Recreated from original plot by Edwin Hubble, March 15, 1929.
This diagram shows the data from Edwin Hubble's original measurement of the velocity and distance of Cepheid variable stars in other galaxies. He measured the distance to the stars by comparing their apparent brightness to their known luminosity, and determined their velocities from the Doppler shift of their spectra. Hubble inferred from this plot that velocity is proportional to distance: galaxies that are farther away are moving away from us faster than nearby galaxies. If the universe were static, the way Einstein had originally assumed, the line through the data points on the Hubble diagram would be horizontal, and the velocity of other galaxies that Hubble observed would not depend on their distance. Instead, Hubble discovered that the universe is expanding. (Unit: 11)