IN STARRY SKIES

Evening Post, Volume CXIII, Issue 53, 4 March 1927, Page 15

IN STARRY SKIES

"BEYOND THE MILKY

WAY"

LATEST NEWS FROM MOUNT

WILSON

(By "Omega Centauri.") In our last notes we regretted that Professor H. H. Turner had not continued his delightful voyage in space far beyond the Milky "Way to the marvellous Spiral Nebulae. This has been done by Professor George Ellery Hale, and his latest conclusions are embodied in the volume before us. Professor Hale is one of the very few fortunate beings who have been privileged to see much further into space than the rest of mankind. He is honorary director of the Mount Wilson Observatory, where the greatest telescope in the w rid is laying bare unimagined marvels and enlarging the hori-on of knowledge in an unexampled way. Professor Hale knows of every astronomical discovery as soon as it is made, and no one is in a better position to jsdge the theories that are made to explain the facts. Our chief regret is that his book is so short. Professor Hale is tantalisingly concise. Every word is weighed. He gives us the latest discoveries mid discloses the latest opinions of those who have made them, and who are now pressing on towards further conquests of the unknown. But all this wealth of information is compressed into a book eight inches by five by half an inch. The printing occupies only a space about live inches by three on each page, and the three articles which are comprised in the volume, together with over forty illustrations, fill 'ss than 105 pages. There are, however, over a dozen pages without a word upon them, and we cannot help wishing that the author had used ; every inch of this space. Here is a man who has seen what few will ever see, who knows what is known only to a favoured few, who is able to interpret what he sees -s few can hope to do, and his picture of the present stati of knowledge in a vast subject is con-, densed into 105 small printed pages. The price of the little book is tea shillings, which comes to tenpence an ounce, but though this is a fairly high rate, even as books go now, we should be glad to buy many more of the name kind at the same rate. We feel that this is an authoritative presentation of the conception of the universe which is held by the present leaders of astronomical thought; but we wish he had allowed us to enter more fully into the workings of the astronomer's mind. The conclusions are clearly rhown, but we should have liked to attempt to follow the reasonings by which they have been reached. The first chapter, on "The -Oriental Ancestry of the Telescope," bears little relation to the other two. Whilst full of historical interest it cannot eompar with them in importance. Professor Hale starts far back in the dim and distant past. Certain circles and crescents in neolithic art suggest that even in the Stone Age observations were made of the sun and the moon, Coming to the more recent times

our knowledge , of Egyptian astronomy is fuller and more definite. Tho appearance of Sirius in advanco of the sun in tho early morning sky marked the beginning of the new year, and it is believed that systematic observation can be traced back to 4241 B.C. The ancient Egyptians were severely practical, and they valued astronomy because it enabled them to subdivide time and to frame a reliable calendar. Amongst the pictures given in this book of Egyptian instruments are Tutankhamen's transit instrument, a Merkhct or sun clock, an Egyptian sun dial, and a water clock. The Great Pyramid was certainly most carefully oriented and served ais an immense gnomon for determining the equinoxes, but Professor Hale declares that it was primarily a tomb for Pharaoh, not an astronomical observatory. Tho tablets of the Babylonians and Assyrians were far more durable than the papyri of the Egyptians, so the ancient records of Chaldean astronomy

which have survived nro more numerous than thoso of Egyptian. One series of tablets known as the D.iy of 80l is ascribed to the wise men of nbout 2800 B.C. An ephemeris for the year 425 B.C. contains for each month the lcngtli . of the month, the date of full moon, the date of the moon's last visibility, the heliacal rising of the planets and stars, and in some cases predictions of coming eclipses. Tho Greeks gained their earliest knowledge of astronomy and astronomical instruments from, the Egyptians and Chaldeans, but they developed the ! scionco more systematically than either. ; They loved knowledge for its own sake. Nearly 300 years B.C. Aristarchus held that tho apparent motions of the sun, moon, and stars are due to the rotation of tho earth on its axis and its revolution round the sun. Ho realised that the stars are too far away (o kliow any sensible change of direction as tho earth, moved. Unfortunately Ptolemy, whose Almagest controlled astronomical thought' for fifteen centuries, went back to tho notion of a fixed earth, and it was left for Copernicus to regain the true point of view. Somo of the Greek instruments, as, for instance, tho Zodiacal armilla, were extremely ingenious and eompli- _ cated, and they served as models for those on the walls of Pekin, and for those used by Tyeho Brahc. the latter, indeed, made somo valuable advances, such as tho method of transversals for subdividing the graduations of circles, but never employed lenses to increase the power of tho eye. Tycho stands out as the last great astronomical observer before the dawn of the telescopic ace.

