As to variable stars, some of these permit of naked-eye estimation, others need a field-glass, and there are some which require to be followed in a good telescope. The observer who enters this department may either desire to find new objects or to obtain further data with regard to old ones. If the former, he cannot do better than watch some of the suspected variables in Gore’s Catalogue of 736 objects, published by the Royal Irish Academy. Whether suspected or known variables are put under surveillance, the plan of comparison will be the same. Several stars near the variable in position, and nearly equal in light, should be compared with it, and the differences in lustre, in tenths of a magnitude, recorded as frequently as possible. The extent and period of the variation will become manifest by a discussion of the results. The comparison-stars should of course be constant in light, and, if naked-eye stars, they may be selected from the Uranometria Nova Oxoniensis or ‘Harvard Photometry.’ If telescopic stars are required, then recourse must be had to comprehensive charts such as Argelander’s Durchmusterung, which includes stars up to 9½ mag. Variable stars of the Algol type are especially likely to escape recognition, as they retain a normal brilliancy except during the few hours near the time of a minimum.
As to star-colours, it must be admitted that our knowledge is in an unsatisfactory condition. The results of past observation show discordances which are difficult to account for. When, however, all the circumstances are considered, we need feel no surprise at this want of unanimity. In certain cases it is probable that actual and periodical changes occur in the colours of stars, though absolute proof is still required. Atmospheric variations unquestionably affect the tints of stars, and some alterations depend upon altitude, for a celestial object seen through the dense lower air-strata near the horizon will hardly preserve the same apparent hues when on the meridian. Telescopes are also liable to induce false impressions of colour, and especially by the employment of different eyepieces not equally achromatic. And the observer’s judgment is sometimes at fault through physiological influences, or he may have a systematic preference for certain hues which little impress another observer. Those engaged in this branch feel the want of a reliable and ready means of comparison, and several have been tried; but there are objections to their use, and it seems that the best objects are furnished by the stars themselves. Let the observer study the colours of well-known stars, and familiarize his eye with the distinctions in various cases (also with the differences due to meteorological effects &c.); he will then gradually acquire confidence, and may use these objects as standards. The difficulty will be that they cannot be directly compared, in the same field, with other stars; but relative differences may be noted by turning the telescope from one object to the other. This will be better than forming estimates on the basis of an artificial method, which will sure to be troublesome to arrange, and probably erroneous in practice. In some stars the colour is so curious as to be attributed with difficulty, and with regard to faint stars colour-estimates are often unreliable; so that it is not desirable to go below the 9th mag. unless a very large instrument is employed.
The necessity of being constantly on the look-out for temporary stars has been already mentioned. There is also the need for further observations of such of these objects as still exist. They are, however, very minute, and the observer will have to be careful as to their identity. Though no great revival in brilliancy is to be expected, these objects exhibited some singular fluctuations during their decline, and it is important to keep them under view as long as possible.
Many other departments of sidereal work are best left to the professional astronomer. The derivation of accurate star-places, proper motions, distances, &c. requires instruments of great refinement and trained hands to use them. Researches such as these do not come within the scope of ordinary amateurs. But a vast field is open to them in respect to double and variable stars; and the physical relations of many of the former greatly intensify the interest in this branch, and make it necessary to secure frequent observations.
[CHAPTER XVII.]
NEBULÆ AND CLUSTERS OF STARS.
Distinction.—Large number of Nebulæ and Clusters visible.—Varieties of form and grouping.—Distribution.—Early Observations.—Variable Nebulæ.—Nebulous Stars.—The Magellanic Clouds.—Double Nebulæ.—Real dimensions of Nebulæ and Clusters.—Round Nebulæ and Clusters.—Description of Objects.—Further Observations required.—Lists of selected Objects.
Distinction.—These objects, though classed together in catalogues, offer some great distinctions which the observer will not be long in recognizing. It was thought at one period that all nebulæ were resolvable into stars[61], and that their nebulous aspect was merely due to the confused light of remote star-clusters. But modern telescopes, backed up by the unequivocal testimony of the spectroscope, have shown that purely nebulous matter really exists in space. The largest instruments cannot resolve it into stars, and it yields a gaseous spectrum. The conjecture has been thrown out that it may be considered as the unformed material of which suns and planets are made.
Large Number visible.—D’Arrest once said that nebulæ are so numerous as to be infinite, and his opinion is supported by the rapid increase in the number known. Let us make a comparison. Messier inserted in the Connaissances des Temps for 1783 and 1784 (published in 1781) a catalogue containing 103 nebulæ and star-clusters. Of these 68 were new. In 1888 a new edition of Sir J. Herschel’s catalogue of 1864 (revised and extended by Dreyer) was printed by the Royal Astronomical Society, and this includes 7840 objects![62] The labours of the Herschels, of Lord Rosse, D’Arrest, Marth, Tempel, Stephan, and Swift have vastly augmented our knowledge in this branch since the time of Messier.
Varieties of Form and Grouping.—A telescope reveals all grades of condensation in stellar groups. Some consist of rather bright, scattered stars, and are easily resolved. Others contain more stars, but they are smaller, and greater power is required to show them. Others again are condensed into globular clusters needing high powers and good instruments to disconnect the mass of stars composing them. Some are faint, and the stars so minute that they are only to be distinguished from nebulæ in the finest telescopes. As to the nebulæ properly so called, they exist in all forms. They may be either round, elliptical, or in the form of a streak. Some are highly condensed in their centres, others present well-defined circular disks like planets, and a small proportion are in the form of rings[63]. Many peculiarities of detail have been remarked, and a curious and complicated spiral structure has been discovered in certain prominent nebulæ. One of these has been termed the “Whirlpool” Nebula from its singular convolution of form. Other objects have received distinctive appellations agreeably to their appearance. Thus, there is the “Dumb-bell” Nebula, the “Crab” Nebula, the “Horseshoe” Nebula, &c. Lord Rosse’s 6-foot reflector is in a large degree responsible for the particular knowledge we possess of many of these objects. The large mirror commands a grasp of light which renders it very effective on forms of this character. An instrument of small diameter is quite inadequate to deal with them. They can be seen, it is true, and the general shape recognized in the most conspicuous examples, but their details of structure are reserved for the greater capacity of large apertures.