Number and Magnitude of the Stars

Absolute Magnitude.

We turn now to consider the real brightness of the stars. How would they compare with the sun in splendour if we could see them at the same distance? To infer the true brightness from the apparent brightness we must first know the distance (for the methods of measuring the distance see PARALLAX). An idea of the variety of brightness may be given by considering the seven most conspicuous stars. Sirius gives 28 times the sun's light, but it has a companion only as bright as the sun. Canopus is so remote that its distance cannot be found accurately, but it can scarcely be less than io,000 times as luminous as the sun. a Centauri consists of two stars, one almost a replica of the sun and the other considerably fainter.

Then follow Vega and Arcturus with luminosities so and ioo in terms of the sun. Capella consists of two stars, with luminosities and 70, having a faint distant companion of luminosity about 0.01. Rigel is remote, but its distance is more certain than that of Canopus; its luminosity is greater than io,000.

This shows that there are stars far surpassing the sun in bril liancy; but, naturally, if we pay attention only to the most con spicuous stars we shall form an exaggerated idea of the general order of brilliance. There would be the same difficulty if we extended our survey to all the naked-eye stars; these would include the most luminous stars contained in a very wide region and the feebler stars in a much smaller region. It is only by rather elaborate investigations that we can get together a fair sample of the stellar population, in which the luminous stars are not over-represented by forcing themselves on our attention. Ac cording to the latest discussions the following table gives the relative proportions of stars of different degrees of intrinsic brightness in any volume of space: Stars fainter than the sun far outnumber those which are brighter.

At present the faintest known star is one to which attention was drawn by A. van Maanen. It was picked out from the mul titudes of apparently similar stars by its rapid motion across the sky, which indicated that it was likely to be especially near to us. It has an absolute visual magnitude 16.5, corresponding to of the light of the sun. Its catalogue designation is Wolf 359. Other very feeble stars are Proxima Centauri (mag. 15.4), van Maanen's star (14.3), Barnard's star (13.3), Groom bridge 34 comes (13.o). All these are cool red stars and their photographic brightness is about two magnitudes fainter.

At the other end of the scale it is difficult to assign an upper limit to the brightness because the objects suspected of great brilliancy are generally too remote for parallax measurement. In the star clusters, however, we see a group of stars all at practically the same distance, and can therefore measure at once the relative brightness of different classes of objects. The star S. Doradus in the Greater Magellanic Cloud is believed to have an absolute magnitude —9(400,000 X sun) ; and other stars in the same cloud are not far short of this. The range in real brightness of the

stars is at least 20 magnitudes—very much the same as the range in apparent brightness so far as it is possible to observe it.

Interferometer Measurements.

One of the striking achieve ments in recent years has been the measurement of the angular diameters of a few favourable stars by means of Michelson's interferometer (q.v.). As adapted to astronomical work this con sisted of a 20 ft. beam carrying two inclined mirrors which could slide along it, and carried on the 1 oo in. telescope at Mt. Wilson observatory; the light from the movable mirrors fell on two fixed mirrors, and was directed by them on to the great telescope mir ror. Virtually the observer was looking simultaneously through two apertures whose separation could be varied up to 20 ft., very much as in a gigantic range-finder. (See BINOCULAR INSTRU MENT: Stereoscope.) The effect of using two apertures is that the spurious (diffraction) disc of the stellar image is crossed by fine diffraction lines, and the lines become closer as the separation of the apertures is increased. If the object looked at is not a mere point of light, its finite size blurrs the diffraction pattern, and the object of using widely separated apertures is to obtain a diffrac tion pattern so fine that even the invisible disc of a star is suf ficient to blurr it out and make it disappear. Seven stars have been found to have angular diameters exceeding 0•02" which is about the limit attainable with this instrument; a 5o ft. inter ferometer was, in 1928, under construction at Mt. Wilson, with which it is hoped to secure more stars, but the great majority must remain beyond the scope of this method. In some ways this application of the interferometer is to be regarded rather as a tour de force than an important advance of knowledge, be cause we can always compute approximately the angular diameter from the apparent magnitude and spectral type (which determines the radiating power of the surface) by a simple theory that can scarcely be doubted. Up to the present the observed diameters have been found to be in close accord with the theory. Of special interest, however, are the observations of the variable star, Mira Ceti; here the interferometer measurements indicate variations in the diameter, and seem to confirm the theory, put forward on other grounds, that this type of variability is due to pulsation.

Another application of the interferometer is to measure the separation of double stars in which the components are so close that ordinary visual measurements are impossible. In particular Capella, which is known to be a double by spectroscopic observa tion, cannot be resolved by any telescope. Nevertheless, by inter ferometer measurement the orbit has been found just as though it were a visual double. We have the rare opportunity of combin ing visual and spectroscopic orbital data, and the result is that Capella is one of the best known stars as regards mass, absolute luminosity, etc.