In tho next chapter, which deals with the heat received from the stars, we leave the ancients and go right to the fighting front, whore the pioneers are still at work conquering new fields of knowledge. The origin of their present activities may be found in Newton's analysis of sunlight in 1666, but it required more than two and a half centuries of progress to bring instrument!! to such a state of sensitiveness anc' perfection that the distribution of energy in a stellar spectrum could be determined. An important step was taken in 1800, when Sir. William Herschel announced the .discovery of tho infra-red radiation. Within a year Bitter discovered the ultra-violet rays, and in 1802 Thomas Young made the first measurement of the lengths of waves of lght. In. the visible spectrum the violet waves are the shortest ana the red the longest. The extreme red rays have a wave length less than twice that of the extreme violet rays. From the analogy of sound we may say that the visible spectrum extends over less than one octave. In recent years rays far shorter than the ultra-violet and others far longer than the improved have been studied and the explanations of Nichols and Tear have made the spectrum complete from the shortest gamma , rays of radium to the longest radio d ways, a total range of more than fifty M octaves. Tho latest astronomical triIV umph is not only to measure the radia- ; ft .tion from faint stars, but to determine j; § how the energy is distributed throughjr. j out tho visible and invisible spectrum. ;; I The earliest attempt to measure the :• heat radiated by a star was made in }"• 1869 by Huggins, who projected the % imago formed by his eight-inch teleSj? scope ou to a sensitive thermo-couple. He thought he detected some indiea-

tions of heat, but these must have been accidental, for twenty years later Professor Boys, with far more sensitive instruments, failed to detect any thermal radiation, even from the brightest stars. In 1898, however, Professor Hale (then director of Yerkes Observatory) invited Ernest Fox Nichols to test the radiometer which he was developing, with the twenty-four-inch telescope. This radiometer proved to be twelve times as sensitive as the radiometer of Professor Boys, and the light-gathering power of the Yerkes's telescope was more than four times as great as that used by Boys, so the new instrument was fifty times as sensitive as the old one. A single candle, 2000 feet away, gave a deflection of 67 millimetres, and an astronomer's head at the same distance a deflection of 25 millimetres. The corresponding movements for Arcturus and Vega were 1.08 millimetres and .52 millimetres respectively. Pfund and Coblentz perfected the thermo-couple for a similar purpose, using Lebedew's discovery that it was more sensitive in a vacuum.

In 1913 Pfund was successfully measuring stellar heat radiation at Alleghenny Observatory, with a thirty-inch telescope, and a year later Coblentz, with the thirty-six-inch Crossley reflector at Lick measured tho heat from stars which are invisible to the naked eye. The installation of the 100-inch Hooker telescope at Mount Wilson put a new power into the astronomer's hands, and Pettit.and Nicholson measured with it the heat of a star as faint as the thirteenth magnitude, which could just have been detected with Herschcl's twenty-foot telescope. But even this is not near the present limit of achievement. It has been found that the radiation from the cooler stars is mostly in the infra-red. A star-like X Cygni, at its minimum, gives fifty thousand times as much heat as a white star of tho same apparent brightness. In the case of certain very red stars it would therefore be possible to measure the radiation if they were as faint as the eighteenth magnitude.. Dr. C. G. Abbot had been so successful in his studies of the solar spectrum with the bolometer that he was asked to try this instrument at Mount Wilson for investigations of the spectra of bright stars. He succeeded in the examination of ten spectra, but the bolometer proved not to be sufficiently sensitive. It was replaced by a new Nichol's radiometer. This was fifteen times as sensitive as the original one used at Yerker, and with the large telescope at a high altitude it secured a thousand-fold gain in effective sensitiveness. This enabled Abbot to determine energy curves for stellar spectra, and to prove the relation which exists between the distribution of energy and the spectral type. The results have thrown fresh light on the life history of a star. ' tin early youth it is seen to bo an enormously distended mass of gas sometimes exceeding 300 million miles in diameter with density in its outer parts comparable with that of the residual gas in a vacuum tube, and with a surface temperature from 2500 to 3000 degrees centigrade. As it radiates heat it decreases in diameter and increases in density, till as a white star it may have a surface temperature of 20 ; 000 deg. C, and a central one of thirty million degrees. With further contraction the surface temperature now declines, and by the time an early dwarf stage, like that of the sun, Is reached, it is only 6000 deg. C. The star becomes still dimmer and redder, and beforo it ceases to bo visible

the surface temperature falls to 2500

dog. C. But what meanwhile is happening inside the star? It is now believed that most of the electrons aro stripped from tho atoms, and that a fraction •of the. .mass is. continually being converted into radiant energy. But this leads to such important and at the same time such startlingly revolutionary ideas that we must givo them fuller consideration in our next article.