Starship Asterisk*

APOD Robot wrote: Andromeda Island Universe

The most distant object easily visible to the eye is M31, the great Andromeda Galaxy some two and a half million light-years away. But without a telescope, even this immense spiral galaxy - spanning over 200,000 light years - appears as a faint, nebulous cloud in the constellation Andromeda.

http://antwrp.gsfc.nasa.gov/diamond_jubilee/1920/cs_nrc.html wrote:
BULLETIN OF THE NATIONAL RESEARCH COUNCIL
Vol. 2, Part 3, May, 1921, Number 11, pp 171-217.
THE SCALE OF THE UNIVERSE
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_DIMENSIONS AND STRUCTURE OF THE GALAXY_ BY HEBER D. CURTIS


<<I hold, therefore, to the belief that the [Andromeda] galaxy is probably not more than 30,000 light-years in diameter; that the spirals are not intra-galactic objects but island universes, like our own galaxy, and that the spirals, as external galaxies, indicate to us a greater universe into which we may penetrate to distances of ten million to a hundred million light-years.>>
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_EVOLUTION OF THE IDEA OF GALACTIC SIZE_ BY HARLOW SHAPLEY


<<Another consequence of the conclusion that the galactic system is of the order of 300,000 light-years in greatest diameter, is the previously mentioned difficulty it gives to the "comparable- galaxy" theory of spiral nebulae ... Since the [island universes] theory probably stands or falls with the hypothesis of a small galactic system, there is little point in discussing other material on the subject...

It seems to me that the evidence, other than the admittedly critical tests depending on the size of the galaxy, is opposed to the view that the spirals are galaxies of stars comparable with our own. In fact, there appears as yet no reason for modifying the tentative hypothesis that the spirals are not composed of typical stars at all, but are truly nebulous objects. Three very recent results are, I believe, distinctly serious for the theory that spiral nebulae are comparable galaxies -

(1) Seares' deduction that none of the known spiral nebulae has a surface brightness as small as that of our galaxy;

(2) Reynold's study of the distribution of light and color in typical spirals, from which he concludes they cannot be stellar systems; and

(3) van Maanen's recent measures of rotation in the spiral M 33, corroborating his earlier work on Messier 101 and 81, and indicating that these bright spirals cannot reasonably be the excessive distant objects required by the theory.

But even if spirals fail as galactic systems, there may be elsewhere in space stellar systems equal to or greater than ours - as yet unrecognized and possibly quite beyond the power of existing optical devices and preset measuring scales. The modern telescope, however, with such accessories as high-power spectroscopes and photographic intensifiers, is destined to extend the inquiries relative to the size of the universe much deeper into space, and contribute further to the problem of other galaxies.>>
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