Brinkley, of Trinity College, Dublin, agree also as to this star, in showing that the annual parallax is imperceptible. Now, admitting the annual parallax of the nearest fixed star, suppose Sirius, to be 2", then its distance from the Earth would be no less than 9,797,587,500,000 miles, or nearly ten million million of miles. But admitting the parallax of the nearest fixed star to be only 1", which is probably too great, then its distance will be nearly twenty million million of miles. The parallax of any fixed star has been, till lately, thought imperceptible. Piazzi, from his observations made at Palermo, suspected a parallax of a few seconds in several stars. Dr. Brinkley, who has paid particular attention to this subject, says, that his observations made with the circle, eight feet in diameter, belonging to the observatory of Trinity College, Dublin, appeared to point out a parallax in several stars; and that the agreement of results, obtained by different sets of observations, seemed to leave no doubt on this head. However, observations made elsewhere do not confirm his results. The distance of the fixed stars, proved by the motion of the Earth, is indeed wonderful, yet there is nothing contrary to reason and experience in admitting it. Why should we, as Dr. Brinkley justly observes, limit the bounds of the universe by the limits of our senses? We see enough in every department of nature, to deter us from rejecting any hypothesis, merely because it extends our ideas of the creation and divine Creator. Our knowledge of the fixed stars must be much more circumscribed than of the planets, since the best telescopes do not magnify the fixed stars so as to submit their diameters to measurement; but it is well ascertained that the apparent diameter of the brightest of them is less than 1". The fixed stars, as we have seen, are at immeasurable distances from us, at distances compared with which the whole solar system is but a point. Their diameters are less than we can measure, yet their light is more intense than that of the planets. We conclude, therefore, that they are self-shining bodies, and according to a high degree of probability, like our Sun, the centre of planetary systems. Admitting this, the multitudes of fixed stars that may be discovered with the most inferior telescopes, show us an extent of the universe that our imagiuation can scarcely comprehend; but what is even this, compared to the extent that the discoveries and conjectures of Dr. Herschel point out? We cease to have distinct ideas, when we numerate by ordinary measures the distances of the fixed stars, and we require the aid of other ⚫ circumstances to enable us to comprehend them. Thus, we compute that the nearest of the fixed stars is so far distant, that light, which moves at the rate of 192,900 miles in a second, will take above a year in coming from the star to the Earth; that the light of many telescopic stars may have been many hundred years in reaching us; and still farther, that, according to Dr. Herschel, the light of some of the nebulæ, just perceptible in his forty feet telescope, has been above a million of years on its passage. The limit of the distance of the nearest fixed star, may be considered as well ascertained; but any thing advanced with respect to the distances of the others, must be in a manner conjectural. The brighter fixed stars have been supposed to be nearer to us than the rest. Besides their superior lustre leading to this conclusion, many of them were discovered to have small motions, called proper motions, that could only be explained by supposing them to arise from a real motion in the stars themselves, or in the Sun and solar system, or from a motion compounded of both these circumstances. Now, whichever of these suppositions was adopted, it was reasonable to suppose, that the cause of the smaller stars not appearing to be affected, could only arise from the greater distance of those stars. However, it is now ascertained that some of the smaller stars appear to have proper motions, much greater than those of the brightest stars. Hence, conclusions deduced from the proper motions of the bright stars, respecting the relative distances of those stars. must tend to weaken conclusions that might be deduced from their brightness and apparent magnitudes. There is a double star of the sixth magnitude, the 61st star of the Swan, which consists of two stars, within a few seconds of each other. Each of these stars are moving nearly at the same rate, that is, at the rate of 6" in a year. It is likely they are also moving about their common centre of gravity. At present they preserve nearly the same distance from each other. This proper motion is far greater than has been observed in any of the brighter stars, or indeed in any star. It might be supposed, on this account, that these stars (61 cygni) are nearer to us than the brighter stars. To ascertain this point, Dr. Brinkley has made observations of the zenith distances, at the opposite seasons, and he could not discover any sensible parallax in these stars. Bessel has compared these and some of the neighbouring stars by observations on the right ascensions, and found no sensible parallax. Still the arguments formerly adduced, for the brighter fixed stars being nearer to us, are considerably weakened by the great proper motions observed in some of the smaller stars. The star 40 Eridani has a proper motion of about 4" in a year. The annual proper motion of Arcturus is about 2". In many of the stars there is no proper motion perceptible. Besides the proper motions, it has been remarked by Dr. Herschel, that in several instances, the line joining two stars very near together, changes its position. This is in some cases explained by a proper motion in the brighter star; in other cases it seems to indicate, as has already been observed, the revolution of one star round another. The double star Castor is a striking instance: during the last fifty years, the line joining the two stars, which are about 5" asunder, has had a motion of rotation at the rate of about one degree in a year, while the interval between the stars has remained nearly the same. Of the three circumstances which explain the apparent motion of a star, that which supposes it to arise from a combination of the motion of the solar system and of the star is most probable. The Sun and nearest fixed stars are probably all in motion round a centre, the centre of gravity, perhaps of a nebula, or cluster of stars, of which the Sun is one, and the milky way a part, as Dr. Herschel supposes, while this nebula revolves with other nebulæ about a common centre. The direction of the motion of our system cannot with certainty be ascertained, because from the whole motion we observe in a fixed star, we have nothing to help us in assigning that which belongs to the Sun. Dr. Herschel has particularly considered this subject, and has concluded that our Sur is moving towards a point in the constellation Hercules, the declination of which is 400, and right ascension 2460. His arguments are very ingenious. but there is necessarily so much hypothetical in them, that the mind cannot feel much confidence in his conclusion. That our system is in motion, there can be no doubt; the difficulty is to ascertain the precise direction and velocity: and from the circumstances of the case, there seems to be little probability that the knowledge will ever be here attained by man. Dr. Herschel conjectures that the distances of the fixed stars are nearly inversely as their apparent magnitudes. From thence, and a train of ingenious reasoning, relative to the faintest nebulæ discoverable by his forty feet telescope, he has concluded that the distances of these nebulæ are so great, that light issuing from them must have been two millions of years in reaching the Earth. But the recent discoveries relative to the proper motions of the smaller fixed stars must, as has been said, in some measure weaken the conclusions formerly adopted respecting the relative distances of the fixed stars. The discoveries of Dr. Herschel have also made us acquainted with many nebulæ, which are not resolvable into stars, but apparently formed of luminous matter, gradually condensing, by the principle of universal attraction, into masses, as if about to form the suns of future systems. Distant ages only can appreciate these conjectures. Dr. Brinkley's Elements of Astronomy. THE END. |