PHILOSOPHICAL PAPERS. Dr. HERSCHEL'S OBSERVATIONS on NEBULOUS STARS, properly fo called. [From the Eighty-first Volume of the Philofophical Tranfactions.] "None None of my late examinations of a fpace in the heavens, which I had not reviewed before, I difcovered a star of about the 8th magnitude, furrounded with a faintly luminous atmosphere, of a confiderable extent. The phænomenon was so striking, that I could not help reflecting upon the circumftances that attended it, which appeared to me to be of a very inftructive nature, and fuch as may lead to inferences which will throw a confiderable light on fome points relating to the conftruction of the heavens. "Cloudy or nebulous ftars have been mentioned by feveral aftronomers; but this name ought not to be applied to the objects which they have pointed out as fuch; for, on examination, they proved to be either mere clusters of ftars, plainly to be diftinguished with my large inftruments, or fuch nebulous appearances as might be reasonably fuppofed to be occafioned by a multitude of ftars at a vaft diftance. The milky way itself, as I have hewn in fome former papers, con fifts intirely of stars, and by imperceptible degrees I have been led on from the most evident congeries of ftars, to other groups in which the lucid points were fmaller, but ftill very plainly to be feen; and from them to fuch wherein they could but barely be suspected, till I arrived at laft to fpots in which no trace of a ftar was to be difcerned. But then the gradations to these latter were by fuch well-connected fteps as left no room for doubt, but that all thefe phænomena were equally occafioned by stars, varioutly difperfed in the immenfe expanfe of the universe. "When I purfued these researches, I was in the fituation of a natural philofopher who follows the various fpecies of animals and infects from the height of their perfection down to the lowest ebb of life; when, arriving at the vegetable kingdom, he can scarcely point out to us the precife boundary where the animal ceafes and the plant be gins; and may even go fo far as to fufpect them not to be effentially different. different. But recollecting himself, "But, that we may not be too precipitate in thefe new decifions, let us enter more at large into the various grounds which induced us to furmife, that every vifible object, in the extended and diftant heavens, was of the ftarry kind, and collate them with those which now offer themselves for the contrary opini on. "It has been obferved, on a former occafion, that all the smaller parts of other great fyftems, fuch as the planes, theirrings and fatellites, the comets, and fuch other bodies of the like nature as may belong to them, can never be perceived by us, on account of the faintnefs of light reflected from small, opaque objects; in my prefent remarks, therefore, all thefe are to be entirely fet afidé. "A well-connected feries of objects, fuch as we have mentioned above, has led us to infer, that all nebulæ confift of stars. This being admitted, we were authorifed to extend our analogical way of reasoning a little farther. Many of the nebula had no other appearance than that whitifh cloudiness, on the blue ground upon which they feemed to be projected, and why the fame caufe fhould not be affigned to explain the most extensive nebulofities, as well as thofe that amounted only to a few minutes of a degree in fize, did not appear. It could not be inconfiftent to call up a telefcopic milky way, at an immenfe distance, to account for fuch phænomena; and if any part of the nebulofity feemed detached from the rest, or contained a vifible ftar or two, the probability of feeing a few near ftars, apparently fcattered over the far diftart regions of myriads of fidereal collections, rendered nebulous by their distance, would also clear up thefe fingularities. "In order be inore eafily underftood in my remarks on the comparative difpofition of the heavenly bodies, I fhall mention fome of the particulars which introduced the ideas of connection and disjunction : for thefe, being properly founded upon an examination of objects that may be reviewed at any time, will H 2 be "On June the 27th, 1786, I saw a beautiful cluster of very fmall ftars of various fizes, about 1'in diameter, and very rich of ftars. On viewing this object, it is impoffible to withhold our affent to the idea which occurs, that these stars are connected fo far one with another, as to be gathered together within a certain fpace, of little extent, when compared to the vast expanfe of the heavens. As this phænomenon has been repeatedly