|
Messier 87 (also known as Virgo A or NGC 4486, and generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local universe, it is notable for its large population of globular clusters—M87 contains about 12,000 compared to the 150-200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends outward at least 1,500 parsecs (4,900 light-years), travelling at relativistic speed. It is one of the brightest radio sources in the sky, and is a popular target for both amateur astronomy observations and professional astronomy study. French astronomer Charles Messier discovered M87 in 1781, cataloguing it as a nebulous feature while searching for objects that would confuse comet hunters. The second brightest galaxy within the northern Virgo Cluster, M87 is located about 16.4 million parsecs (53.5 million light-years) from Earth. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the centre. Forming around one sixth of M87's mass, the stars in this galaxy have a nearly spherically symmetric distribution, their density decreasing with increasing distance from the core. At the core is a supermassive black hole, which forms the primary component of an active galactic nucleus. This object is a strong source of multiwavelength radiation, particularly radio waves. M87's galactic envelope extends out to a radius of about 150 kiloparsecs (490,000 light-years), where it has been truncated—possibly by an encounter with another galaxy. Between the stars is a diffuse interstellar medium of gas that has been chemically enriched by elements emitted from evolved stars. ==Observation history== In 1781, French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance. This list was intended to identify objects that might be confused with comets. In subsequent use, each item in the catalogue was prefixed with an 'M'. Thus, M87 was the eighty seventh member of Messier's catalogue.〔 During the 1880s, this nebula was included in the ''New General Catalogue'' as NGC 4486. This compilation of nebulae and star clusters was assembled by the Danish-Irish astronomer John Dreyer based primarily on the observations of English astronomer John Herschel.〔 In 1918, American astronomer Heber Curtis of Lick Observatory observed that there was no spiral structure in M87 and he noticed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest at the inner end.〔 The following year, a supernova within M87 reached a peak photographic magnitude of 21.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.〔〔 American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure but appeared to belong to the same family of non-galactic nebulae as spiral nebulae.〔 In 1926 he produced a new categorization of nebulae, with M87 being classified as a type of elliptical extra-galactic nebula with no apparent elongation (class E0).〔 By 1931, Hubble had identified M87 as a member of the Virgo cluster, for which he gave a provisional estimate of 1.8 million parsecs from Earth. At the time it was the only known example of an elliptical nebula for which individual stars could be resolved.〔 M87 continued to be called an extragalactic nebula for many years thereafter, but by 1956 it had been identified as an E0-type galaxy.〔 In 1947, a prominent radio source was identified overlapping the location of M87, and this was labeled Virgo-A.〔 This source was confirmed to be M87 by 1953, and the linear relativistic jet emerging from the core of the galaxy was suggested as the cause. This jet extended from the core at a position angle of 260° to an angular distance of 20″ with an angular width of 2″.〔 In 1969-70, a strong component of the radio emission was found to closely align with the optical source of the jet.〔 In April 1965, the US Naval Research Laboratory group launched an Aerobee 150 equipped with a pair of geiger counters.〔 This flight discovered seven candidate X-ray sources, including the first extragalactic X-ray source; Virgo X-1 was designated as the first X-ray source detected in Virgo.〔 A later Aerobee rocket launched from White Sands Missile Range on July 7, 1967, yielded further evidence that the source Virgo X-1 was the radio galaxy M87.〔 Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87.〔 However, there is little central concentration of the X-ray emission.〔 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「Messier 87」の詳細全文を読む スポンサード リンク
|