The Mystery of epsilon Aurigae
Do you like mystery? This month, one of the most puzzling variable stars in the sky will dim for the first time in 27 years. It is epsilon Aurigae in the constellation Auriga, the Charioteer. With an estimated distance of 2,000 light years, epsilon Aurigae is a variable star. But, the star’s variability isn’t the odd part.
Astronomers have cataloged thousands of variable stars with cycles ranging from hours to days at a time. Some stars, like the Cephied variables, pulsate in brightness due to changes in opacity of their outer layers. Other variable stars are eclipsing binary systems. The star Algol, in the constellation Perseus, is a famous example. With the regularity of clockwork, Algol’s luminosity plunges every 2.9 days. It then rebounds to full brightness in just a few hours.
Observed dips in a star’s brightness are plotted as light curves. In the case of eclipsing binaries, the amount of dimming, changes in color and the length of time for a return to full brightness provides information about the size, composition and orbit of the occluding object. Algol serves as the prototype for eclipsing binary systems.
Discovered in 1821 by Johann Fristch, epsilon Aurigae’s variability has baffled astronomers for more than a century. Once its eclipse begins, it doesn’t return to normal brightness for over 2 years! So, the eclipsing body must be very large. The light curve shows a moderate rise in brightness mid-eclipse suggesting that the occluding body has a central gap like a doughnut. Spectral analysis reveals another interesting feature. Epsilon Aurigae’s color doesn’t change during the eclipse meaning that whatever passes in front of the star emits no light of its own.
Over the years, efforts to develop a model to explain epsilon Aurigae have led to some interesting ideas, but answers remain elusive. German astronomer Hans Ludendorff proposed that a disk of meteroids orbited epsilon Aurigae. In 1937, Yerkes Observatory astronomers Gerard Kuiper, Otto Struve, and Bengt Strömgren theorized that the invisible body emits infrared radiation creating a haze of charged particles that blocks the star’s light. Later in 1955, Otto Struve suggested that the orbiting body is a dense disk of gas clouds.
Data collected during the last two eclipses of 1955-1957 and 1982-1984 have reinforced Struve’s idea. When the data sets are compared, shifts in timing for the mid-point and end of the eclipse are revealed. These timing differences tell us that the cloud of material is changing in both shape and density. These changes have caused some to suggest that the companion object may be a proto-planetary disk!
A competing hypothesis offered by Peter Eggleton and James Pringle suggests that shifts in eclipse timing might be due to the evolution of the star as it shrinks from a red giant to a white dwarf. As the star begins to contract, the size and shape of the disk cloud is altered. So, we may be seeing a process that, thousands of years from now, will create an object resembling the famous Ring Nebula.
Epsilon Aurigae is visible to the unaided eye as the point of an isosceles triangle of stars next to Auriga’s brightest star, Capella. Because epsilon Aurigae is too bright for most professional telescopes, a campaign to collect observations from the general public is underway. Learn more about the project and how you can help unlock the secrets of this enigmatic variable at http://citizensky.org.