feen in a thoufand cafes, I may juftly Jay great ftrefs on the idea of fuch ftars being connected.. "In the year 1779, the 9th of September, I difcovered a very fmall ftar near Bootis. The queftion Tiere occurring, whether it had any connection with or not, was determined in the negative; for confidering the number of ftars fcattered in a variety of places, it is far from being uncommon, that a ftar at a great distance thould happen to be nearly in a line drawn from the fun through, and thus conftitute the obferved double ftar. "The 7th of September, 1-81, when I firft faw the planetary nebula near Aquarii, I pronounced it to be a fyftem whofe parts were connected together. Without entering into any kind of calculation, it is evident, that a certain equal degree of light within a very fmall space, joined to the particular flape this object prefents to us, which is nearly found, and even in its deviation confiftent with regularity, being a little elliptical, ought naturally to give us the idea of a conjunction in the things that produce it. And a confiderable addition to this argument may be derived from a repeti tion of the fame phænomenon, in nine or ten more of a fimilar construction. "When I examined the cluster of stars, following the head of the great dog, I found on the 19th of March, 786, that there was within this clufter a round, refolvable nebula, of about two minutes in diameter, and nearly of an equal degree of light throughout. Here, confidering that the cluster was free from nebulofity in other parts, and that many fuch clufters, as well as many fuch nebulæ, exift in divers parts of the heavens, it appeared to me very probable, that the nebula was unconnected with the cluster; and that a fimilar reafon would as eafily account for this appearance as it had refolved the phænomenon of the double ftar near Bootis; that is a cafual fituation of our fun and the two other objects nearly in a line. And though it may be rather more remarkable, that this fhould happen with two compound fyftems, which, are not by far fo numerous as fingle ftars, we have, to make up for this fingularity, a much larger fpace in which it may take place, the cluster being of a very confiderable extent. "On the 15th of February, 1786, I difcovered that one of my planetary nebulæ, had a fpot in the centre, which was more luminous than the reft, and with long attention, a very bright, round, well defined centre became vifible. I remained not a fingle moment in doubt, but that the bright centre was connected with the rest of the apparent difk. "In the year 1785, the 6th of October, I found a very bright, round nebula, of about 1 minute in diameter. It has a large, bright nucleus in the middle, which is undoubtedly connected with the luminous parts about it. And though we mult muft confefs, that if this phænomenon, and many more of the fame nature, recorded in my catalogues of nebulæ, comfift of clustering ftars, we find ourselve, involved in fome difficulty to account for the extraordinary condenfation of them about the centre; yet the idea of a connection between the outward parts and thefe very condenfed ones within is by no means leffened on that account. "There is a telefcopic milky way, which I have traced out in the heavens in many sweeps made from the year 178 to 1789. It takes up a fpace of more than 60 fquare degrees of the heavens, and there are thousands of stars fcattered over it: among others, four that form a trapezium, and are fituated in the well known nebula of Orion, which is included in the above extent. All thefe ftars, as well as the four I have mentioned, I take to be entirely unconnected with the nebulofity which involves them in appearance. Among them is alfo Orionis, a cloudy ftar, improperly fo called by former aftronomers; but it does not seem to be connected with the milkinefs any more than the reft. "I come now to fome other phænomena, that, from their fingularity, merit undoubtedly a very full difcuffion. Among the reafons which induced us to embrace the opinion, that all very faint milky nebulofity ought to be afcribed to an affemblage of stars is, that we could not eafily affign any other caufe of fufficient importance for fuch luminous appearances, to reach us at the immenfe diftance we must fuppofe ourfelves to be from them. But if an argument of confiderable force fhould now be brought forward, to fhew the existence of a luminous matter, in a state of modification very different from the construction of a fun or star, all objections, drawn from our incapacity of accounting for new phænomena upon old