Determining distances to cosmic objects is a challenging business. Most techniques give only relative distances. X-rays detected by the Chandra space telescope from a star in our Galaxy provide a direct way to measure distances to galactic objects, and perhaps even other galaxies.

The star shown in red, Cygnus X-3, is a variable source of X-rays whose emission varies over a period of 4.8 hours. Interstellar dust scatters the X-rays by varying amounts, leading to a ‘halo’ of radiation. By measuring the difference in arrival times between radiation from the inner halo (green ring) and outer halo (blue ring) a team of astronomers, led by Peter Predehl of the Max-Planck Institute in Garching, Germany, has worked out the distance to Cygnus X-3 to be 30,000 light years.

The uncertainty in the new measurement, to be published in the journal Astronomy and Astrophysics, is 20%. The accuracy was limited by the short observation time (3.5 hours) and the authors hope to improve this by observing the star over the whole of its 4.8-hour cycle.

The consequences of this work may be far greater if the technique can be applied to variable X-ray sources in other galaxies. In particular, the distance to the Large Magellanic Cloud is crucial for estimating the age and expansion rate of the Universe, but there has to be enough dust around to produce a useful halo.