The evolution of nebulae

In his two papers on the construction of the heavens in 1784 and 1785, Herschel echoed the language, if not the scientific approach, of physical geographers and geologists. He articulated a view of the Milky Way system as a branched ''stratum'' of stars, and the nebulae as variously shaped beds of stars, more or less distant in space. In two papers that appeared in 1789 and 1791, he borrowed his metaphors more from zoologists and botanists interested in the evolution of living things. Although he again over-simplified the picture presented by an array of observations, he deserves credit for emphasizing, as no one had before him, the dynamic nature of nebulae and star clusters, evolving with time under the influence of gravity.

In his 1789 paper, Herschel proposed that specimens of the many varieties he had noted among the nebulae — globular, double or triple, ''narrow but much extended,'' comet-like, fan-shaped, etc.—would, if properly arranged, form a temporal sequence that would illuminate a basic organizing principle. Diffuse, irregularly shaped nebulae, he thought, were just beginning to draw themselves together, while globular clusters seemed to have settled into their symmetrical, centrally condensed shape through the action of a central power (which he tentatively equated with Newton's gravitational force) over a long period of time.

Although many of the nebulae he fit into this scheme did not belong in the same temporal sequence — he lumped galaxies and relatively small star-forming regions in our own galaxy together—he showed a keen intuition for the formation of globular clusters. More generally, he demonstrated an apt approach to a central problem in astronomy: that of inferring the course of the physical evolution of stars and stellar systems, which occurs on extremely long timescales, from observations recorded over the span of a human lifetime. His concluding paragraph in the 1789 paper is widely quoted for beautifully expressing this approach. ''This method of viewing the heavens seems to throw them into a new kind of light,'' he wrote. ''They are now seen to resemble a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds; and one advantage we may reap from it is, that we can, as it were, extend the range of our experience to an immense duration. For, to continue the simile I have borrowed from the vegetable kingdom, is it not almost the same thing, whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?''50

About 18 months after he presented this paper to the Royal Society, on a cold, clear night in November 1790, Herschel came across an object in the constellation Taurus (now known by its catalog number NGC 1514) that completely changed his view of nebulae and prompted him to add a new twist to his ideas on the evolution of stars and stellar systems. He saw a star apparently embedded in a spherical cloud, like an extended atmosphere. The nebulous cloud seemed to be associated with the star, and therefore at the same distance. But the cloud itself would not resolve into a cluster of stars, as Herschel would have predicted.

''A most singular phaenomenon!'' he commented in his observing notebook. If the round nebulosity was actually a collection of stars, but at a great distance, the central ''star'' of this object must be superlatively bright, and unlike any other known star. It seemed much more reasonable to admit, instead, the existence of a ''shining fluid,'' here and probably elsewhere. Herschel could only guess at the nature of this self-luminous matter; he wondered if it might resemble ''the electrical fluid in the aurora borealis [the northern lights].''51

No matter what the detailed nature of the shining fluid, Herschel understood at once that its existence might serve to ''unravel'' other mysterious phenomena. In particular, he had never been sure how to classify what he called ''planetary nebulae.'' These objects looked a little like planets in his telescopes: large and round, but of uniform milky brightness, not brighter in the middle like globular clusters. He recognized that an orb of shining fluid surrounding a star, like the ''singular phenomenon'' he noticed in the constellation Taurus, accounted very well for the appearance of the planetary nebulae. Never one to curb his speculative impulses, he further surmised that the shining fluid might constitute a first stage in the formation of a star: ''If [...] this matter is self-luminous, it seems more fit to produce a star by its condensation than to depend on the star r • rr^ 52

for its existence,'' he wrote.52

As in so many instances before, Herschel had hit on kernels of truth but applied his theories too widely. A planetary nebula consists of a cloud of heated material ejected from a central star. But the gaseous material is not self-luminous as Herschel understood the term; it glows as a result of the intense ultraviolet heating it receives from the star. And planetary nebulae such as the one he saw in Taurus consist of stars in their later stages of evolution, not stars caught in the act of forming. The gaseous envelopes have come from the star and are not, as Herschel thought, condensing to create the central body.

These misunderstandings notwithstanding, in broad outline Herschel's theory anticipated our current view of star formation. A proto-stellar cloud of hot gas and dust, as we find in the Orion nebula and other emission or star-forming types of nebula, contracts and heats up further, until the proto-star reaches a high enough density to begin the thermonuclear processes that characterize a full-fledged star. And in Herschel's own time, his account of the evolution of nebulae was favorably viewed by the greatest exponent of the theory of nebular evolution, the French astronomer Pierre-Simon Laplace.

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