Great Observatories program

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Great_Observatories_program

Four Great Observatories

NASA's series of Great Observatories satellites are four large, powerful space-based astronomical telescopes launched between 1990 and 2003. They were built with different technology to examine specific wavelength/energy regions of the electromagnetic spectrum: gamma rays, X-rays, visible and ultraviolet light, and infrared light.

The Hubble Space Telescope (HST) primarily observes visible light and near-ultraviolet. It was launched in 1990 aboard the Space Shuttle Discovery during STS-31, but its main mirror had been ground incorrectly, resulting in spherical aberration that compromised the telescope's capabilities. The optics were corrected to their intended quality by the STS-61 servicing mission in 1993. In 1997, the STS-82 servicing mission added capability in the near-infrared range, and in 2009 the STS-125 servicing mission refurbished the telescope and extended its projected service life. It remains in active operation as of July 2024.

The Compton Gamma Ray Observatory (CGRO) primarily observed gamma rays, though it extended into hard x-rays as well. It was launched in 1991 aboard Atlantis during STS-37. It was de-orbited in 2000 after a gyroscope failed.

The Chandra X-ray Observatory (CXO) primarily observes soft X-rays. It was launched in 1999 aboard Columbia during STS-93 into an elliptical high-Earth orbit, and was initially named the Advanced X-ray Astronomical Facility (AXAF). It remains in active operation as of July 2024.

The Spitzer Space Telescope (SST) observed the infrared spectrum. It was launched in 2003 aboard a Delta II rocket into an Earth-trailing solar orbit. Depletion of its liquid helium coolant in 2009 reduced its functionality, leaving it with only two short-wavelength imaging modules. It was removed from service and placed into safe-mode on January 30, 2020.

Origins of the Great Observatory program

The concept of a Great Observatory program was first proposed in the 1979 NRC report "A Strategy for Space Astronomy and Astrophysics for the 1980s". This report laid the essential groundwork for the Great Observatories and was chaired by Peter Meyer (through June 1977) and then by Harlan J. Smith (through publication). In the mid-1980s, it was further advanced by all of the astrophysics Division Directors at NASA headquarters, including Frank Martin and Charlie Pellerin. NASA's "Great Observatories" program used four separate satellites, each designed to cover a different part of the spectrum in ways which terrestrial systems could not. This perspective enabled the proposed X-ray and InfraRed observatories to be appropriately seen as a continuation of the astronomical program begun with Hubble and CGRO rather than competitors or replacements. Two explanatory documents published by NASA and created for the NASA Astrophysics Division and the NASA Astrophysics Management Working Group laid out the rationale for the suite of observatories and questions that could be addressed across the spectrum. They had an important role in the campaign to win and sustain approval for the four telescopes.

Great Observatories

Hubble Space Telescope

Hubble Space Telescope

Main article: Hubble Space Telescope

The history of the Hubble Space Telescope can be traced back to 1946, when the astronomer Lyman Spitzer wrote the paper Astronomical advantages of an extraterrestrial observatory. Spitzer devoted much of his career to pushing for a space telescope.

The 1966–1972 Orbiting Astronomical Observatory missions demonstrated the important role space-based observations could play in astronomy. In 1968, NASA developed firm plans for a space-based reflecting telescope with a 3-meter mirror, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979. Congress eventually approved funding of US$36 million for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. During the early 1980s, the telescope was named after Edwin Hubble.

Hubble was originally intended to be retrieved and returned to Earth by the Space Shuttle, but the retrieval plan was later abandoned. On 31 October 2006, NASA Administrator Michael D. Griffin gave the go-ahead for a final refurbishment mission. The 11-day STS-125 mission by Space Shuttle Atlantis, launched on 11 May 2009, installed fresh batteries, replaced all gyroscopes, replaced a command computer, fixed several instruments, and installed the Wide Field Camera 3 and the Cosmic Origins Spectrograph.

Compton Gamma Ray Observatory

Compton Gamma Ray Observatory

Main article: Compton Gamma Ray Observatory

Gamma rays had been examined above the atmosphere by several early space missions. During its High Energy Astronomy Observatory Program in 1977, NASA announced plans to build a "great observatory" for gamma-ray astronomy. The Gamma Ray Observatory (GRO), renamed Compton Gamma-Ray Observatory (CGRO), was designed to take advantage of the major advances in detector technology during the 1980s. Following 14 years of effort, the CGRO was launched on 5 April 1991. One of the three gyroscopes on the Compton Gamma Ray Observatory failed in December 1999. Although the observatory was fully functional with two gyroscopes, NASA judged that failure of a second gyroscope would result in inability to control the satellite during its eventual return to Earth due to orbital decay. NASA chose instead to preemptively de-orbit Compton on 4 June 2000. Parts that survived reentry splashed into the Pacific Ocean.

