M87
Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe,[a] it is notable for its large population of globular clusters—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 at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.
The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. 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 center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.
The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.
In 1781, the French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance as part of a list intended to identify objects that might otherwise be confused with comets. In subsequent use, each catalogue entry was prefixed with an "M". Thus, M87 was the eighty-seventh object listed in Messier's catalogue. During the 1880s, the object was included as NGC 4486, in the New General Catalogue of nebulae and star clusters assembled by the Danish-Irish astronomer John Dreyer, which he based primarily on the observations of the English astronomer John Herschel.
In 1918, the American astronomer Heber Curtis of Lick Observatory noted M87's lack of a spiral structure and observed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest near the galactic center. 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 [fr; ru] in 1922.
In 1922, the American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but like spiral nebulae, appeared to belong to the family of non-galactic nebulae. In 1926 he produced a new categorization, distinguishing extragalactic from galactic nebulae, the former being independent star systems. M87 was classified as a type of elliptical extragalactic nebula with no apparent elongation (class E0).
In 1931, Hubble described M87 as a member of the Virgo Cluster, and gave a provisional estimate of 1.8 million parsecs (5.9 million light-years) from Earth. It was then the only known elliptical nebula for which individual stars could be resolved, although it was pointed out that globular clusters would be indistinguishable from individual stars at such distances. In his 1936 The Realm of the Nebulae, Hubble examines the terminology of the day; some astronomers labeled extragalactic nebulae as external galaxies on the basis that they were stellar systems at far distances from our own galaxy, while others preferred the conventional term extragalactic nebulae, as galaxy then was synonym for the Milky Way. M87 continued to be labelled as an extragalactic nebula at least until 1954.
