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Pleione (28 Tauri, BU Tauri) is a binary star in the Pleiades star cluster (M45), located roughly 390 light years away in the constellation of Taurus. Pleione was not given a Bayer designation, but did receive a Flamsteed number—hence its designation 28 Tauri. Since the star is located close to Atlas, it's difficult for stargazers to distinguish with the naked eye, even though it's a hot type B star 190 times more luminous than the Sun. Pleione rotates even faster than Achernar on its axis, close to its breakup velocity. The brighter star of the binary pair, component A, is a classical Be star with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. Although some research on the system has been performed, stellar characteristics of the orbiting B component are not well known.〔See the section Star system for further information〕 Pleione is the seventh brightest star in the Pleiades, after Taygeta. ==Visibility== With an apparent magnitude of +5.05 in V, the star is rather difficult to make out with the naked eye, especially since its close neighbour Atlas is 3.7 times brighter and located less than 5 arcminutes away.〔The brightness ratio of Atlas versus Pleione is derived from the formula for apparent magnitude and is based on their respective visual magnitudes: Atlas () at 3.62 and Pleione () at 5.05. Therefore: :〕 Beginning in October of each year, Pleione along with the rest of the cluster can be seen rising in the east in the early morning before dawn.〔 〕 To see it after sunset, one will need to wait until December. By mid-February, the star is visible to virtually every inhabited region of the globe, with only those south of 66° unable to see it. Even in cities like Cape Town, South Africa, at the tip of the African continent, the star rises almost 32° above the horizon. Due to its declination of roughly +24°, Pleione is circumpolar in the northern hemisphere at latitudes greater than 66° North. Once late April arrives, the cluster can be spotted briefly in the deepening twilight of the western horizon, soon to disappear with the other setting stars.〔 〕 Pleione is classified as a Gamma Cassiopeiae type variable star, with brightness fluctuations that range between a 4.8 and 5.5 visual magnitude.〔 〕 Its variable star designation is BU Tauri. The SIMBAD astronomical database lists its spectral class as B8IVev,〔 although the current classification recognized by many researchers is B8IVpe.〔〔〔 The suffix "ev" stands for ''"Spectral emission that exhibits variability"'' whilst the suffix "pe" refers to ''"Emission lines with peculiarity"''. In the case of Pleione, the "peculiar" emissions come from gaseous circumstellar disks formed of material being ejected from the star. There has been significant debate as to the star's actual distance from Earth. The debate revolves around the different methodologies to measure distance—parallax being the most central, but photometric and spectroscopic observations yielding valuable insights as well.〔〔 〕 Before the Hipparcos mission, the estimated distance for the Pleiades star cluster was around 135 parsecs or 440 light years. However, when the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about 119 ± 1.0 pc (388 ± 3.2 ly), triggering substantial controversy among astronomers.〔〔 〕〔 〕 If the Hipparcos estimate were accurate, some astronomers contend, then stars in the cluster would have to be fainter than Sun-like stars—a notion that would challenge some of the fundamental precepts of stellar structure. Interferometric measurements taken in 2004 by the Hubble Telescope's Fine Guidance Sensors and corroborated by studies from Caltech and NASA's Jet Propulsion Laboratory showed the original estimate of 135 pc or 440 ly to be the correct figure.〔 However a recent study published in 2009 has argued otherwise, publishing a new parallax measurement of 8.32 mas with a very tight error factor of 0.13 mas yielding a distance of 120.2 ± 1.9 pc or 392.0 ± 6.0 ly.〔 Which distance estimate future astrometric calculations will corroborate remains to be seen, although the upcoming Gaia mission with its expected launch in late 2012 could well prove to be the ultimate arbiter in this debate.〔 〕 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「Pleione (star)」の詳細全文を読む スポンサード リンク
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