THE STELLAR UNIVERSE. 



391 



In some parts of the firmament appearances have been observed which have 

 led to the conjecture that nebulous patches may be in a state of progressive 

 formation into stellar clusters. An illustration of this is presented in fig. 15, 

 annexed, in which a patch in the constellation of Virgo is represented. This 

 portion of the heavens is strewn over with small round telescopic clusters, the 

 stars of which seem to be closely condensed together. 



" Planetary nebula," says Sir John Herschel, M are very extraordinary ob- 

 jects. They have, as their name imports, exactly the appearance of planets ; 

 round or slightly oval disks, in some instances quite sharply terminated, in 

 others a little hazy at the borders, and of a light exactly equable or only a very 

 little mottled, which, in some of them, approaches in vividness to that of actual 

 planets. Whatever be their nature, they must be of enormous magnitude. 

 One of them is to be found in the parallel of * Aquarii, and about 5m. preceding 

 that star. Its apparent diameter is about 20". Another, in the constellation 

 Andromeda, presents a visible disk of 12", perfectly defined and round. Grant- 

 ing these objects to be equally distant from us with the stars, their real dimen- 

 sions must be such as would fill, on the lowest computation, the whole orbit 

 of Uranus. It is no less evident that, if they be solid bodies of a solar nature, 

 the intrinsic splendor of their surfaces must be almost infinitely inferior to that 

 of the sun's. A circular portion of the sun's disk, subtending an angle of 20", 

 would give a light equal to 100 full moons; while the objects in question are 

 hardly, if at all, discernable with the naked eye. The uniformity of their 

 disks, and their want of apparent central condensation, would certainly augur 

 their light to be merely superficial, and in the nature of a hollow spherical 

 shell ; but whether filled with solid or gaseous matter or altogether empty, it 

 would be waste of time to conjecture. 



" The nebulae furnish, in every point of view, an inexhaustible field of spec- 

 ulation and conjecture. That by far the larger share of them consist of stars 

 there can be little doubt ; and in the interminable range of system upon sys- 

 tem, and firmament upon firmament, which we thus catch a glimpse of, the 

 imagination is bewildered and lost. On the other hand, if it be true, as, to say 

 the least, it seems extremely probable, that a phosphorescent or self-luminous 

 matter also exists, disseminated through extensive regions of space, in the 

 manner of a cloud or fog — now assuming capricious shapes, like actual clouds 

 drifted by the wind, and now concentrating itself like a cometic atmosphere 

 around particular stars ; what, we naturally ask, are the nature and destination 

 of this nebulous matter ? Is it absorbed by the stars in whose neighborhood 

 it is found, to furnish, by its condensation, their supply of light and heat ? or 

 is it progressively concentrating itself by the effect of its own gravity into 

 masses, and so laying the foundation of new sidereal systems or of insulated 

 stars ? It is easier to propound such questions than to offer any probable reply 

 to them. Meanwhile, appeal to fact, by the method of constant and diligent 

 observation, is open to us ; and, as the double stars have yielded to this style 

 of questioning, and disclosed a series of relations of the most intelligible and 

 -interesting description, we may reasonably hope that the assiduous study of 

 the nebulae will, ere long, lead to some clearer understanding of their intimate 

 nature." 



Having thus given examples of the principal varieties of form and condition 

 exhibited by these objects, we may add that their number seems to be almost 

 as unlimited as that of the stars, and that, like the stars, they are not equally 

 distributed over every part of the firmament, but prevail most in particular re- 

 gions of the heavens. The catalogue of nebulae published by Sir John Her- 

 schel in the Philosophical Transactions for 1833, contains a list of 2,306 

 nebulae visible from the observatory at Slough, which do not include the large 



