A quarter of a century ago, Sir John Herschel expressed his opinion that those nebulæ which were not resolved into individual stars by the highest powers then used, might be hereafter completely resolved by a further increase of optical power:
In fact, this probability has almost been converted into a certainty by the magnificent reflecting telescope constructed by Lord Rosse, of 6 feet in aperture, which has resolved, or rendered resolvable, multitudes of nebulæ which had resisted all inferior powers. The sublimity of the spectacle afforded by that instrument of some of the larger globular and other clusters is declared by all who have witnessed it to be such as no words can express.[23]
Although, therefore, nebulæ do exist, which even in this powerful telescope appear as nebulæ, without any sign of resolution, it may very reasonably be doubted whether there be really any essential physical distinction between nebulæ and clusters of stars, at least in the nature of the matter of which they consist; and whether the distinction between such nebulæ as are easily resolved, barely resolvable with excellent telescopes, and altogether irresolvable with the best, be any thing else than one of degree, arising merely from the excessive minuteness and multitude of the stars of which the latter, as compared with the former, consist.--_Outlines of Astronomy_, 5th edit. 1858.
It should be added, that Sir John Herschel considers the “nebular hypothesis” and the above theory of sidereal aggregation to stand quite independent of each other.
Professor Helmholtz, assuming that at the commencement the density of the nebulous matter was a vanishing quantity, as compared with the present density of the sun and planets, calculates how much work has been performed by the condensation; how much of this work still exists in the form of mechanical force, as attraction of the planets towards the sun, and as _vis viva_ of their motion; and finds by this how much of the force has been converted into heat.
The result of this calculation is, that only about the 45th part of the original mechanical force remains as such, and that the remainder, converted into heat, would be sufficient to raise a mass of water equal to the sun and planets taken together, not less than 28,000,000 of degrees of the centigrade scale. For the sake of comparison, Professor Helmholtz mentions that the highest temperature which we can produce by the oxy-hydrogen blowpipe, which is sufficient to vaporise even platina, and which but few bodies can endure, is estimated at about 2000 degrees. Of the action of a temperature of 28,000,000 of such degrees we can form no notion. If the mass of our entire system were of pure coal, by the combustion of the whole of it only the 350th part of the above quantity would be generated.