Chandra X-ray Observatory

Chandra X-ray Observatory

Main article: Chandra X-ray Observatory

In 1976 the Chandra X-ray Observatory (called AXAF at the time) was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at Marshall Space Flight Center (MSFC) and the Smithsonian Astrophysical Observatory (SAO). In the meantime, in 1978, NASA launched the first imaging X-ray telescope, Einstein Observatory (HEAO-2), into orbit. Work continued on the Chandra project through the 1980s and 1990s. In 1992, to reduce costs, the spacecraft was redesigned. Four of the twelve planned mirrors were eliminated, as were two of the six scientific instruments. Chandra's planned orbit was changed to an elliptical one, reaching one third of the way to the Moon's at its farthest point. This eliminated the possibility of improvement or repair by the Space Shuttle but put the observatory above the Earth's radiation belts for most of its orbit.

Spitzer Space Telescope

Spitzer points its high-gain antenna towards the Earth.

Main article: Spitzer Space Telescope

By the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects of atmosphere of Earth. Most of the early concepts, envisioned repeated flights aboard the NASA Space Shuttle. This approach was developed in an era when the Shuttle program was presumed to be capable of supporting weekly flights of up to 30 days duration. In 1979, a National Research Council of the National Academy of Sciences report, A Strategy for Space Astronomy and Astrophysics for the 1980s, identified a Shuttle Infrared Telescope Facility (SIRTF) as "one of two major astrophysics facilities [to be developed] for Spacelab," a Shuttle-borne platform.

The launch of the Infrared Astronomical Satellite, an Explorer-class satellite designed to conduct the first infrared survey of the sky led to anticipation of an instrument using new infrared detector technology. By September 1983, NASA was considering the "possibility of a long duration [free-flyer] SIRTF mission". The 1985 Spacelab-2 flight aboard STS-51-F confirmed the Shuttle environment was not well suited to an onboard infrared telescope, and a free-flying design was better. The first word of the name was changed from Shuttle so it would be called the Space Infrared Telescope Facility.

Spitzer was the only one of the Great Observatories not launched by the Space Shuttle. It was originally intended to be so launched, but after the Challenger disaster, the Centaur LH2/LOX upper stage that would have been required to push it into a heliocentric orbit was banned from Shuttle use. Titan and Atlas launch vehicles were canceled for cost reasons. After redesign and lightening, it was launched in 2003 by a Delta II launch vehicle instead. It was called the Space Infrared Telescope Facility (SIRTF) before launch. The telescope was deactivated when operations ended on 30 January 2020.

Timeline

Timeline of NASA Great Observatories Program

Strengths

Chandra, Hubble, and Spitzer composite image of the Crab Nebula (2009)

Since the Earth's atmosphere prevents X-rays, gamma-rays and far-infrared radiation from reaching the ground, space missions were essential for the Compton, Chandra and Spitzer observatories. Hubble also benefits from being above the atmosphere, as the atmosphere blurs ground-based observations of very faint objects, decreasing spatial resolution (however brighter objects can be imaged in much higher resolution than by Hubble from the ground using astronomical interferometers or adaptive optics). Larger, ground-based telescopes have only recently matched Hubble in resolution for near-infrared wavelengths of faint objects. Being above the atmosphere eliminates the problem of airglow, allowing Hubble to make observations of ultrafaint objects. Ground-based telescopes cannot compensate for airglow on ultrafaint objects, and so very faint objects require unwieldy and inefficient exposure times. Hubble can also observe at ultraviolet wavelengths which do not penetrate the atmosphere.

Each observatory was designed to push the state of technology in its region of the electromagnetic spectrum. Compton was much larger than any gamma-ray instruments flown on the previous HEAO missions, opening entirely new areas of observation. It had four instruments covering the 20 keV to 30 GeV energy range, which complemented each other's sensitivities, resolutions, and fields of view. Gamma rays are emitted by various high-energy and high-temperature sources, such as black holes, pulsars, and supernovae.

