William Herschel and The Planetary Nebulae

At the close of the paper I give the location of half-a-dozen planetary nebulae, as I call them. These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well-defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude.Herschel to Jerome Lalande, 17 March 1785 (in French). 1An Unexpected DiscoveryIn the summer of 1782 King George III appointed William Herschel (1738-1822) astronomer to the court at Windsor Castle. On 7 September, a month after he took up residence at nearby Datchet, Herschel was engaged in his long-established search for double stars when he came across an object near v Aquarii that was unlike anything he had ever seen before:A curious Nebula, or what else to call it I do not know, it is of a shape somewhat oval, nearly circular, and with this power [460] appears to be 10 or 15" diameter. It is of the same shape with 278, but much less in appearance, with 932 it is still the same shape but much larger. So that its appearance seems to follow the law of magnifying, from whence it is clear that it is of some real magnitude in the heavens and not a glare of light. The brightness in all the powers does not differ so much as if it were of a planetary nature, but seems to be of the starry kind, tho' no star is visible with any power. It is all over of nearly the same brightness. The compound eyepiece will not distinguish it from a fixt star, at least not sensibly.2It had the pale light of a nebula but the disk of a planet, and eventually he would name it a 'planetary nebula', the term that astronomers have used ever since for this class of objects.But what was it? This was a puzzle that would haunt Herschel for the forty years remaining to him.A planetary nebula is in fact an emission nebula consisting of an expanding glowing shell of ionized gas ejected from an old red giant star late in its life. (In this example, as more often than not, Herschel was unable to discern the central star that remained.) Such a quasi-spherical shell of luminous gas will appear brightest at the outer edges, which are relatively well-defined as it is the shock-front of the expanding cloud, in contrast to a globular cluster of stars which is brightest at the centre but has poorly-defined outer limits. In the years to come, as he became increasingly familiar with clusters of stars, Herschel quickly developed his great interpretive tool - the concept of gravitational attraction operating over time to produce the clusters - but planetary nebulae would refuse incorporation into this scheme of things. And when in 1790 he came to accept that, in addition to stars, the universe also contained clouds of luminous ('true') nebulosity, the planetary nebulae would again prove incalcitrant.Is It Moving? Eighteen months earlier he had encountered "a nebulous star or perhaps a comet", and after four days this object (which we know as the planet Uranus) had shown by its movement that it was close at hand, a member of the solar system. Was the object near v Aquarii also a member of the solar system? Ninety minutes after he discovered it, Herschel checked its position, but - not surprisingly - it had not (yet) moved.3Three weeks later, on 30 September, he checked again: "Nebula near v Aquarii is in the same place and of the same shape. "4Ten months then passed, and on 30 July 1783 - with his search for double stars almost complete and his new and larger 20-ft reflector soon to be commissioned - Herschel interrupted his current programme of examination of Messier nebulae to revisit the mysterious object:Nebula. …