柱一（御夫座ε）是在北天御夫座内的一颗恒星，在拜耳命名法中的名称是御夫座ε。它在西方的固有名称是Almaaz、Haldus、或Al Anz。柱一是颗不寻常的食双星，系统包含一颗F0的超巨星和一颗未知的伙伴，通常被认为是个有着黑暗盘面的小B型恒星。大约每27年，柱一的光度会从视星等+2.92等降至+3.83等，这种变暗会持续640-730天。除了这种食变，这个系统还有约66天周期的低振幅变动。这个系统与地球的距离仍有争议，但现在的估计大约是2000光年。

德国天文学家约翰·海因里希·弗里奇在1821年首度观测与怀疑柱一是一颗变星。稍后，爱德华·海斯和弗里德里希·阿格兰德确认弗里奇最初的怀疑，并对这颗恒星特别关注。然而，汉斯·鲁登道夫才是第一位对它仔细研究的天文学家。他的工作表明这个系统是食变星，由于它的伴星掩蔽，才使它的光度变暗。

Epsilon Aurigae (ε Aur, ε Aurigae) is the Bayer designation for a star in the northern constellation of Auriga. It is traditionally known as Almaaz, Haldus, or Al Anz. Epsilon Aurigae is an unusual eclipsing binary system comprising an F0 supergiant and a companion which is generally accepted to be a huge dark disk orbiting an unknown object, possibly a binary system of two small B-type stars. About every 27 years, Epsilon Aurigae's brightness drops from an apparent visual magnitude of +2.92 to +3.83. This dimming lasts 640–730 days. In addition to this eclipse, the system also has a low amplitude pulsation with a non-consistent period of around 66 days. The distance to the system is still a subject of debate, but modern estimates place it approximately 2,000 light years from Earth.