Chandra similarly had no ground predecessors. It followed the three NASA HEAO Program satellites, notably the highly successful Einstein Observatory, which was the first to demonstrate the power of grazing-incidence, focusing X-ray optics, giving spatial resolution an order of magnitude better than collimated instruments (comparable to optical telescopes), with an enormous improvement in sensitivity. Chandra's large size, high orbit, and sensitive CCDs allowed observations of very faint X-ray sources.

Spitzer also observes at wavelength largely inaccessible to ground telescopes. It was preceded in space by NASA's smaller IRAS mission and European Space Agency (ESA)'s large ISO telescope. Spitzer's instruments took advantage of the rapid advances in infrared detector technology since IRAS, combined with its large aperture, favorable fields of view, and long life. Science returns were accordingly outstanding. Infrared observations are necessary for very distant astronomical objects where all the visible light is redshifted to infrared wavelengths, for cool objects which emit little visible light, and for regions optically obscured by dust.

Synergies

A labeled space image comparing views of a supernova remnant by three different Great observatories.

Aside from inherent mission capabilities (particularly sensitivities, which cannot be replicated by ground observatories), the Great Observatories program allows missions to interact for greater science return. Different objects shine in different wavelengths, but training two or more observatories on an object allows a deeper understanding.

High-energy studies (in X-rays and gamma rays) have had only moderate imaging resolutions so far. Studying X-ray and gamma-ray objects with Hubble, as well as Chandra and Compton, gives accurate size and positional data. In particular, Hubble's resolution can often discern whether the target is a standalone object, or part of a parent galaxy, and if a bright object is in the nucleus, arms, or halo of a spiral galaxy. Similarly, the smaller aperture of Spitzer means that Hubble can add finer spatial information to a Spitzer image. Reported in March 2016, Spitzer and Hubble were used to discover the most distant-known galaxy, GN-z11. This object was seen as it appeared 13.4 billion years ago.

Ultraviolet studies with Hubble also reveal the temporal states of high-energy objects. X-rays and gamma rays are harder to detect with current technologies than visible and ultraviolet. Therefore, Chandra and Compton needed long integration times to gather enough photons. However, objects which shine in X-rays and gamma rays can be small, and can vary on timescales of minutes or seconds. Such objects then call for followup with Hubble or the Rossi X-ray Timing Explorer, which can measure details in angular seconds or fractions of a second, due to different designs. Rossi's last full year of operation was 2011.

The ability of Spitzer to see through dust and thick gases is good for galactic nuclei observations. Massive objects at the hearts of galaxies shine in X-rays, gamma rays, and radio waves, but infrared studies into these clouded regions can reveal the number and positions of objects.

Hubble, meanwhile, has neither the field of view nor the available time to study all interesting objects. Worthwhile targets are often found with ground telescopes, which are cheaper, or with smaller space observatories, which are sometimes expressly designed to cover large areas of the sky. Also, the other three Great Observatories have found interesting new objects, which merit diversion of Hubble.

One example of observatory synergy is Solar System and asteroid studies. Small bodies, such as small moons and asteroids, are too small and/or distant to be directly resolved even by Hubble; their image appears as a diffraction pattern determined by brightness, not size. However, the minimum size can be deduced by Hubble through knowledge of the body's albedo. The maximum size can be determined by Spitzer through knowledge of the body's temperature, which is largely known from its orbit. Thus, the body's true size is bracketed. Further spectroscopy by Spitzer can determine the chemical composition of the object's surface, which limits its possible albedos, and therefore sharpens the low size estimate.

At the opposite end of the cosmic distance ladder, observations made with Hubble, Spitzer and Chandra have been combined in the Great Observatories Origins Deep Survey to yield a multi-wavelength picture of galaxy formation and evolution in the early Universe.

Milky Way Galactic Center as seen by the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory

Impact

All four telescopes have had a substantial impact on astronomy. The opening up of new wavebands to high resolution, high sensitivity observations by the Compton, Chandra and Spitzer has revolutionized our understanding of a wide range of astronomical objects, and has led to the detection of thousands of new, interesting objects. Hubble has had a much larger public and media impact than the other telescopes, although at optical wavelengths Hubble has provided a more modest improvement in sensitivity and resolution over existing instruments. Hubble's capability for uniform high-quality imaging of any astronomical object at any time has allowed accurate surveys and comparisons of large numbers of astronomical objects. The Hubble Deep Field observations have been very important for studies of distant galaxies, as they provide rest-frame ultraviolet images of these objects with a similar number of pixels across the galaxies as previous ultraviolet images of closer galaxies, allowing direct comparison.

