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Epsilon Eridani
Epsilon Eridani (ε Eri, ε Eridani) is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows it to be viewed from most of Earth's surface. At a distance of 10.5 light years (ly), it has an apparent magnitude of 3.73. It is the third closest individual star or star system visible to the unaided eye and was the closest star known to host a planet until the unconfirmed discovery of Alpha Centauri Bb. Its age is estimated at less than a billion years. Because of its youth, Epsilon Eridani has a higher level of magnetic activity than the present-day Sun, with a stellar wind 30 times as strong. Its rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a comparatively lower level of elements heavier than helium.〔 It is a main-sequence star of spectral class K2, which means that energy generated at the core through nuclear fusion of hydrogen is emitted from the surface at a temperature of about 5,000 K, giving it an orange hue.
The motion of Epsilon Eridani along the line of sight to Earth, known as the radial velocity, has been regularly observed for more than twenty years. Periodic changes in this data yielded evidence of a giant planet orbiting Epsilon Eridani, making it one of the nearest extrasolar systems with a candidate exoplanet.〔 This object, Epsilon Eridani b, was formally announced in 2000 by a team of astronomers led by Artie Hatzes.〔 Current data indicate that this planet orbits with a period of about 7 years at a mean separation of 3.4 astronomical units (AU), where 1 AU is the mean distance between Earth and the Sun.〔 Although this discovery has been controversial because of the amount of background noise in the radial velocity data,〔 many astronomers now regard the planet as confirmed.
The system includes two belts of rocky asteroids: one at about 3 AU and a second at about 20 AU, whose structure may be maintained by a hypothetical second planet, Epsilon Eridani c.〔 Epsilon Eridani harbors an extensive outer debris disk of remnant planetesimals left over from the system's formation.〔
Epsilon Eridani's designation was established in 1603 by Johann Bayer. It may be a member of the Ursa Major Moving Group of stars that share a similar motion through the Milky Way, implying these stars shared a common origin in an open cluster. Its nearest neighbor, the binary star system Luyten 726-8, will have a close encounter with Epsilon Eridani in approximately 31,500 years when they will be separated by about 0.93 ly.〔 As one of the nearest Sun-like stars with the potential for a planet that may harbor life,〔 Epsilon Eridani has been the target of SETI searches. Epsilon Eridani appears in science fiction stories and has been suggested as a destination for interstellar travel.〔 From Epsilon Eridani, the Sun would appear as a 2.4-magnitude star in Serpens.〔
== Observation history ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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