|
The Kreutz sungrazers (, pronounced kroits) are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, who first demonstrated that they were related.〔 A Kreutz sungrazers's aphelion is about 170 AU from the Sun; these sungrazers make their way from the distant outer Solar System from a patch in the sky in Canis Major, to the inner Solar System, to their perihelion point near the Sun, and then leave the inner Solar System in their return trip to their aphelion. Several members of the Kreutz family have become great comets, occasionally visible near the Sun in the daytime sky. The most recent of these was Comet Ikeya–Seki in 1965, which may have been one of the brightest comets in the last millennium.〔 It has been suggested that another cluster of bright Kreutz system comets may begin to arrive in the inner Solar System in the next few years to decades.〔 Many hundreds of smaller members of the family, some only a few meters across, have been discovered since the launch of the SOHO satellite in 1995. None of these smaller comets have survived its perihelion passage. Larger sungrazers such as the Great Comet of 1843 and C/2011 W3 (Lovejoy) have survived their perihelion passage. Amateur astronomers have been successful at discovering Kreutz comets in the data available in real time via the Internet.〔 == Discovery and historical observations == The first comet whose orbit had been found to take it extremely close to the Sun was the Great Comet of 1680. This comet was found to have passed just 200,000 km (0.0013 AU) above the Sun's surface, equivalent to about half the distance between the Earth and the Moon. It thus became the first known sungrazing comet. Its perihelion distance was just 1.3 solar radii. Astronomers at the time, including Edmond Halley, speculated that this comet was a return of a bright comet seen close to the Sun in the sky in 1106.〔 163 years later, the Great Comet of 1843 appeared and also passed extremely close to the Sun. Despite orbital calculations showing that it had a period of several centuries, some astronomers wondered if it was a return of the 1680 comet.〔 A bright comet seen in 1880 was found to be travelling on an almost identical orbit to that of 1843, as was the subsequent Great Comet of 1882. Some astronomers suggested that perhaps they were all one comet, whose orbital period was somehow being drastically shortened at each perihelion passage, perhaps by retardation by some dense material surrounding the Sun.〔 An alternative suggestion was that the comets were all fragments of an earlier Sun-grazing comet.〔 This idea was first proposed in 1880, and its plausibility was amply demonstrated when the Great Comet of 1882 broke up into several fragments after its perihelion passage. In 1888, Heinrich Kreutz published a paper showing that the comets of 1843 (C/1843 D1, the Great March Comet), 1880 (C/1880 C1, the Great Southern Comet), and 1882 (C/1882 R1, Great September Comet) were probably fragments of a giant comet that had broken up several orbits before.〔 The comet of 1680 proved to be unrelated to this family of comets. After another Kreutz sungrazer was seen in 1887 (C/1887 B1, the Great Southern Comet of 1887), the next one did not appear until 1945.〔 Two further sungrazers appeared in the 1960s, Comet Pereyra in 1963 and Comet Ikeya–Seki, which became extremely bright in 1965, and broke into three pieces after its perihelion.〔 The appearance of two Kreutz Sungrazers in quick succession inspired further study of the dynamics of the group. The group generally has an Inclination of roughly 140 degrees, a perihelion distance of around 0.01 AU, and a Longitude of ascending node of 340–10°. 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「Kreutz sungrazer」の詳細全文を読む スポンサード リンク
|