Successors to Great Observatories

Primary mirror size comparison of Spitzer, Hubble, and Webb telescopes

• The James Webb Space Telescope (JWST) launched in December 2021 and works simultaneously with Hubble. Its segmented, deployable mirror is over twice as wide as the Hubble's, increasing angular resolution noticeably, and sensitivity dramatically. Unlike Hubble, JWST observes in the infrared, in order to penetrate dust at cosmological distances. This means it continues some Spitzer capabilities, while some Hubble capabilities are lost in the visible and especially the ultraviolet wavelengths. JWST exceeds Spitzer's performance in near-infrared. The European Space Agency's Herschel Space Observatory, operational from 2009 to 2013, has exceeded Spitzer in the far-infrared. The SOFIA (Stratospheric Observatory for Infrared Astronomy) airborne platform observed in near- and mid-infrared. SOFIA had a larger aperture than Spitzer, but lower relative sensitivity.
• The Fermi Gamma-ray Space Telescope (FGRST), formerly known as the Gamma Ray Large Area Space Telescope, is a follow-on to Compton launched on 11 June 2008. FGRST is more narrowly defined, and much smaller; it carries only one main instrument and a secondary experiment, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). FGRST is complemented by Swift, launched in 2004, and previously by HETE-2, launched in 2000.
• The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in 2002, observes in some Compton and Chandra wavelengths, but is pointed at the Sun at all times. Occasionally it observes high-energy objects which happen to be in the view around the Sun.
• Another large, high-energy observatory is INTEGRAL, Europe's INTErnational Gamma Ray Astrophysics Laboratory, launched in 2002. It observes in similar frequencies to Compton. INTEGRAL uses a fundamentally different telescope technology, coded-aperture masks. Thus, its capabilities are complementary to Compton and Fermi.

Later programs

• The Beyond Einstein program will seek to develop new areas of science. Constellation-X and the Laser Interferometer Space Antenna (LISA) have been referred to by NASA as the Einstein Great Observatories, to differentiate them from the current generation. However, they are not a part of the Great Observatories program.

• The International Solar-Terrestrial Physics Science Initiative (ISTP), in the spirit of the Great Observatories program, is a group of instruments to study the Sun and related electromagnetic phenomena near Earth.

Next Great Observatory

In 2016, NASA began considering four different Flagship space telescopes, they are the Habitable Exoplanet Imaging Mission (HabEx), Large UV Optical Infrared Surveyor (LUVOIR), Origins Space Telescope (OST), and Lynx X-ray Observatory. In 2019, the four teams will turn their final reports over to the National Academy of Sciences, whose independent Decadal Survey committee advises NASA on which mission should take top priority.

NASA announced the Habitable Worlds Observatory (HWO) in 2023, a successor building on the Large UV Optical Infrared Surveyor (LUVOIR) and Habitable Exoplanet Imaging Mission (HabEX) proposals. The administration also created the Great Observatory Maturation Program for the development of the Habitable Worlds Observatory

Revolutionary terror

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Revolutionary_terror Revolutionary terror, also referred to as revolutionary terrorism or reign of terror, refers to the institutionalized application of force to counter-revolutionaries, particularly during the French Revolution from the years 1793 to 1795 (see the Reign of Terror). The term "Communist terrorism" has also been used to describe the revolutionary terror, from the Red Terror in Russia and Cultural Revolution in China to the reign of the Khmer Rouge and others. In contrast, "reactionary terror", often called White Terrors, has been used to subdue revolutions.

Origins, evolution and history

The Drownings at Nantes, anonymous period painting

German Social Democrat Karl Kautsky traces the origins of revolutionary terror to the Reign of Terror of the French Revolution. Vladimir Lenin considered the Jacobin use of terror as a needed virtue and accepted the label Jacobin for his Bolsheviks. However, this distinguished him from Karl Marx.

The deterministic view of history was used by Communist regimes to justify the use of terror. Terrorism came to be used by communists, both the state and dissident groups, in both revolution and in consolidation of power. The doctrines of anarchism, Marxism, Marxism–Leninism and Maoism have all spurred dissidents who have taken to terrorism. Communist leaders used the idea that terror could serve as the force which Marx said was the "midwife of revolution" and after World War I communist groups continued to use it in attempts to overthrow governments. For Mao Zedong, terrorism was an acceptable tool.

