Epsilon Aurigae was first given serious, systematic, scientific scrutiny in 1821. Early modern astronomers correctly classified it as an eclipsing binary variable star, with an invisible partner that will periodically dim the light as it eclipses the main star from the perspective of Earth. This happens every 27 years, and Epsilon Aurigae's apparent brightness drops for a period of more than a year. The nature of this partner has remained a mystery, even though we've been observing the star for nearly two centuries.

Over the years, different ideas have come and gone. Early hypotheses as to the nature of Epsilon Aurigae was that it is a F-type supergiant star with a mass of over 15 solar masses. For the darker companion, people have proposed that it is an infrared star, a black hole complete with accretion disk, or (most recently) a disk of opaque material orbiting the companion star. The difficulty with the most recent interpretation is the improbability of the orbits.

For the latter case to be correct, then the orbit of the disk around the darker companion star would have to be in the same plane as the orbit of the darker object (companion star) around Epsilon Aurigae, which would in turn have to be the exact same plane as Earth's vantage point in order to produce the sequence of events we observe here on Earth. New, direct, observations show this is indeed the case—look at enough stars, and you'll apparently see the improbable. (Insert something about a large number of monkeys randomly banging away on a large number of typewriters knocking out this exact Nobel Intent article given enough time.)

New observations of the system are reported in a letter in last week's edition of Nature based on data collected using Georgia State University's Center for High Angular Resolution Astronomy interferometer with the Michigan Infra-Red Combiner. They've produced a series of direct images of the 2009 Epsilon Aurigae eclipse (along with a snapshot from 2008 to use as a baseline).

Combining this data and some that is in press from other research groups, the authors report that the main star has a mass of 3.63±0.68 solar masses (much less massive than earlier estimates), its dark companion has a mass of 5.9±0.1 solar masses. The disk of dust that orbits around the companion has a negligible mass, 0.07 time the mass of the Earth. From the images, they are able to describe the disk, improbable orbit and all, as a cylinder with a radius of 3.81±0.01 AU (the distance from the Earth to the Sun) and a height of 0.76±0.02 AU. The authors conclude that the disk is an optically thick but geometrically thin, suggesting it is a debris disk as opposed to a young stellar object.

Nature, 2010. DOI: 10.1038/nature08968 (About DOIs).