An American Scientist classic article
This article by Harlow Shapley originally appeared in the January 1942 issue of American Scientist,the first issue of the then-quarterly magazine to carry the name American Scientist. Virginia Trimble's commentary begins on page 7.
Like the galaxies themselves, the field of inquiry concerning galaxies is large and not easily surveyed in a brief article. It will be well to restrict the assignment and write concerning only a few selected topics.
Let us first try a bird’s-eye view of our own Galaxy. The bird whose eye we would use needs to be a remarkable creature to reach the remoteness necessary for an outside look. We cannot use Cygnus, the Swan, that heads in full flight along northern Milky Way, nor Aquila, the Eagle, nor the big-billed Toucan, the Flamingo, the Phoenix, the Goose, the Bird of Paradise, nor Corvus, the Crow. All these constellation birds are composed of stars that are bright neighbors of the Sun and distinctly localized far inside our own Galaxy.
What we need is an observation point something like a million light-years distant, well outside the bounds of the enormous Milky Way system. It would be pretty satisfactory to settle our bird comfortably in the outer haze of stars of the Andromeda Nebula. If the observer be a contemporary of ours, he will be looking at our system in terms of eight thousand centuries ago. It has been that long since the radiation left the Sun and its neighboring stars on its way to the retina of the all-comprehending but quite imaginary bird now surveying us from the Andromeda galaxy.
Such a temporal disparity, 8 x 105 years, is of no particular moment in our considerations of the galaxies; and short-term enterprises like the current Western civilization, or even the whole history of mankind, can be neglected in the cosmic panorama as too momentary, too fleeting, for a clear recording.
It is well known that the Milky Way star system is a much flattened organization and that the Sun and planets are well inside. This interpretation of the Milky Way was pointed out one hundred and ninety years ago by Thomas Wright, a pioneer “bird’s-eye viewer’ of Durham, England. He saw that the hypothesis of a flattened stellar system with the Earth near the central plane would satisfactorily explain the Milky Way band as a phenomenon of projection in such a system.1 Our hypothetical observer in Andromeda would see this flattened wheel-shaped system not from the direction of its rim, nor from the direction of its axis, but from an intermediate position, galactic latitude –21°. It would appear in projection, therefore, as an elongated object, perhaps with the axes of the rough ellipse in the ratio of about three to one. There would be a conspicuous globular nucleus of naked-eye brightness.
1 For an account of the early cosmic interpretations by Thomas Wright and Immanuel Kant see the highly interesting account by F. A. Paneth, The Observatory (June, 1941), pp. 71 ff.; also H. Shapley, chap. v, Immanuel Kant, 1724–1924 (New Haven, Yale University Press, 1925), E. C. Wilm, ed.
» Post Comment