Fading Star

In Stars by Brian Koberlein

Two thousand light years from Earth is a star known as Epsilon Aurigae. It’s a third magnitude star most of the time, but about every 27 years it dims to about half its brightness for nearly two years. The cause of the dimming is a bit of a mystery. 

It’s long been thought that the dimming is the result of Epsilon Aurigae being a binary system. With a companion star in a large orbit, it could pass in front of the primary star, making it appear to dim. The star is indeed a binary star (if not a multiple star) but the details of the dimming mechanism have been difficult to pin down.

Historically there have been two main ideas. The first is that Epsilon Aurigae is yellow supergiant about 15 times the mass of the Sun, with a companion of similar mass obscured somewhat by dust. This idea is supported by the fact that the spectrum of Epsilon Aurigae has many of the signatures common to yellow supergiants. However the companion star has a spectral signature more similar to a B-type main sequence star.

The other idea is that Epsilon Aurigae is much smaller, with a mass of 2 to 4 solar masses. This would make it smaller than the B-type companion with a mass of about 6 solar masses. In order for the companion to be much dimmer than Epsilon Aurigae, it would have to be surrounded by a thick disk of dust, and that disk would have to be aligned edge on when seen from Earth. It would be odd for a main sequence star to have a thick dusty disk, since they are more commonly seen around young stars that are still forming.

When the most recent dimming occurred in 2009 – 20011, both amateur astronomy groups and the Spitzer infrared telescope made observations of the transit. It now seems that both models were at least partly right. The model that now seems to best fit the data assumes Epsilon Aurigae is only about 10 solar masses, but it moving toward the end of its life. This means it is much brighter than a main sequence star of similar mass. The B-type companion is therefore much dimmer by comparison. With smaller masses, the two stars would be close enough that the companion would capture gas and dust pushed away from Epsilon Aurigae, thus explaining the companion’s dusty disk.