What does it mean? Part 1
The set of images published in Nature on April 8th 2010 represent only a few nights of observing, mainly during ingress phases of this eclipse. Brian and I will, in tag team form, blog about a number of facets about the observation and its implication, and provide a sense of what's next in this process.
First, this direct detection of the disk is a wonderful demonstration of the scientific method: long theorized to be there, and at long last it is observationally confirmed.
You might ask: The disk must be large, but how large? And how massive? How far is the companion from Epsilon Aurigae?
How large? It's big - nearly reaching the orbit of Jupiter around the Sun if we moved it into our solar system, and as thick as earth's orbit in the vertical dimension. This dimensional estimate is dependent on distance assumptions, but we'll come back to that.
How massive? Surprisingly lightweight! In the Nature paper we estimated that only an earth mass of gravel would be required to optically darken a volume that size! Some of you may remember what dark spots cometary fragments were able to produce on Jupiter a few years ago. Even the Zodial dust cloud occupies a huge volume in our solar system but is a tiny amount of mass compared to Earth. Finally, debris disks like those around beta Pictoris and Vega are detected but similarly lightweight. Photons are easy to stop - even your thin eyelids are sufficient.
How far from the companion star that is being eclipsed now? The mass ratio and estimated masses suggest that the orbital separation is 19 AU, about the orbit of Uranus. For completeness, the bright companion star itself is about as wide as the orbit of Venus, making this a "close binary" even if the distances are huge.
What else have we learned so far? Over to Brian for Part 2...