
Fig12_10 Discovery expanding universe
Fig. 12.10 . A plot of the distance of extragalactic nebulae, or galaxies, versus the radial velocity at which each galaxy is receding from Earth, published in 1929 by the American astronomer Edwin Hubble (18891953). The linear relationship between the distance and radial velocity indicates that the Universe is expanding. Vesto M. Slipher (18751969) determined most of these velocities more than a decade before this diagram was drawn. Here the velocity is in units of kilometers per second, abbreviated km s1, and the distance is in units of millions of parsecs, or Mpc, where 1 Mpc is equivalent to 3.26 million lightyears. Hubble underestimated the distances of the spiral nebulae, so the distance scale for modern versions of this diagram is about seven times larger. The filled circles and solid line represent the solution for individual nebulae; the open circles and dashed line are for groups of them, and the cross represents the mean distance of 22 nebulae whose distances could not be determined individually.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

