Stellar light show, and magic act, to begin

Epsilon Aurigae's unusual dimming remains mysterious, but help from amateur astronomers could change that.
By | Published: July 31, 2009 | Last updated on May 18, 2023
Epsilon Aurigae
Epsilon (ε) Aurigae is one of the brightest stars in the constellation Auriga the Charioteer. It dims every 27.1 years as seen from Earth, but scientists are still unsure what causes the change in brightness.
Bill and Sally Fletcher
July 31, 2009
Next week marks the return of both a cosmic mystery and a great opportunity for amateur astronomers.

Starting in August, the normally bright star Epsilon (ε) Aurigae in the constellation Auriga the Charioteer will begin to dim. After around 6 months, it’ll stay dim for about a year, then slowly brighten until it regains its usual shine. And, adding more confusion, in the middle of its darkest moment, Epsilon Aurigae will temporarily brighten almost two-tenths of a magnitude. This unusual light show happens every 27.1 years and has been a continual source of amazement to every new generation of stargazers.

Amateur astronomers can make significant contributions to this International Year of Astronomy by closely monitoring Epsilon Aurigae’s varying brightness. Even naked-eye observations will confirm its drop from a magnitude of 3.0 to 3.8, though more sophisticated tools would help, too. At this point, astronomers need all the help they can get.

The closest equivalent to this dimming and brightening is a binary star system eclipse: When one of the two stars passes in front of the other, the whole thing looks dimmer to us on Earth because only one of its star’s light reaches us, instead of the usual both. (Binary stars are typically so far away they appear as a single star to the naked eye.)

Epsilon Aurigae finder chart
Use this finder chart to record changes in
Epsilon (ε) Aurigae’s brightness. The magnitudes (decimal points have been removed) of the comparison stars will help with your estimates.
Astronomy: Roen Kelly
But binary eclipses usually occur every few days and last only hours, a far cry from the years-long activity of Epsilon Aurigae. And what of that surprising mid-eclipse peak in brightness? Some astronomers currently theorize Epsilon Aurigae may be a binary system where the companion star brings with it a huge accretion disk of dust and gas, blocking the main star’s light (the hole in the disk would account for the peak). Other models put two stars within that disk, making the whole thing a trinary system. Recent observations also suggest ongoing changes within the system, as might be consistent with a red giant evolving to a white dwarf.

This is one of the few genuine mysteries left to us in the galaxy, and it might be changing before our eyes. “[It’s happening] on a time scale of decades rather than centuries or millions of years,” said Robert Stencel of the University of Denver. Each eclipse happens a little differently, so we never really know what to expect. This year’s observations might provide all the answers, or just leave us with new questions.

That’s why the amateur astronomers are so essential. “If we have people in Canada or Finland watching this thing during the high summer,” Stencel said, “they might be able to help us fill in some blanks.” So observers of the northern latitudes take note: Astronomers need you to help figure out Epsilon Aurigae’s mystery. With any luck, we’ll have it all figured out by next time, in 2036. And if not, well, we’re getting used to that.

Be sure to check a closer look at Epsilon Aurigae by Robert Zimmerman in Astronomy‘s October issue, on newsstands September 1.