Part 3 (1/2)

The distance of Antares, though not very accurately known, is probably not far from 350 light-years. Its angular diameter of 0.040 of a second would thus correspond to a linear diameter of about 400 million miles.]

We may thus form a new picture of the two branches of the temperature curve, long since suggested by Lockyer, on very different grounds, as the outline of stellar life. On the ascending side are the giants, of vast dimensions and more diffuse than the air we breathe. There are good reasons for believing that the ma.s.s of Betelgeuse cannot be more than ten times that of the sun, while its volume is at least a million times as great and may exceed eight million times the sun's volume. Therefore, its average density must be like that of an attenuated gas in an electric vacuum tube. Three-quarters of the naked-eye stars are in the giant stage, which comprises such familiar objects as Betelgeuse, Antares, and Aldebaran, but most of them are much denser than these greatly inflated bodies.

The pinnacle is reached in the intensely hot white stars of the helium cla.s.s, in whose spectra the lines of this gas are very conspicuous. The density of these stars is perhaps one-tenth that of the sun. Sirius, also very hot, is nearly twice as dense. Then comes the cooling stage, characterized, as already remarked, by increasing density, and also by increasing chemical complexity resulting from falling temperature. This life cycle is probably not followed by all stars, but it may hold true for millions of them.

The existence of giant and dwarf stars has been fully proved by the remarkable work of Adams and his a.s.sociates on Mount Wilson, where his method of determining a star's distance and intrinsic luminosity by spectroscopic observations has already been applied to 2,000 stars. Discussion of the results leads at once to the recognition of the two great cla.s.ses of giants and dwarfs. Now comes the work of Michelson and Pease to cap the climax, giving us the actual diameter of a typical giant star, in close agreement with predictions based upon theory. From this diameter we may conclude that the density of Betelgeuse is extremely low, in harmony with Russell's theory, which is further supported by spectroscopic a.n.a.lysis of the star's light, revealing evidence of the comparatively low temperature called for by the theory at this early stage of stellar existence.

TWO OTHER GIANTS

The diameter of Arcturus was successfully measured by Mr. Pease at Mount Wilson on April 15. As the mirrors of the interferometer were moved apart, the fringes gradually decreased in visibility until they finally disappeared at a mirror separation of 19.6 feet.

Adopting a mean wave-length of 5600/10000000 of a millimetre for the light of Arcturus, this gives a value of 0.022 of a second of arc for the angular diameter of the star. If we use a mean value of 0.095 of a second for the parallax, the corresponding linear diameter comes out 21,000,000 miles. The angular diameter, as in the case of Betelgeuse, is in remarkably close agreement with the diameter predicted from theory. Antares, the third star measured by Mr. Pease, is the largest of all. If it is actually a member of the Scorpius-Centaurus group, as we have strong reason to believe, it is fully 350 light-years from the earth, and its diameter is about 400,000,000 miles.

[Ill.u.s.tration: Fig. 28. Diameters of the Sun, Arcturus, Betelgeuse, and Antares compared with the orbit of Mars.

Sun, diameter, 865,000 miles.

Arcturus, diameter, 21,000,000 miles.

Betelgeuse, diameter, 215,000,000 miles.

Antares, diameter, 400,000,000 miles.]

It now remains to make further measures of Betelgeuse, especially because its marked changes in brightness suggest possible variations in diameter. We must also apply the interferometer method to stars of the various spectral types, in order to afford a sure basis for future studies of stellar evolution. Unfortunately, only a few giant stars are certain to fall within the range of our present instrument. An interferometer of 70-feet aperture would be needed to measure Sirius accurately, and one of twice this size to deal with less brilliant white stars. A 100-foot instrument, if feasible to build, would permit objects representing most of the chief stages of stellar development to be measured, thus contributing in the highest degree to the progress of our knowledge of the life history of the stars. Fortunately, though the mechanical difficulties are great, the optical problem is insignificant, and the cost of the entire apparatus, though necessarily high, would be only a small fraction of that of a telescope of corresponding aperture, if such could be built. A 100-foot interferometer might be designed in many different forms, and one of these may ultimately be found to be within the range of possibility. Meanwhile the 20-foot interferometer has been improved so materially that it now promises to yield approximate measures of stars at first supposed to be beyond its capacity.

