compare