The New Heavens

by George Ellery Hale

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.

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.

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 illustrated 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 passages, 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, surpassed 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 shipment abroad. Extracted from the natural gas of Texas wells by new and ingenious processes, this substitute for hydrogen, almost as light and absolutely uninflammable, produced in quantities 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 quantities, would have sufficed to reverse the situation.