Direct detection of pulsations of the Cepheid star xi Gem and an independent calibration of the period-luminosity relation

Cepheids are a class of variable (pulsating) stars whose absolute luminosit ies are related in a simple manner to their pulsational periods. By measuri ng the period and using the `period-luminosity' relationship, astronomers c an use the observed visual brightness to determine the distance to the star . Because these stars are very luminous, they can be observed in other gala xies, and therefore can be used to help determine the expansion rate of the Universe(1) (the Hubble constant). Calibration of the period-luminosity re lation is a necessary first step, but the small number of sufficiently near by Cepheids has forced the use of a number of indirect means, with associat ed systematic uncertainties. Here we present a distance to the Cepheid zeta Geminorum, determined using a direct measurement (by an optical interferom eter) of its changes in diameter as it pulsates. Within our uncertainty of 15 per cent, our distance is in agreement with previous indirect determinat ions. Planned improvements to the instrument will allow us to calibrate dir ectly the period-luminosity relation to better than a few per cent.