The Hubble constant, which is the rate at which the velocity of a galaxy increases with distance, is one of the most fundamental constants in cosmology. Astronomers can only obtain an accurate value for the constant using 'standard candles', which give reliable and accurate measurements of the distance to receding galaxies. One such class of standard candle is the Cepheid variable stars, which vary in size and brightness with a period that is correlated with their intrinsic luminosity. But astronomers also need a further independent measure of the distance to closer Cepheid variables to calibrate this relationship between period and luminosity.

Shri Kulkarni and co-workers from the Palomar Observatory determined the distance to Zeta Geminorum by measuring variations in its angular diameter that were a thousand times smaller than the resolution of the Hubble Space Telescope. They achieved this remarkable resolution using a technique known as interferometry, which has been used in radio astronomy for decades but has only recently been available to optical astronomy. Using the Palomar Testbed Interferometer, they combined the light from two telescopes 110 metres apart to achieve the same resolution as a single telescope with a diameter of 110 metres.

Several teams are now using optical interferometry, but this is the first conclusive observation of variations in the diameter of a star. The team has obtained preliminary results that agree with other techniques for measuring the distance of Zeta Geminorum. They hope to increase the accuracy of their measurements to within five percent, at which point they will be able to make a significant contribution to fundamental cosmology.