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Galactic distance by triangulation.

Triangulation is a common method of measuring distances on the face of the earth. Now it has been used for the first time in determining the distance from earth to a faraway galaxy. The achievement introduces a method that may in the long run prove more certain than other methods now is use. It also gives Hubble's constant, the number that measures the rate of expansion of the universe.

The triangulation method depends on radio and optical observations of a supernova in a distant galaxy, specifically a supernova that exploded in 1979 in the galaxy M100 located in the Virgo cluster. A supernova is a stellar explosion. It starts from virtually zero diameter and produces an expanding ball of debris.

Optical observation can give the linear speed of expansion, in this instance up to 12,000 kilometer per second. From this the diameter of the ball at any time can be calculated. Radio observation can measure the angle that the diameter makes, as viewed from the earth. From these two data triangle can be set up that gives the distance to the galaxy, in this case 19 megaparsecs. Essential to the procedure is precise, very-long-baseline interferometry, the combining of simultaneous observations by radiotelescopes located across the earth. Participants in the observation are Norbert Bartel of the Harvard-Smithsonian Center for Astrophysics (CFA) in Cambridge, Mass., A.E.E. Rogers of the Haystack Observatory in Westford, Mass., Irwin I. Shapiro, Marc V. Gorenstein and C.R. Giwnn of the cFA, J.M. Marcaide of the Max Planck Institute for Radioastronomy in Bonn, West Germany, and K.W. Weiler of the National Science Foundation in Washington, D.C. They reported their findings in the Nov. 7NATURE.

The value of the Hubble constant determined is 65 kilometers per second per megaparsec with uncertainties of plus 35 and minus 25 km/sec/Mpc. The large uncertainties arise because a long string of assumptions had to be made about the geometric development of a supernova -- whether it expands evenly and spherically, whether it is a thin sell or a filled sphere, etc. Further observations of supernovas in general and of this one in particular ought to lessen some of the uncertainties. The value of the Hubble constant lies between the two earlier "standard" ones, 50 and 100, but the uncertainties do not allow it to distinguish between them. Such a distinction, settling on a single value for the constant, is the ultimate goal.
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Author:Thomsen, Dietrick E.
Publication:Science News
Date:Nov 30, 1985
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