Since the early 19th century, astronomers have observed this extremely long-period eclipsing binary located in the constellation Auriga, the charioteer. In 1928, astronomer  Harlow Shapley correctly concluded that the two stars were about equal in mass. Based on this information they should be about equal in brightness as well. But the spectrum of the system showed no light from the companion at all. The visibly bright first star (called the primary) was being eclipsed by a massive, invisible second star (called the secondary).

Epsilon Aurigae is bright enough to be seen with the unaided eye even in the most light-polluted cities, and it is visible every fall, winter and spring. The change in brightness that this star undergoes is called an eclipse (a process of fading and coming back to its usual brightness).