STARRY HEAVENS

Southland Times, Issue 22728, 2 November 1935, Page 11

STARRY HEAVENS

VARIABLE STARS LIGHT FLUCTUATIONS (By R. D. Thompson, M.A., M.Sc., F.R.A.S.) Although the stars differ greatly among themselves in brightness, yet to the casual observer each individual star appears to shine regularly night by night with the same steady and uniform luminosity. To the astronomers, ot ancient times the Heavens were unchanging and unchangeable,” and it is only within comparatively recent years that investigation has shown that the brightness of many stars is anything but unchanging. Most stars, it is true, do shine with a perfectly steady light, but many thousands of stars are now known whose light is continuously fluctuating—now bright, now dim—in all sorts of peculiar and characteristic ways. To these the name of “variable stars has been applied, and the study of these variable stars, in order to find the true explanation of their changes in brightness, is one of the most important branches of modern astronomy. . In some cases the light from a variable star fluctuates in a _ very steady and orderly way, just as if a lamp, as one writer puts it, were being slowly turned up and down at regular intervals; thus, as a result of past observations it is possible to predict just exactly when the light from such a star will be at a maximum or at a minimum. Many other stars, however, show quite irregular fluctuations in brightness, and the light from them seems to wax and wane in a truly inexplicable - manner. Moreover, the range in brightness from maximum to minimum varies greatly with individual stars; in some cases the star fades away until it is quite invisible to the naked eye only to reappear again as its brightness once more increases, while with other stars the light fluctuations are so small as to be quite imperceptible without the aid of delicate telescopic measurements. Algol—The Demon. This variability in the light from certain stars appears to have escaped detection in the early days of astronomy, and no record of any such phenomenon is found in ancient writings. There is good reason to believe, however, that one star at least was known to be variable as long ago as the early Middle Ages. This star is now catalogued under the name of Beta Persei, and is not remarkable either for colour or brightness, being a very ordinarylooking white star of the second magnitude, and less conspicuous than several of its near neighbours. To this star the Arabian astronomers gave the name of Algol—“the Demon”— a designation quite out of keeping with those which were allotted to the other stars, and which would, therefore, seem to indicate something mysterious or uncanny about this particular member of the starry skies. In the year 1669 Montanari redirected the attention of astronomers to what was then regarded as an entirely new discovery. He found that Algol, although usually of normal brightness, yet on occasions would undergo a distinct fading in luminosity lasting only for a few hours. Early Discoveries. The history of stellar variability really dates from 1596, in which year Fabricius, a Dutch astronomer, announced the discovery of a new star of the third magnitude in the constellation of Cetus (the Whale). The new star, however, faded rapidly in brightness and within a few weeks had disappeared completely. Some ten years later a star was again observed in much the same position as that which had been previously occupied by the vanished star, and was this time catalogued by Bayer under the name of Omicron Ceti. This star, too, disappeared in its turn, and it was not until 30 years later, when the star was once again “discovered,” that the amazing fact was disclosed that this particular star remained quite invisible for the greater part of the year, and flared up into prominence only for a, short period every eleven months. Thus Omicron Ceti—known popularly as Mira—differs greatly from Algol in the manner of its variability, for whereas Algol is usually at its normal brightness and fades away only for about 10 hours every three days, Mira, on the other hand, remains invisible for many months throughout the year. Other variable stars exhibiting this peculiar property of light variation were discovered soon afterwards, chief among them being Beta Lyrae, which resembles Algol in its light changes, except that the amount of its periodic fading is not uniform and Delta Cephei in the constellation of Cepheus, which differs from the other three variables so far mentioned in that its brightness never remains the same for any length of time, but is changing continuously in a regular series of up and down fluctuations. Many thousands of stars have now been found to behave in a similar manner, and to these the name of “Cepheid” variables has been given. As a class these Cepheid variables present a fascinating problem to the modem astronomer, and although the cause of their regular periodic variations in brightness is as yet unknown, nevertheless they have already yielded a tremendous wealth of information regarding stellar distances and the dimensions of the universe in general. Long Period Variables. Many classifications for variable stars have been suggested, some depending on the length and frequency’ of their periodic changes in brightness, and others again on the causes, _ so far as they are known, underlying these fluctuations. The usual practice is now to group them into one or other of the following divisions: (1) Short period variables, (2) long period variables, or (3) irregular variables. A long period variable is a star whose fluctuations in brightness occur at intervals exceeding 150 days; Mira—the best known star of this class—has a period of 333 days, and the majority of them average from 300 to 400 days. These stars are usually reddish in colour, and the redder the tint, the longer is the period. The cause of the light variation in long period variables is as yet unknown, but at the time of maximum brightness there is a definite eruption of hydrogen gas at a high temperature, caused possibly by some deep-seated pulsation or intermittent explosion taking place within the star at regular intervals.