Messier 87 is a giant elliptical galaxy, the largest and brightest member of the Virgo-Coma Galaxy cluster. M 87 is famous for two peculiar and perhaps unique features: a huge globular cluster system, and a spectacular jet of turbulent gas extending several thousand light years from its core. This galaxy is also the strong radio source known as Virgo A or 3C 274. Since it is the largest nearby elliptical galaxies, and one of the brightest radio sources in the sky, M 87 is a popular target for both amateur astronomers and professional studies.
Charles Messier discovered this object in 1781, along with eight other nebulous objects. Seven of those objects are also members of the Virgo Cluster; the eighth is the globular cluster M 92.
Amateur Observation
Visually, Messier 87 has a diameter of 7' and an apparent magnitude of 8.6. M 87 is nearly spherical (type E0 or E1 in Hubble's classification); it has a very bright, circular halo containing a bright nucleus. Very deep photographic plates taken by David Malin of the Anglo-Australian Observatory showed that M 87 extends into a vast elongated shape over half a degree wide - more than the diameter of the Full Moon!
M 87's giant jet was discovered by Heber D. Curtis of the Lick Observatory in 1918, which he described as "a curious straight ray." Extending toward the northeast from the galaxy's bright nucleus, the jet is better seen on short-exposure photographs. In 1966, Halton Arp discovered a second jet, significantly less conspicuous, pointing in the opposite direction. Because of its peculiar jets, Arp included M 87 as No. 152 in his Catalogue of Peculiar Galaxies.
The only supernova recorded in M 87 appeared in February 1919, but was not detected until 1922 on photographic plates. Its maximum brightness of magnitude 11.5 corresponds to an absolute magnitude of nearly -20 at M 87's distance.
This giant galaxy has a number of apparent neighbors. Some of them are probably physical satellites, while others may be apparent neighbors because of projection effects only. The brightest of these neighboring galaxies are NGC 4476, NGC 4478, NGC 4486A, and NGC 4486B.
Properties and Globulars
M 87 lies very near the dynamical center of the Coma-Virgo Supercluster, the point around which other supercluster members (including our own Local Galaxy Group) are orbiting. At the cluster's distance - about 60 million light-years - M 87's apparent diameter corresponds to 120,000 light years, larger than the disk of the Milky Way. The vast elongated shape revealed by David Malin's study extends more than half a million light years. Its outer layers are noticeably distorted, probably because of gravitational interactions with other Virgo Cluster galaxies and because of material acquired during cannibalistic encounters.
As M 87 is nearly spherical, it fills a far greater volume than does our own galaxy, and thus contains many more stars. M 87's has been estimated to contain 2 to 3 trillion solar masses, and is also of extreme luminosity, with an absolute magnitude of about -22.
This magnificent galaxy is perhaps the one with most known globular clusters. While our Milky Way is surrounded by 150 to 200 globulars, M87 possesses more than 12,000, as estimated from a 2006 survey out to 25' from its core.
Jet and Evolution
M 87's giant jet extends at least 5000 light-years from the nucleus, and consists of a string of knots and clouds of gas ejected from the core in violent turbulence. The jet appears blue in short-exposure color photos; its light is strongly polarized and exhibits a continuous spectrum, typical of synchrotron radiation. In 1999 observations by the Hubble Space Telescope, the motion of M 87's jet was measured at four to six times the speed of light. But this apparent superluminal motion is probably an illusion caused by the fact that the jet is pointing towards us. However, the detection of such motion supports the theory that quasars and active radio galaxies may all be the same phenomenon viewed from different perspectives.
In 1984, Walter Baade and Rudolph Minkowski identified M 87 with the strong radio source Virgo A. M 87 is also a powerful source of X-rays, and sits near the center of a hot, X-ray emitting cloud that extends over vast reaches of the Virgo Cluster. M 87 is also a very strong source of gamma rays, observed since the 1990s.
Hubble Space Telescope images of M 87's violent, active nucleus have revealed a central object of about 2 to 3 billion solar masses. Concentrated within a spherical region just 60 light-years across, and surrounded by a rapidly rotating gaseous disk, a supermassive black hole almost certainly lies at the very heart of this object. About the size of our solar system, this black hole is probably responsible for the jets of matter ejected from the galaxy.
The Chandra X-ray Observatory revealed loops and rings in the hot X-ray-emitting gas surrounding M 87. These loops and rings are caused by variations in the rate at which material is ejected from the supermassive black hole. Their distribution suggests that minor eruptions occur every six million years. The regular eruptions prevent gas from cooling and forming stars, seriously affecting M 87's evolution, and preventing it from becoming a large spiral galaxy.