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Barnard's Star is a very-low-mass red dwarf about six light-years away from Earth in the constellation of Ophiuchus. It is the fourth-closest known individual star to the Sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Hemisphere.〔http://shiva.uwp.edu/p120/astro_survey.html〕 Despite its proximity, at a dim apparent magnitude of about nine, it is not visible with the unaided eye; however, it is much brighter in the infrared than it is in visible light. The star is named for American astronomer E. E. Barnard. He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion as 10.3 arcseconds per year, which remains the largest known proper motion of any star relative to the Solar System. Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard's Star has focused on measuring its stellar characteristics, its astrometry, and also refining the limits of possible extrasolar planets. Although Barnard's Star is an ancient star, it still experiences star flare events, one being observed in 1998. The star has also been the subject of some controversy. For a decade, from the early 1960s to the early 1970s, Peter van de Kamp claimed that there were one or more gas giants in orbit around it. Although the presence of small terrestrial planets around Barnard's Star remains a possibility, van de Kamp's specific claims of large gas giants were refuted in the mid-1970s. == Overview == Barnard's Star is a red dwarf of the dim spectral type M4, and it is too faint to see without a telescope. Its apparent magnitude is 9.5. This compares with a magnitude of −1.5 for Sirius – the brightest star in the night sky – and about 6.0 for the faintest objects visible with the naked eye (this magnitude scale is logarithmic, so the magnitude of 9.54 is only about 1/27th of the brightness of the faintest star that can be seen with the naked eye under good viewing conditions). At 7–12 billion years of age, Barnard's Star is considerably older than the Sun, which is 4.5 billion years old, and it might be among the oldest stars in the Milky Way galaxy. Barnard's Star has lost a great deal of rotational energy, and the periodic slight changes in its brightness indicate that it rotates once in 130 days〔 (the Sun rotates in 25). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. However, in 1998, astronomers observed an intense stellar flare, surprisingly showing that Barnard's Star is a flare star. Barnard's Star has the variable star designation V2500 Ophiuchi. In 2003, Barnard's Star presented the first detectable change in the radial velocity of a star caused by its motion. Further variability in the radial velocity of Barnard's Star was attributed to its stellar activity.〔 The proper motion of Barnard's Star corresponds to a relative lateral speed of 90 km/s. The 10.3 seconds of arc it travels annually amount to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon. The radial velocity of Barnard's Star towards the Sun is measured from its blue shift to be 110 km/s. Combined with its proper motion, this gives a true velocity relative to the Sun of 143 km/s. Barnard's Star will make its closest approach to the Sun around AD 11,800, when it approaches to within about 3.75 light-years. However, at that time, Barnard's Star will not be the nearest star, since Proxima Centauri will have moved even closer to the Sun. Barnard's Star will still be too dim to be seen with the naked eye at the time of its closest approach, since its apparent magnitude will be about 8.5 then. After that it will gradually recede from the Sun. Barnard's Star has a mass of about 0.14 solar masses (),〔 and a radius 15% to 20% of that of the Sun.〔 Thus, although Barnard's Star has roughly 150 times the mass of Jupiter (), its radius is only 1.5 to 2.0 times larger, due to its much higher density. Its effective temperature is 3,100 kelvins, and it has a visual luminosity of 0.0004 solar luminosities.〔 Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would appear only 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 astronomical units. Barnard's Star's has 10–32% of the solar metallicity.〔 Metallicity is the proportion of stellar mass made up of elements heavier than helium and helps classify stars relative to the galactic population. Barnard's Star seems to be typical of the old, red dwarf population II stars, yet these are also generally metal-poor halo stars. While sub-solar, Barnard's Star's metallicity is higher than that of a halo star and is in keeping with the low end of the metal-rich disk star range; this, plus its high space motion, have led to the designation "intermediate population II star", between a halo and disk star.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Barnard's Star」の詳細全文を読む スポンサード リンク
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