Of course, not all eclipsing binaries can be analysed in the way we have described above; often a certain amount of guess-work has to be employed. A case of particular interest is the star £ Aurigae. This is an eclipsing binary system consisting of a small, blue, bright B star with radius 5 solar radii and a large, red, K giant with radius 245 solar radii. Eclipses occur every 972 days and last for about 35 days. The B star is much smaller than its companion and is the brighter star in the system.. Therefore we can analyse the eclipses as being due to a point source of light (the B star) disappearing behind a large opaque disc (the K giant). If the disc had a sharp edge we would expect the onset of the eclipse to be almost instantaneous.

The most likely explanation for these phenomena is that the outer layers of the giant star’s atmosphere are so diffuse that even after the onset of the eclipse we see the bright star gradually dis¬appearing behind denser and denser layers of the giant’s atmosphere. The eclipse starts earlier (and ends later) when viewed in ultraviolet, rather than optical, light because ultraviolet photons are more easily absorbed in the atmosphere of the giant star than optical ones. In fact it is possible to use the blue star as a probe to shine through the various outer layers of the giant star and to give us information on its atmospheric structure.

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