After World War II, Marxist–Leninist groups seeking independence, like nationalists, concentrated on guerrilla warfare along with terrorism. By the late 1950s and early 1960s, there was a change from wars of national liberation to contemporary terrorism. For decades, terrorist groups tended to be closely linked to communist ideology, being the predominant category of terrorists in the 1970s and 1980s, but today they are in the minority. Their decline is attributed to the end of the Cold War and the fall of the Soviet Union.

French historian Sophie Wahnich distinguishes between the revolutionary terror of the French Revolution and modern day Islamic terrorism and the September 11 attacks:

Revolutionary terror is not terrorism. To make a moral equivalence between the Revolution's year II and September 2001 is historical and philosophical nonsense[.] [...] The violence exercised on 11 September 2001 aimed neither at equality nor liberty. Nor did the preventive war announced by the president of the United States.

Revolutionary violence in Marxism

In his article "The Victory of the Counter-Revolution in Vienna" in the Neue Rheinische Zeitung (No. 136, 7 November 1848), Karl Marx wrote:

The purposeless massacres perpetrated since the June and October events, the tedious offering of sacrifices since February and March, the very cannibalism of the counterrevolution will convince the nations that there is only one way in which the murderous death agonies of the old society and the bloody birth throes of the new society can be shortened, simplified and concentrated, and that way is revolutionary terror.

In his biography of Joseph Stalin, Edvard Radzinsky, a Russian author of popular history books, noted that Stalin wrote a nota bene—"Terror is the quickest way to new society"—beside the above passage in a book by Karl Kautsky.

Vladimir Lenin, Leon Trotsky and other leading Bolshevik ideologists viewed mass terror as a necessary weapon during the dictatorship of proletariat and the resulting class struggle. In his The Proletarian Revolution and the Renegade K. Kautsky (1918), Lenin wrote: "One cannot hide the fact that dictatorship presupposes and implies a "condition", one so disagreeable to renegades [such as Kautsky], of revolutionary violence of one class against another ... the "fundamental feature" of the concept of dictatorship of the proletariat is revolutionary violence".

The Bolsheviks engaged in a form of social determinism that was hostile to bourgeoisie and wealthier classes. Martin Latsis, one of the Soviet leaders directing the Cheka, stated his intentions for those classes who were considered reactionary and incapable of being reeducated. Latsis wrote:

We are engaged in exterminating the bourgeoisie as a class. You need not prove that this or that man acted against the interests of the Soviet power. The first thing you have to ask an arrested person is: To what class does he belong, where does he come from, what kind of education did he have, what is his occupation? These questions are to decide the fate of the accused. That is the quintessence of the Red Terror.

On the other hand, they opposed individual terror, which has been used earlier by the People's Will organization. According to Trotsky: "The damaging of machines by workers, for example, is terrorism in this strict sense of the word. The killing of an employer, a threat to set fire to a factory or a death threat to its owner, an assassination attempt, with revolver in hand, against a government minister—all these are terrorist acts in the full and authentic sense. However, anyone who has an idea of the true nature of international Social Democracy ought to know that it has always opposed this kind of terrorism and does so in the most irreconcilable way".

France

Main article: Reign of Terror

The French Revolution began in 1789, but by 1793 the new government began to search for new means to defend itself. The Sans-Culottes had demanded government action against enemies and the remains of the Old Regime, spanning from the General Maximum (which guaranteed the price of staple commodities) to the execution of several dozen prisoners. The murder of the radical republican writer Jean-Paul Marat in July of 1793 in his own bath intensified the situation.The Jacobin Government adopted policies of Terror in the most dire days of the civil and foreign wars against the Revolution: September, 1793. French Historian Albert Soboul writes: "On 5 September the Terror was made official policy." For the rest of that September more laws were made that targeted counterrevolutionaries, and granted and enforced the demands of the Sans-Culottes. The next year was dominated by the hunt for and execution or imprisonment of enemies the Revolution, the Jacobins, and France. Maximilien Robespierre, the leader of the Jacobins, justified the violence by saying: “Subdue by terror the enemies of liberty, and you will be right, as founders of the Republic. The government of the revolution is liberty's despotism against tyranny. Is force made only to protect crime? And is the thunderbolt not destined to strike the heads of the proud?”

