A GIANT STAR

Otago Witness, Issue 3509, 14 June 1921, Page 25

A GIANT STAR

273,000,000 MILES IN DIAMETER.. A NOTABLE ADVANCE IN ASTRONOMY. One of tlie most important advances 11 mcctern astronomy was carried through jus before tho close of last yea r. On the night °t December 13 Messrs Pease and Anderson 01 tho .Mount AVilson Observatory in Cali fernia, succeeded in measuring the diametei of Betelgeux, the bright red star in the con sfellation Orion (says the Manchester Guar di-ail). Hitherto it has been impossible tc measure the diameters of stars. Sun, moon a : ; c planets, ana even the larger members of the various satellite systems within tli£ oolar system, present appreciable discs, and the scale of the Solar system being known tneir angular diameters can easily be transi kited, into linear. _ Thus we know "the sun tc be 866,000 miles in diameter, the moon 21C0. Jupiter about 90,000, and Mars a little over 400 >. Ihe stars, however, even in tlie largest telescopes, appear as points of light. Stupendous as the larger stars are in actual size, they are plunged at such vast distances 111 space that they show no appreciable disc ; even in telescopes of the highest magnifying power. As far back as 1890 Professor A. A. Michelson, of the University cf Chicago, called attention 10 the possibility oi measuring very minute angles He himself applied the method in 1892 to the measurements of the diameters of the satellites of Jupiter. He pointed out also its applicability to the measurement of s teller diameters; but as very good seeing is required for so delicate an investigation, it 14 only within t’.. o last two years Hint Professor Micheison himseli, and latterly .Messrs Pease and Anderson, of tlie Mount AVilson Observatory, have made experiments cn tlie stars with the " interferometer " method. Professor Eddington, of Cambridge, has thus described the method as applied at Mount Wilson:—"A plane surface, 20 feet long, has been mounted nt tlie top end of the 100-inch telescope. Two mirrors, which can move along the line joining them, are mounted on the surface. Light is retDcte.-l from the mirrors on to a second pair of mirrors closer together, which reflect them agai n on to the 100-incli mirror. Thus two beams are brought together and allowed to produce interference fringes, which can be viewed with an eye-piece. If the star disc is C( mperable to tlie width of the fringes their visibility will alter for the length of the base-line." The length of the base-line at which tlie fringes disappear determines the anguiar diameter. Using this method nn the night of December 13, Messrs Pease ar.d -Anderson found the angular diameter of Betelgeux to be O.Olosee, or forty-five thousandths of a second of arc. Tiie distance of the star is not known so accurately as that <1! several other distant suns, but a mean of the various measures makes it about 159 light years—a distance which light requires 150 years to traverse—which is probably not far from the truth. The angular diameter and the distance of Betelgeux being known, it is easy to computes its linear diameter, which comes out at 273,000,000 miles, about 390 times tho diameter of the sun. Tills achievement of Messrs Pease and Anderson is an epoch-making one for several reasons. It is the first measurement of the diameter of a star. It must not be supposed, cf course, that tlie diameters of al! the other stars can be similarly measured. Betelgeux is one of the largest bodies in the universe, and even by tlie interferometer method only the diameters of the very largest car. bemeasured, ar.d it is not expected that many ■are large enough to yield appreciable results. Nevertheless, the result for Betelgeux is of the highest importance. In October last Professor Eddington, in his address to the British Association, remarked: ‘‘AA T e believe wo know with fair accuracy the apparent surface brightness corresponding to each spectral type; then, all that is necessary is to divide the total apparent brightness by this surface brightness, and the result is the angular area subtended by the star.” And. making this assumption, Professor Eddington hazarded tlie statement that the angular diameter of Betelgeux was 0.51 see, an estimate which is very near to the value just found by Messrs Pease and Anderson. The achievement at Mount AVilson is of importance, too. in indicating that our sun, which seems to us of such enormous size and mass, is in reality one of an inferior order of stars. The important work of Professor H. N. Russell, cf Princeton, New Jersey, and of Professor Hetzspmng, of Potsdam, and several other astronomers, has familiarised us with the id- a Ihat tlie stars are to be divided into two classes—the giants, which are much larger and brighter than the sun. and the dwarfs. The measurement of the diameter of Betelgeux is a final confirmation of the theory. Our sun, it is tine, is not one of the smallest or faintest stars; it may be defined as a large dwarf, but, as compared with Betelgeux. Rigcl, Canopus, Akleliaian, Arctnrus, and other familiar luminaries of our midnight skies our ‘‘olb of day ” is a very insignificant body indeed. incidentally, the measurement of the diameter cf Betelgeux confirms Professor Russell's theory of stellar evolution. According to that theory the giant icd stars, such as Betelgeux and' Autares, are at an early stage of evolution; they are of enormous size and low- density. After passing the red stage a- star contracts, the density increases, and its temperature rises, and the white and blue stages are reached, provided the mass is great enough to attain the highest possible temperature. A period of decline sets in, and the star becomes a cooling dwarf —white, yellow, ,and finally red. According to this theory, our sun is a dwarf star, on tlie down grade. Tlie hypothesis mav not be true in its entirely—there are, indeed, still several serious objections to it—but it has. received substantial confirmation in recent years, and the proof of the enormous diameter of a red star adds greatly to its probability.