Jeans nebular theory

Sometime between May 1922 and April 1923, after his return from the IAU meeting in Rome and the publication of his classification system, Hubble must have studied the nebular theory developed earlier by Jeans at Cambridge, for the theory began to influence his research.

Jeans began with the premise at the heart of Laplace's nebular hypothesis, that the solar system originated out of a rotating nebulous mass of gas, but he sought to apply this premise to the origin of the spiral nebulae, which he believed were island universes like our own galaxy.

The nebulous mass in Jeans's theory—which, he admitted, came into existence ''in an entirely unknown way'' — contracted under the influence of gravity.18 Initially it assumed a spherical form, but distortions from nearby masses or nebulae and further contraction conspired to flatten the nebula somewhat. Eventually it would take on a ''lenticular'' or lens-like form, bulging in the middle and tapering at the edges. Jeans compared this theoretical form to photographs of certain extra-galactic nebulae seen edge-on.

Further contraction of the nebula would lead to matter being thrown off its equatorial rim, ''much in the same way in which water would gradually drip over the edge of a slowly shrinking cup,'' Jeans explained.19 The ejected matter would form a chain of small nebular ''satellites,'' which would cool and condense into clusters of stars. This chain of condensing material could be compared to the arms of spirals, Jeans suggested. The process of the central nebular mass ejecting matter would continue, with material streaming outward along the arms. Overall, the nebula would appear more distinct at the edges, where clusters of stars would form first, and more nebulous in the middle.

At least one other scientist besides Hubble, a geophysicist and mathematician at Cambridge, took note of Jeans' theory. In 1923, Harold Jeffreys suggested, though rather vaguely, that the spiral nebulae might represent the end point of Jeans' nebular evolution, while the ''lenticular'' nebulae, those Hubble called spindles, represented an earlier stage.

Whether Hubble learned the details of Jeans' theory from such published opinions or from Jeans himself, he wasted no time incorporating the theory into his scientific agenda. In April 1923, Hubble mentioned it explicitly in a letter to Slipher at Lowell Observatory. He wrote, ''I have been trying to construct a classification of non-galactic nebulae analogous to Jeans' evolution sequence but from purely observational material. The basis is a distinction between amorphous nebulosity and the granular beaded arms of spirals.'' He noted that the amount of material in the various ''amorphous'' and ''spiral'' nebulae he had looked at seemed to be roughly equal, judging from their luminosities. Thus it was ''quite possible to conceive of them as representing different stages of an evolutional sequence.''20

In accordance with this effort to relate the non-galactic nebulae to Jeans' theoretical forms, Hubble directed his attention to some of the more prominent non-galactic nebulae, trying to see individual stars, which had never been resolved in spiral systems, in their outskirts. One such nebula is the elliptical galaxy M87 in the constellation Virgo, a conspicuous member of a vast cluster of galaxies. Between 1920 and 1923, Hubble collected images of this nebula with the 100-inch telescope. These images, far better than any that could have been taken before, tantalized him. Extending in a line from the nucleus of the nebula Hubble saw ''a remarkable chain of nebulous objects,'' a series of five small round nebulae. He described them as ''almost stellar condensations.'' He also noted—again, in conformity with Jeans' theory, although he did not mention Jeans in his 1923 report on M87—that faint stars seemed to ''cluster about the outskirts of the nebula,'' just outside the central regions of amorphous nebulosity.21 Here might be evidence of ''condensations'' at the outer limits of the nebula, as predicted by Jeans. Then again, the bright points might be unresolved clusters of stars rather than single stars embedded in some remnant nebulosity, so Hubble expressed himself cautiously. He was right to be cautious—those bright points were, in fact, globular clusters attendant upon the M87 galaxy. The situation tugged at his most basic tendencies: his ambition to discover observational clues to the physical nature of the nebulae, and his desire to investigate the phenomena without theoretical bias and hence without the possibility of being led astray if the theory proved incorrect.

The M87 question was still unsettled that summer when he obtained a different sort of clue. Comparing photographic plates of the irregularly shaped nebula known as NGC 6822 — a cloudy blob in the constellation Sagittarius, discovered by his friend Barnard in 1884 — Hubble thought he saw brightness changes associated with variable stars. If real, the light variations would not only signal the presence of individual stars, but might also afford an estimate of the distance to the nebula, based on a comparison of similar variable stars at known distances.

In July, Hubble dashed off a letter to Shapley, with whom he had maintained a civil if not warm relationship after the latter became director of the Harvard College Observatory. Hubble wanted to know if Shapley could provide comparison plates from an earlier epoch, from Harvard's archives. Shapley could, and published his own analysis of the plates, suggesting that

NGC 6822 lay more than a million light-years away. Here, for any who paid attention, was a hint that the barrier of inter-galactic distances would soon be breached.

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