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Rho Cassiopeiae (; ρ Cas, ρ Cassiopeiae) is a yellow hypergiant star in the constellation Cassiopeia. It is about from Earth, yet can still be seen by the naked eye as it is 500,000 times brighter than the Sun. On average it has an absolute magnitude of −9.5, making it visually one of the brightest stars known. Its diameter measures 450 times that of the Sun, approximately 630,000,000 kilometers, or about twice the size of the Earth's orbit. Rho Cassiopeiae is a single star, and is categorized as a semiregular variable. As a yellow hypergiant, it is one of the rarest types of stars. Only around a dozen are known in the Milky Way, but it is not the only one in its constellation which also contains V509 Cassiopeiae. ==Observation== The Bayer designation for this star was established in 1603 as part of the ''Uranometria'', a star catalog produced by Johann Bayer. The star catalog by John Flamsteed published in 1712, which orders the stars in each constellation by their right ascension, gave this star the Flamsteed designation 7 Cassiopeiae. Rho Cas was first described as variable in 1901. It was classified only as "pec." with a small but definite range of variation. Its nature continued to be unclear during the deep visual minimum in 1946, although it was presumed to be related to the detection of an expanding shell around the star. The spectrum developed lower excitation features described as typical of an M star rather than the previous F8 class. The nature of Rho Cas was eventually clarified as a massive luminous unstable star, pulsating and losing mass, and occasionally becoming obscured by strong bouts of mass loss. Rho Cas usually has an apparent magnitude near 4.5, but in 1946 it unexpectedly dimmed to 6th magnitude and cooled by over 3,000 Kelvin, before returning to its previous brightness. A similar eruption was recorded in 1893, suggesting that it undergoes these eruptions approximately once every 50 years. This happened again in 2000–2001, when it was observed by the William Herschel Telescope〔 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「Rho Cassiopeiae」の詳細全文を読む スポンサード リンク
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