Epsilon Aurigae

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Epsilon Aurigae (ε Aur / ε Aurigae) is a star in the constellation Auriga. It is traditionally known as Almaaz, Haldus, or Al Anz. Epsilon Aurigae is an eclipsing binary system composed of a bright, older star of spectral class F0 and an invisible companion now estimated to be a B class star. About every 27 years, epsilon Aurigae's brightness drops from an apparent visual magnitude of +2.92 to +3.83 [2]. This dimming lasts between 640-730 days [3]. In addition to this eclipse, the system also has a low amplitude pulsation with a period around 66 days[4]. The system lies approximately 2,000 light years from Earth.

Epsilon Aurigae was first suspected to be a variable star when German astronomer Johann 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 the star in great detail. His work revealed that the system was an eclipsing binary variable star, or a star that dims when its partner obscures its light.

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.[5] 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.

System components and variability

The Epsilon Aurigae system is now under intense observation and investigation due to the Spitzer and Citizen Sky programs. Therefore, the composition of the star system is currently in question.
The pair was formerly believed to be composed of one supergiant F-type star and a massive, though dim, eclipsing component whose exact nature was not known, although a 1985 model proposed it to be a disk of dust.[5] The two entities eclipse each other every 27.1 years, and each eclipse lasts approximately two years.[9] Midway through the eclipse, the system brightens slightly, due to the disk being edge-on, reducing its profile and blocking less of the light from Epsilon Aurigae A. The F-type supergiant and the dust disk are nearly thirty AU apart,[9] which is approximately the distance of the planet Neptune from the Sun.[11]
[edit]Visible component
The visible component, Epsilon Aurigae A, is a semiregular pulsating post-asymptotic giant branch star belonging to the spectral class F0.[5] This F-type star has around 135 times the diameter of the Sun,[1] and is 40,000 to 60,000 times as luminous. (Reliable sources vary considerably in their estimates of both quantities.) If the star were in the position of the Sun, it would envelop Mercury and possibly Venus. F-type stars like Epsilon Aurigae tend to glow "white" and display strong ionized calcium absorption lines and weak hydrogen absorption lines; being a class above the Sun (which is a G-type star), F-type stars are typically hotter than Sunlike stars.[12] Other F-type stars include Procyon's primary star, the brightest star in the Canis Minor constellation;[13] and Canopus, the second brightest star in the night sky and the brightest star in the Carina constellation.[14]
[edit]Eclipsing component
The eclipsing component emits a negligible amount of light, and is not visible to the naked eye. A heated region, however, has been discovered in the center of the object. The exact form of Epsilon Aurigae's eclipsing component is not known. Hypotheses concerning the nature of this second object have been proposed.[5] Three of these hypotheses have garnered attention from the scientific community.
The first hypothesis, set forth in 1937 by astronomers Gerard Kuiper, Otto Struve, and Bengt Strömgren, suggested that Epsilon Aurigae was a binary star system containing an F2 supergiant and an extremely cool "semitransparent" star that would completely eclipse its companion. However, the eclipsing star would scatter light emitted by its eclipsed companion resulting in the observed decrease in magnitude. The scattered light would be detected on Earth as a star visible to the naked eye, although this light would be significantly dimmed.[5] However, astronomer Su-Shu Huang published a paper in 1965 that outlined the defects of the Kuiper-Struve-Strömgren model, and proposed that the companion is a large disk system, edge-on from the perspective of Earth.[5] Robert Wilson, in 1971, proposed that a "central opening" lay in the disk, a possible reason for the system's sudden brightening mid-way through the eclipse.[5] In 2005, the system was observed in the ultraviolet by the Far Ultraviolet Spectroscopic Explorer (FUSE); as the star system was not emitting energy at rates characteristic of objects such as the neutron star binary system Circinus X-1 or black hole binary system Cygnus X-1, the object occupying the center of the disk is not expected to be anything of the sort; in contrast, a new hypothesis has suggested that the central object is actually a B5-type star.[5][1]

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