1. Palomar Observatory 105-24,
2. Infrared Processing and Analysis Center,
3. Jet Propulsion Laboratory 171-113, California Institute of Technology, Pasadena

Correspondence to: S. R. Kulkarni¹ Correspondence and requests for materials should be addressed to B.F.L. (e-mail: Email: bfl@astro.caltech.edu).

Abstract

Cepheids are a class of variable (pulsating) stars whose absolute luminosities are related in a simple manner to their pulsational periods. By measuring the period and using the 'period–luminosity' relationship, astronomers can use the observed visual brightness to determine the distance to the star. Because these stars are very luminous, they can be observed in other galaxies, and therefore can be used to help determine the expansion rate of the Universe¹ (the Hubble constant). Calibration of the period–luminosity relation is a necessary first step, but the small number of sufficiently nearby Cepheids has forced the use of a number of indirect means, with associated systematic uncertainties. Here we present a distance to the Cepheid Geminorum, determined using a direct measurement (by an optical interferometer) of its changes in diameter as it pulsates. Within our uncertainty of 15 per cent, our distance is in agreement with previous indirect determinations. Planned improvements to the instrument will allow us to calibrate directly the period–luminosity relation to better than a few per cent.

1. Palomar Observatory 105-24,
2. Infrared Processing and Analysis Center,
3. Jet Propulsion Laboratory 171-113, California Institute of Technology, Pasadena

Correspondence to: S. R. Kulkarni¹ Correspondence and requests for materials should be addressed to B.F.L. (e-mail: Email: bfl@astro.caltech.edu).