principles, will lofe their validity. Hitherto I have been fhewing, by various inftances in objects whofe places are given, in what manner we may form the ideas of connection and its contrary by an attentive infpetion of them only: I will now relate a feries of obfervations, with remarks upon them as they are delivered, from which I shall afterwards draw a few fimple conclufions, that feem to be of confiderable import ance. "To diftinguish the observations from the remarks, the former are given in italics, and the date annexed is that on which the objects were difcovered; but the defcriptions are extracted from all the obfervations that have been made upon them. “October 16, 1784. A ftar of about the 9th magnitude furrunded by a milý nebulofits, or chevelure, of about 3 minutes in diameter. The nebulofity is very faint, and a little extended or elliptical, the extent being not far from the meridian, or a little from north preceding to South following. The chevelure involves a small flar, which is about 1 minute north of the cloudy far: other Stars of equal magnitude are perfectly free from this appearance. My prefent judgment concerning this remarkable object is, that the nebulofity belongs to the star which is fituated in its center. The fmall one, on the contrary, which is mentioned as involved, being one of many that are profufely fcattered over this rich neighbourhood, Į fuppofe to be quite unconnected with this phænomenon. A circle of H3 three three minutes in diameter is fufficiently large to admit another fmall ftar, without any bias to the judgment I form concerning the one in question. "It must appear fingular, that fuch an object fhould not have immediately fuggefted all the remarks contained in this paper; but about things that appear new we ought not to form opinions too hastily, and my obfervations on the conftruction of the heavens were then but entered upon. In this cafe, therefore, it was the fafeft way to lay down a rule not to reafon upon the phænomena that might offer themfelves, till I fhould be in poffeffion of a fufficient stock of materials to guide my refearches. “October 16, 1-84. A fmall far of about the 11th or 12th magnitude, very faintly affected with milky nebulofity; other stars of the jame magnitude are perfectly free from this appearance. Another obfervation mentions or 6 Small 5 flars within the space of 3 or 4', all very faintly affected in the fame mannner, and the nebulofity fufpelled to be a little ftronger about each ftar. But a third obfervation rather oppofes this increase of the faintly luminous appearance. "Here the connection between the stars and the nebulofity is not fo evident as to amount to conviction; for which reafon we fhall pafs on to the next. "January the 6th, 1785. A bright ftar with a confiderable milky chevelure; a little extended, 4 or 5' in length, and near a broad; it lofes itself infenfibly. Other fars of equal magnitude are perfectly free from this chevelure. "The connection between the ftar and the chevelure cannot be doubted, from the infenfible gradą. tion of its luminous appearance, decreafing as it receded from the center. "January 31, 1785. A pretty confiderable flar, with a very faint and very small, irregular, milky ch:velure; other ftars of the fame fize are perfectly free from fuch appearance. "I can have no doubt of the connection between the ftar and its chevelure. "October 5, 1785. Aftar with a ftrong bur all around. A fecond obfervation calls it a very bright nucleus, with a milky nebulofity, of no great extent. A third fufpects the milkine to belong to more of the fame, which is diffused over the whole fweep in that place; but a fourth fays, that the milky nebulofity is much ftronger than what the nebulous ground, on which the ftar is placed, intitles it to. "The connection, therefore, between the nebulofity and the ftar is evident. "January 1, 1786. Aftar furrounded with milky chevelure; the ftar is not central. A fecond obfervation calls it affected with a very faint, and extenfive, milky chevelure. A third only mentions aftar affected with milky chevelure. "As by the word chevelure I always denoted fomething relating to a center, the connection cannot be doubted. "February 24, 1786. A confiderable far, very faintly affected with milky chevelure. A fecond obfervation, much the fame. "November 27, 1786. Aftar. involved in milky chevelure. "January 17, 1787. A far with a pretty ftrong milky nebulofiry, equally difperfed all around: the ftar is of about the 9th magnitude. A memorandum to the obfervation fays, that, having but just begun, I |