[Ill.u.s.tration: Fig. 29. Aldebaran, the ”leader” (of the Pleiades), was also known to the Arabs as ”The Eye of the Bull,” ”The Heart of the Bull,” and ”The Great Camel” (Hubble).

Like Betelgeuse and Antares, it is notable for its red color, which accounts for the fact that its image on this photograph is hardly more conspicuous than the images of stars which are actually much fainter but contain a larger proportion of blue light, to which the photographic plates here employed are more sensitive than to red or yellow. Aldebaran is about 50 light-years from the earth.

Interferometer measures, now in progress on Mount Wilson, indicate that its angular diameter is about 0.020 of a second.]

While the theory of dwarf and giant stars and the measurements just described afford no direct evidence bearing on Laplace's explanation of the formation of planets, they show that stars exist which are comparable in diameter with our solar system, and suggest that the sun must have shrunk from vast dimensions. The mode of formation of systems like our own, and of other systems numerously ill.u.s.trated in the heavens, is one of the most fascinating problems of astronomy.

Much light has been thrown on it by recent investigations, rendered possible by the development of new and powerful instruments and by advances in physics of the most fundamental character. All the evidence confirms the existence of dwarf and giant stars, but much work must be done before the entire course of stellar evolution can be explained.

CHAPTER III

COSMIC CRUCIBLES

”Shelter during Raids,” marking the entrance to underground pa.s.sages, was a sign of common occurrence and sinister suggestion throughout London during the war. With characteristic ingenuity and craftiness, ostensibly for purposes of peace but with bomb-carrying capacity as a prime specification, the Zeppelin had been developed by the Germans to a point where it seriously threatened both London and Paris. Searchlights, range-finders, and anti-aircraft guns, surpa.s.sed by the daring ventures of British and French airmen, would have served but little against the night invader except for its one fatal defect--the inflammable nature of the hydrogen gas that kept it aloft. A single explosive bullet served to transform a Zeppelin into a heap of scorched and twisted metal. This characteristic of hydrogen caused the failure of the Zeppelin raids.

Had the war lasted a few months longer, however, the work of American scientists would have made our counter-attack in the air a formidable one. At the signing of the armistice hundreds of cylinders of compressed helium lay at the docks ready for s.h.i.+pment abroad. Extracted from the natural gas of Texas wells by new and ingenious processes, this subst.i.tute for hydrogen, almost as light and absolutely uninflammable, produced in quant.i.ties of millions of cubic feet, would have made the dirigibles of the Allies masters of the air. The special properties of this remarkable gas, previously obtainable only in minute quant.i.ties, would have sufficed to reverse the situation.

SOLAR HELIUM

Helium, as its name implies, is of solar origin. In 1868, when Lockyer first directed his spectroscope to the great flames or prominences that rise thousands of miles, sometimes hundreds of thousands, above the surface of the sun, he instantly identified the characteristic red and blue radiations of hydrogen. In the yellow, close to the position of the well-known double line of sodium, but not quite coincident with it, he detected a new line, of great brilliancy, extending to the highest levels. Its similarity in this respect with the lines of hydrogen led him to recognize the existence of a new and very light gas, unknown to terrestrial chemistry.

Many years pa.s.sed before any chemical laboratory on earth was able to match this product of the great laboratory of the sun. In 1896 Ramsay at last succeeded in separating helium, recognized by the same yellow line in its spectrum, in minute quant.i.ties from the mineral uraninite. Once available for study under electrical excitation in vacuum tubes, helium was found to have many other lines in its spectrum, which have been identified in the spectra of solar prominences, gaseous nebulae, and hot stars. Indeed, there is a stellar cla.s.s known as helium stars, because of the dominance of this gas in their atmospheres.

[Ill.u.s.tration: Fig. 30. Solar prominences, photographed with the spectroheliograph without an eclipse (Ellerman).

In these luminous gaseous clouds, which sometimes rise to elevations exceeding half the sun's diameter, the new gas helium was discovered by Lockyer in 1868. Helium was not found on the earth until 1896.

Since then it has been shown to be a prominent const.i.tuent of nebulae and hot stars.]