Epsilon Aurigae (ε Aurigae, abbreviated Epsilon Aur, ε Aur) is a multiple star system in the northern constellation of Auriga, the charioteer. It is an unusual eclipsing binary system comprising an F0 supergiant (officially named Almaaz , the traditional name for the system) 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. The distance to the system is still a subject of debate, but data from the Gaia spacecraft puts its distance at around light years from Earth.
Epsilon Aurigae was first suspected to be a variable star when German astronomer Johann Heinrich Fritsch observed it in 1821. Later observations by Eduard Heis and Friedrich Wilhelm Argelander reinforced Fritsch's initial suspicions and attracted attention to the star. Hans Ludendorff, however, was the first to study it in great detail. His work revealed that the system was an eclipsing binary variable, a star that dims when its partner obscures its light.
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.
Epsilon Aurigae's eclipsing companion has been subject to much debate since the object does not emit as much light as is expected for an object its size. As of 2008, the most popularly accepted model for this companion object is a binary star system surrounded by a massive, opaque disk of dust; theories speculating that the object is a large, semitransparent star or a black hole have since been discarded.
Nomenclature
ε Aurigae (Latinised to Epsilon Aurigae) is the system's Bayer designation. It also bears the Flamsteed designation 7 Aurigae. It is listed in several multiple star catalogues as ADS 3605 A, CCDM J05020+4350A, and WDS J05020+4349A.
Richard Hinckley Allen reported that Oxford scholar Thomas Hyde recorded the traditional name Almaaz in his 1665 translation of the catalogue of Ulugh Beg, which he identified with the Arabic Al Maʽaz "the billy goat", corresponding to the name of the star Capella (Latin for "nanny goat"). Allen's spelling corresponds to the plural المعز al-maʽaz "goats". Allen also reported that medieval Persian astronomer Zakariya al-Qazwini knew it as Al Anz. Ptolemy in the Almagest said that the star marked the charioteer's left elbow.
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN to catalogue and standardize proper names for stars. For such names relating to members of multiple star systems, and where a component letter (from e.g. Washington Double Star Catalog) is not explicitly listed, the WGSN says that the name should be understood to be attributed to the brightest component by visual brightness. The WGSN approved the name Almaaz for the brightest component of this system on February 1, 2017 and it is now so included in the List of IAU-approved Star Names.
In Chinese, (), meaning Pillars, refers to an asterism consisting of Epsilon Aurigae, Zeta Aurigae, Eta Aurigae, Upsilon Aurigae, Nu Aurigae, Tau Aurigae, Chi Aurigae and 26 Aurigae. Consequently, the Chinese name for Epsilon Aurigae itself is (, "First Star of Pillars").
Observational history
Although the star is easily visible to the naked eye, Johann Fritsch's 1821 observations suggest he was the first to notice that the system was a variable. Eventually, from 1842 to 1848, German mathematician Eduard Heis and Prussian astronomer Friedrich Wilhelm Argelander began observing it once every few years. Both Heis' and Argelander's data revealed that the star had become significantly dimmer by 1847, attracting the full attention of both men at that point. Epsilon Aurigae had brightened significantly, and had returned to "normal" by the following September. As it attracted more attention, more and more data was compiled. The observational data revealed that Epsilon Aurigae did not just vary over a long period, but also experienced short-term variations in brightness as well. Later eclipses took place between 1874 and 1875 and, nearly thirty years later, between 1901 and 1902.
Hans Ludendorff, who had also been observing Epsilon Aurigae, was the first to conduct a detailed study of the star. In 1904, he published in Astronomische Nachrichten an article titled Untersuchungen über den Lichtwechsel von ε Aurigae (Investigations of the Light Changes of Epsilon Aurigae), where he suggested the star was an Algol variable and an eclipsing binary.