Galactocentrism

In astronomy, Galactocentrism is the theory that our sun is at or near the center of the galaxy. Observations by William Herschel in 1785 indicated that the Milky Way was a separate disk-shaped galaxy with the sun in a central position. Although proved incorrect by Harlow Shapley in 1918, the theory was a step in the development of cosmological models as the speculation of the existence of other galaxies, comparable in size and structure to our own, placed the earth in its proper perspective with respect to the rest of the universe.

History

Work by Thomas Wright and Kant indicated that fuzzy patches of light called nebulae were actually distant "island universes" consisting of many stellar systems.^[1] The shape of our own galaxy was expected to resemble that of the nebulae. In 1783, amateur astronomer William Herschel attempted to determine the shape of the galaxy by examining stars through his handmade telescope. Seeing that the stars belonging to the Milky Way galaxy appeared to encircle the Earth, Herschel guessed the galaxy was arranged like a flattened disk.^[2] He carefully counted stars of given apparent magnitudes, and after finding the numbers were the same in all directions, concluded Earth must be close to the center of the galaxy. However, there were two flaws in Herschel's methodology: magnitude is not a reliable index to the distance of stars, and some of the areas that he mistook for empty space were actually dark, obscuring nebulae that blocked his view toward the center of the Milky Way.^[3]

The Herschel model remained relatively unchallenged for the next hundred years, with minor refinements. Jacobus Kapteyn introduced motion, density, and luminosity to Herschel's star counts, which still implied a near-central location of the Sun.^[4] In 1918, as Kapteyn was refining his model, the galactocentric theory was overthrown by astronomer Harlow Shapley's work on globular clusters.

Shapley had been studying the asymmetrical distribution of globular clusters, estimating the distance and location of individual objects by using variable stars as standard candles. Globular clusters contain many cepheid variable stars, whose precise relationship between luminosity and variability period was established by Henrietta Leavitt in 1908.^[4] Using cepheid and RR Lyrae variables to systematically chart the distribution of globular clusters, Shapley discovered that the stars in the Milky Way orbited a common center thousands of light years away from the Sun.^[5] The galactic center was determined to be in the direction of the Sagittarius constellation, approximately 50,000 light-years from us.^[5]

When astronomers realized that starlight can be absorbed by clouds of gas and dust, infrared radiation was used to penetrate the dust clouds and locate our solar system at 9 to 15 thousand parsecs from the center of the Milky Way galaxy.^[2]

References