MEASURING STAR-HEAT.
LAST year the scientific world hailed with interest the dis- covery that heat comes to us from beyond the infinite depths which separate our earth from the fixed stars, bringing us a message not less full of interest than that conveyed by the stellar light-rays. It seemed wonderful indeed that any contrivance man could devise should enable him to render sensible the heat sent forth on all sides from those distant suns. We know that at midday the summer sun pours his beams so fiercely on the earth that we compare their heat with that of a fire near at hand. But we recognize the fact that even within the known bounds of the solar system the sensible heat his direct rays can produce is diminished more than a thousandfold. And knowing this, it seems as though the physicist in Neptune—supposing that arctic world to be the abode of reasoning creatures—would have a problem of some difficulty in the measurement of the warmth received from the tiny sun which rules the Neptunian day. But the distance which separates Neptune from the central luminary of the planetary scheme sinks into utter nothingness beside the vast gap which lies between that scheme and the nearest of the fixed stars. Seven thousand times farther from the sun than Neptune stands the advanced guard of the stellar host, the famous star which marks the raised fore-foot of the Centaur. Light and heat from beyond that enormous distance are reduced fifty-million I Grote. Ut supra.
times more than the solar light and heat which shed their faint rays over the Neptunian ice-fields.
Strange, indeed, and difficult was the problem which had thus been mastered. And yet the lesson taught us was one which in another form we had already learned. We had been able by the aid of a new and wonderful instrument—the spectroscope—to assure ourselves that the stars are suns in all essential respects resembling our own. We know that around them hang sus- pended the vapours of metals which only the fiercest heat can melt, for we saw that the light which came to us from them had been robbed of the waves which those metallic vapours alone have the power of selectively absorbing.
But now a new and more difficult task has been achieved. Astronomy has not been content with the discovery that the stellar heat can be felt, but has faced the more arduous problem of »teasuring that heat. Worthy of the task has been the instrument with which it has been undertaken. The great equatorial of the Greenwich Observatory is perhaps surpassed by no telescope in the world as regards the optical qualities which the astronomer delights in. Constructed by the same eminent opticians who made both the famous Poulkova refractor and the telescope which has done such distinguished service in the cause of science at Cambridge, U.S., and not inferior to either of those telescopes either in size or quality, this splendid instrument has been mounted in a manner which does infinite credit to the mechanical ingenuity of our Astronomer Royal. A corps of observers and mathematicans unsurpassed perhaps in the world garrisons the noble Observatory in which it is placed. With a liberality which cannot be too highly praised (and which might with advantage, perhaps, be extended more widely), Government has taken care that the chief national Observatory shall so be provided for, iu all respects, as worthily to support the national reputation. Future ages will doubtless record a long list of physical researches in which the powers of the noble equatorial of our nobly furnished Observatory shall have been employed by the skilful and practised astronomers now gathered at Greenwich.
Its first great achievement in this special direction is one of which our country may well be proud. Many months since, Mr. Stone, F.R.S., the chief assistant at Greenwich, and already known to fame for his successful attacks upon the problem of the sun's dis- • tance, turned his thoughts towards the application of the powers of the great equatorial to the determination of stellar heat. The results he then attained, though highly interesting, did not become widely known, Mr. Huggins having anticipated their publication by communicating to the Royal Society his own successful treat- ment of the same problem. But, as we have said, it was the simple fact that we do receive heat from the stars, not a quantita- tive estimate of their heating powers, which was then laid before- the scientific world. Mr. Stone hoped to be able to announce the actual amount of heat which the first-class stars send to this globe on which we live.
A little consideration will show the enormous difficulty of the problem. A very delicate thermometer placed at night in the open air responds to a thousand influences which the ordinary observer would be inclined wholly to disregard. Long ago, Gilbert White, of Selborne, noticed how the thermometer rises at night as clouds pass overhead, and sinks as the heavens grow clearer. But as the science of meteorology has progressed, men have recog- nized the fact that changes much less obvious than these affect the thermometer. On each of two different nights the sky may be beautifully clear, the floor of heaven " thick inlaid with patines of bright gold," and yet the condition of the air may be so different that on one night the earth's heat may be rapidly radiating into space, while on the other an unseen presence in mid-air checks the escaping heat and makes the weather warm and genial. Though on each night " the heavens break open to their highest," there is thus the most essential difference in the circumstances under which the stellar heat would be measured, since the same aqueous but unseen veil which cuts off the escaping earth-heat, must in part, at least, influence the heat which is sent to the earth from beyond the stellar spaces.
By an ingenious arrangement Mr. Stone was enabled to over- come this difficulty in great part, and to secure that which so delighteth the soul of the man of science, a reliable zero. Let not the uninitiated be confounded by this mystic word,—it implies merely that which may be compared to the end of a rule or measur- ing-tape. To know if a star sends us heat at any moment we must know what heat our instruments would show before receiving the star's heat ; otherwise, what are we to measure from ? But, as we have said, this initial heat is continually varying. What Mr. Stone had first to do, then, was to master this difficulty. Others
remained which we have not space to specify, but these also he overcame.
At last, after enormous labour, the heat received from two well- known stars has been measured. Arcturus, the leading brilliant of the Herdsman, and Vega, the chief star of the Lyre, are the two orbs dealt with by Mr. Stone. From a careful measurement of their light, Sir John Herschel long since determined that these stars are of equal splendour ; but Arcturus shines with a ruddy yellow light, while Vega exhibits a colour which has been compared to the gleam of highly-polished steel. The estimates of their heat corre- spond with the aspect of these orbs. The fiery Arcturus sends us about twice as much heat as the bluish Vega. Minute indeed is the quantity of heat received from either star, even Arcturus having a direct heating effect corresponding to but about the 800,000th part of a degree Fahrenheit. Or, Mr. Stone remarks, the result may be otherwise stated as follows :—The heat received from Arcturus is sensibly the same as that from the face of a three-inch iron cube full of boiling water at a distance of 383 yards.
To the worlds which circle around these brilliant stars our sun doubtless supplies no larger a degree of heat; nay, we have good reason to believe that he is relatively an insignificant orb. Around Arcturus are well-warmed worlds, nourished by the rays which belong to the red end of the spectrum. Those which circle around Vega, if equally distant, are less plentifully supplied with heat. On the other hand, if one may speculate so confidently as to the state of these worlds as to regard photography as an art practised among their inhabitants, then must the people warmed by Arcturus sit longer for their portraits than those on whom the brilliant Vega pours his powerful actinic rays. Seriously, the researches we have been dealing with suggest strange thoughts for our considera- tion. The question of the plurality of worlds had seemed per- plexing enough when we considered merely the strangely various conditions under which living creatures must subsist in the different orbs which circle round our sun. But when we contemplate the varieties presented among the fixed stars, the mind is lost in the attempt to conceive the enormous range of variety which must characterize the races of living creatures subsisting in the systems of which those stars are the central luminaries.
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