In August 2009 the bright star Epsilon Aurigae begun to lose brightness for the first time in 27 years. Today astronomers announce that this critical phase ended on New Year's Day and take new steps towards determining the nature of this eclipsing system.

This eclipsing event occurs every 27 years, and the dimming is thought to be as a result of an object passing in front of the star, but the nature of both Epsilon Aurigae and the eclipsing object has remained unclear. The star's spectral features indicate that it is a F-class supergiant star with 20 times the mass and 300 times as wide as our Sun, and two competing theories suggest either that Epsilon Aurigae is a massive supergiant, periodically eclipsed by two tight-knit stars inside a swirling, dusty disc, or that it is in fact a dying star with a lot less mass, periodically eclipsed by just a single star inside a disc.

Professional and amateur astronomers have been monitoring the eclipse since it underwent a dramatic drop in brightness in August, and with the support of the National Science Foundation 'Citizen Sky' has been coordinating public participation in the observation campaign, which has seen 120 observers from 19 countries submitting over 1,500 data points so far.

“We have increasing evidence that a dark disc of material has moved in front of our view of Epsilon Aurigae,” says Robert Stencel, scientific advisor for the project. “But the exact shape and make up of the disc has been unknown, but will be better defined soon. To make things even more challenging for us, some think there are multiple stars in the system, and perhaps planets spiraling into one of the stars.”

Stencel – also known as Dr Bob by the amateur astronomical community – also observed the last eclipse of the star in 1982-1984. “Just looking at the coverage in the visual data alone, I can already see interesting changes in the star that have never been seen so clearly before.”

This time around, space-based telescopes have contributed to the observations, with new and archived data from NASA's Spitzer Space Telescope pointing to one of the two competing theories for the nature of Epsilon Aurigae and its eclipsing companion – that of a dying star being eclipsed by a second, single star.

“We’ve really shifted the balance of the two competing theories,” says Donald Hoard of NASA’s Spitzer Science Center at the California Institute of Technology in Pasadena. “Now we can get busy working out all the details.”

Because the star was too bright for Spitzer's delicate infrared arrays to observe directly, the team pointed the star at the corner of four of Spitzer’s pixels, instead of directly at one, to effectively reduce its sensitivity. It took the observations using fast exposures lasting just one-hundredth of a second. Combining the data with earlier observations confirmed the presence of the companion star's disc, which has a radius of roughly four times the distance between the Earth and the Sun. To explain all the features of the system the team had to assume that the F star was a less massive dying star, and that the eclipsing object was a single B class star embedded in the disc.

“It was amazing how everything fell into place so neatly,” says Steve Howell of the National Optical Astronomy Observatory in Tucson, Arizona. “All the features of this system are interlinked, so if you tinker with one, you have to change another. It’s been hard to get everything to fall together perfectly until now.”

But there are still many more details to figure out and vigilant monitoring during the forthcoming 18 month-long dark phase, and as the star regains brightness, will be vital to fill in the missing pieces.

Both the early results of the Citizen Sky project, and the new Spitzer results were presented today at the American Astronomical Meeting in Washington.