Messier 87, or M87, is a giant elliptical galaxy located about 53.5 million light-years from Earth. It is known for the supermassive black hole in its core and its exceptionally bright radio emissions. M87 is also known as Virgo A and NGC 4486.
M87 was first documented by French astronomer Charles Messier in 1781, as he prepared the so-called Messier catalogue. Messier, however, did not realize that he was observing a distant galaxy: he was simply trying to identify all of the fixed objects that might be mistaken for comets by his colleagues. The "Messier objects" are thus a mixture of nebulae, star clusters, and actual galaxies, like M87. The actual nature of M87 was not known until the 20th century, when the modern theory of galaxies was developed.
Astronomers now realize that Messier 87, which appears to be located within the constellation Virgo when viewed from Earth, is one of the Milky Way's largest galactic neighbours, with an estimated size of about 2.4 trillion solar masses (that is, Sun-equivalent stars). It is one of the dominant features of the Virgo cluster of galaxies, and has a dense elliptical shape, rather than the multi-armed spiral shape characteristic of galaxies like the Milky Way. According to recent research, M87 also lacks the usual halo of outer stars, suggesting that some as-yet-unknown event - perhaps a close encounter with another galaxy, perhaps some other unknown phenomenon - has stripped the galaxy of a large number of its stars.
There are three main characteristics of the M87 galaxy which mark it as special interest to astronomers. First, like other galaxies, M87 is believed to have a supermassive black hole in its core. However, M87's black hole appears to be unusually large, absorbing a mass equivalent to that of Sol in just a decade. In 2011, new research suggested that the black hole was twice as big as previously speculated, perhaps with a radius equivalent to the orbit of Neptune in Earth's own solar system (just under three billion miles).
Secondly, the black hole has created an enormous jet of ejected matter, thousands of light-years long. The jet was first observed at the Lick Observatory in 1918, and consists of gases or subatomic particles travelling at an exceptionally high velocity. The jet is an effect of the black hole, and there may also be a second jet, less visible or invisible from Earth, travelling in the opposite direction. Currently, according to Scientific American, astronomers are using the jet to deduce characteristics of the black hole, since the black hole itself absorbs rather than emits light, and therefore cannot be seen directly.
Third, since the late 1940s, astronomers have also tracked unusually bright radio-spectrum emissions coming from M87, leading to its designation as "Virgo A" - that is, the brightest radio emitter in Virgo. The emissions are believed to be another effect of the unusually large supermassive black hole in its core. According to F. Owen of the National Radio Astronomy Observatory, M87 is "one of the strongest radio sources in the sky," and is therefore of interest both to amateur astronomers and professional researchers.