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UNIVERSITY

SPECTRUM

ANALYSIS

239

hand, when the earth is 180° from that position, it is moving slower or faster towards Sirius than the sun is moving. When the earth is so placed that Sirius and the sun are nearly on the same or on opposite sides of it, it is moving transversely to the line joining the sun and Sirius, and its motion relatively to the sun produces no modification of the observed phenomenon. We should have in such a case the full effect due to the relative motion of Sirius and the sun. Correcting, then, for the velocity of the earth relatively to the sun, Mr. Huggins found that the velocity of Sirius relatively to the sun is about twenty miles per second in a direction tending to increase their distance; so that ever since. the time when Sirius was first observed, it has been steadily moving away from the solar system at the rate of something like twenty miles per second, and yet we have not the least documentary or other proof that the brightness or apparent magnitude of Sirius has become at all diminished in consequence. It has been leaving us at that tremendous rate, and yet so far is it, or has it been, from us all this time, that even this increment of distance, growing at such a tremendous rate, has made during historical periods no perceptible change in the amount of light that we receive from it.

The next application that was made of this principle was to verify the fact of the sun's rotation about its axis. It is obvious that, as the sun rotates about its axis in the same direction as the earth rotates, one portion of the solar equator, the portion to the left as we look at the sun-the left-hand side of the sunis coming towards us, and the right-hand side of the sun is going away from us. The sun's rotation about its axis takes place in what is called the positive

direction; that is, the opposite direction to that of the hands of a watch, as looked at from the north pole side of the plane of the ecliptic. Now, although the sun's rotation is very slow, that is to say, though the sun takes about twenty-six days to execute a whole revolution,-still, because of its enormous diameter, the linear velocity of all parts of its equator is very considerable. Therefore if we examine, by means of a spectroscope, the light which comes from, let us say, incandescent hydrogen at different parts of the solar equator, it should correspond to rather higher light (more refrangible rays-more waves per second) from the left-hand side of the sun's equator which is approaching us, than from the right-hand side, which is retiring from us; and, therefore, if we could by a proper optical combination place side by side, as coming through the same spectroscope slit, the light given out by incandescent hydrogen at these two extreme ends of the sun's equator as seen by us, then we should find of the two hydrogen lines, the one from the left-hand side shifted a little up in the scale, and the one from the right-hand shifted a little downwards. Therefore we should find, of course, the hydrogen line in different places of the two spectra; and by measuring the amount of displacement between the two, we could calculate what is the rate of the motion of these points in the sun's equator to us or from us, compared with the whole velocity of light in space.

Now, carry this just a step further, especially thinking of the enormous velocities (which I discoursed upon in last lecture) with which these masses of flaming hydrogen are thrown out in explosions or eruptions from below the visible surface of the sun. Think of a rate of

several hundred miles per second, or something like it, with which these masses of glowing gas are thrown out, and you can easily see that if something of the nature of, but incomparably superior in dimensions to, a cyclone, such as we have in our tropical regions, were taking place, accompanied by down-rushes of colder gas, and up-rushes of warmer gas, both of these being incandescent hydrogen, the general down-rush of the cold will correspond to absorption, and the up-rush of the hot to radiation. There will be cold gas absorbing, but going from us, and an up-rush of (on the whole) radiating gas which is coming towards us; and therefore we should find the absorption correspond to a lower position in

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the spectrum than the natural hydrogen line, while the bright line corresponding to the gas coming towards us will belong to a higher position in the spectrum; and so we account for the double line referred to in my last lecture, the lower half of it nearest the red being dark or due to absorption, and the other side being bright or due to radiation. Thus, even with a slit, the motion of these hydrogen clouds is easily seen by the blurred and broken form presented, whether by their absorption lines as seen on the spectrum of the solar surface; or their radiation lines as seen in the spectrum of the regions round the edge of the disc. Curious examples of these two phenomena are shown in the diagrams

Q

before you. Both represent appearances presented by the green line of hydrogen-in the first partly absorbent, partly radiating, the line is on the disc-in the second it is seen in a prominence, parts of which are moving with very great velocity.

If we think for a moment of the whole light sent us by the sun, in which absorption by hydrogen far exceeds radiation by hydrogen, and think of the different relative rates of motion of different parts of the surface, we see a physical reason for broadening of the hydrogen lines altogether independent of pressure and cyclone

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are very broad may not necessarily have a dense atmosphere, but may be merely rotating rapidly about its axis. Thus caution is requisite in interpreting such appearances. And all the more so because Lord Rayleigh has called attention to the fact that even when a mass of incandescent gas is at rest as regards the spectator, its individual particles are in motion with sufficient relative rapidity to render a very narrow bright or dark line an impossibility. Even very rare hydrogen, if very hot, will therefore give broad absorption bands or bright lines. Other two causes, which may in

certain cases lead to similar results, I must presently point out to you.

I may mention, before leaving this part of the subject, that Fox Talbot has proposed to apply the same principle to double stars, in order to find what is the distance of a physical system of two stars from us; at least when they have one common absorbing constituent in their atmospheres. If we can observe a double star, the plane of whose relative orbit passes (let us say, for example) nearly through the earth, then we may perform upon these two stars precisely the same operation as I have described with reference to the light coming from the two ends of the solar equator; and therefore of course we shall be able to tell what is the actual velocity of the one star in its orbit relatively to the other. We shall be able to calculate the relative velocity of the two, which is in fact the actual velocity of the one star in its orbit round the other; and knowing that actual velocity, we shall be able to calculate, from the observed periodic time, from the actual velocity thus determined, and from the apparent size of the orbit, not only what the actual size of the orbit is, but also how far that orbit is removed from us in order to appear so small as it does. So that by the help of this method, when properly applied, we shall be able to get perhaps a much closer approximation to the distance of various fixed stars from us than we can get by the only method hitherto employed, namely, by the determination of what is called their annual parallax. In fact, we may conceivably thus obtain a measure of the distance of stars so far off as to show no measurable annual parallax at all.

You see, then, that the light from a heavenly body

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