EIU Astro

The Strange case of Epsilon Aurigae July 12, 2009

When I was a freshman in high school and first developed my interest in Astronomy, two of the more fascinating sources of knowledge I had were the books, “The Universe” by Issac Asimov and the “Guinness book of World Records”.

I still remember running across, in Guinness, the record “the largest star” ….which refers to the diameter of the star, not the mass of the star. Back then the record holder, according to Guinness,  was Epsilon Aurigae B, the second member of the binary system (hence the B). The brighter member of the system, Epsilon Aurigae A , is a FO supergiant star visible to the naked eye as a 3.0 magnitude star. Given the temperature from its spectra,  and at a distance of about 700 parsecs or 2300 light years, that means its about 100 times the diameter of the Sun and about 50,000 time more luminous.

Epsilon Aurigae in mid July before dawn

You can find the star in the East before dawn, just to the right and slightly above the bright star Capella.

The real interesting object , is not the FO star, but its companion. The system is what astronomers call an eclipsing binary. The system first caught the eye of astronomers when it was noticed that it was a variable star. A star that varied in brightness. In this case, it change between 3.0, and dims to 3.8 magnitude and back again to 3.0, over a cycle of 27.1 years. Now some star are what are called intrinsic variables, meaning the stars pulsate and actually change in brightness, not so here.

Light Curve from the last eclipse

The companion of the FO star happens have its orbit alligned to our eye so it passes in front of the primary star, blocking some of the light … hence eclipsing binary. Now eclipsing binary stars not uncommon, but in this case, the eclipse last for over 2 years! Meaning, whatever the companion is, it’s VERY big.

Notice I’ve stopped calling the companion a star, since it’s also very dark. Much darker than any star it’s size has a right to be. So dark, that astronomers don’t know for sure what it is. The best theory is it’s a large disk of gas and dust surrounding a hidden star that orbits Epsilon Aurigae. If you look at the light curve, above,  you’ll notice it brightens in mid-eclipse. Some speculate there might be a double star rotating in the center of the disk that clears out a hole for the light of the main star to shine through.

Much of this is speculations, since 27 years ago astronomers weren’t able to get a good spectra of the object. So one of the projects, at the EIU observatory, are students trying to get spectra before and going into the eclipse. We hope the edges of the disk will be thin enough that we can see a change in the spectra as light starts to dim. You don’t need a big telescope since at 3rd magnitude the object is quite bright. So wish us luck, and if we see something we’ll let you know.

Listen to the PODCAST about Epsilon Aurigae at 365 days of astronomy

For More information on how you can contribute go to web site: citizensky.org