Despite the efforts to subdue the enemies of the Revolution, the situation continued to deteriorate until the Law of 22 Prairial, Year II (June 10, 1794) was enacted intensifying state-violence at home and beginning what is refereed to by historians as the "Great Terror." 1,376 people were killed by July 26, 1794. However, the French victory at the Battle of Fleurus (June 1794) had all but secured the Revolution's safety from imminent foreign invasion and gave the Revolutionaries room to breath and reassess the domestic situation. This led to the conservative backlash in late July 1794 (called Thermidor for the month it was according to the Revolutionary Calendar) and the fall of the Jacobin Government with the execution of Robespierre (July 28, 1794).

Soviet Union

Execution of the Romanov family in 1918, Le Petit Journal

Red Terror

Main article: Red Terror

Lenin, Trotsky and other leading Bolshevik ideologists promulgated mass terror as a necessary weapon during the dictatorship of proletariat and the resulting class struggle. Similarly, in his book Terrorism and Communism (1920), Trotsky emphasized that "the historical tenacity of the bourgeoisie is colossal [...] We are forced to tear off this class and chop it away. The Red Terror is a weapon used against a class that, despite being doomed to destruction, does not want to perish". Trotsky also argued that the reign of terror began with the White Terror under the White Guard forces and the Bolsheviks responded with the Red Terror.

Many later Marxists, in particular Karl Kautsky, criticized Bolshevik leaders for terrorism tactics. He stated that "among the phenomena for which Bolshevism has been responsible, Terrorism, which begins with the abolition of every form of freedom of the Press, and ends in a system of wholesale execution, is certainly the most striking and the most repellent of all". Kautsky argued that that Red Terror represented a variety of terrorism because it was indiscriminate, intended to frighten the civilian population and included taking and executing hostages.

The Red Terror (1917-1920) opposed the forces of the White Armies who wanted to reverse the Russian Revolution. It saw the encouraging of peasant seizure of land, the discovery of foreign agents, and the rooting out of old Czarist officials. Estimates of the death toll vary widely, but academic estimates range from 50,000-140,000.

Black Terror

Main article: Kontrrazvedka

The anarchist Kontrrazvedka, the intelligence section of the Revolutionary Insurgent Army of Ukraine, resorted to methods of terror as seen with the "Black Terror" campaign. Nestor Makhno, leader of the Makhnovist movement, listed 80 targets to be liquidated in Alexandrovsk, including Mensheviks, Narodniks and Right Socialist-Revolutionaries. The scale of the Black Terror was insignificant compared to the Red or White Terror with only 70 victims of the extra-judicial organs in Yekaterinoslav.

State terror in the Soviet Union

Main articles: Collectivization in the Soviet Union, Great Purge, and Population transfer in the Soviet Union

The Great Purge refers collectively to several related campaigns of political repression and persecution in the Soviet Union orchestrated by Joseph Stalin during the 1930s, which removed all of his remaining opposition from power. It involved the purge of the Communist Party of the Soviet Union and the persecution of unaffiliated persons, both occurring within a period characterized by omnipresent police surveillance, widespread suspicion of "saboteurs", imprisonment and killings. In the Western World, this was referred to as "the Great Terror".

China

Main articles: Campaign to Suppress Counterrevolutionaries, Land Reform Movement, and Cultural Revolution

 

During the Chinese Communist Revolution, the Chinese Communist Party (CCP) had encouraged and overseen the execution and imprisonment of landlords by their former tenants in the countryside. Upon the Kuomintang's retreat to Taiwan, Mao Zedong and the other leaders of the CCP oversaw a terror in line with their Marxist-Leninist principles. According to the official statistics from the People's Daily of the CCP Central Committee in 1954, at least 1.3 million people were imprisoned in the Campaign to Suppress Counterrevolutionaries in 1950–1953, and 712 thousand people were executed. The tactics of the Terror was also used by Red Guards during the Cultural Revolution of 1966-1976. Mao encouraged this by telling his followers to "Bombard the Headquarters" to remove bureaucrats from power.

Messier 87

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Messier_87 

 

Messier 87
The galactic core of Messier 87, with its blue plasma jet clearly visible (composite image of observations by the Hubble Space Telescope in visible and infrared light)
Observation data (J2000 epoch)
Constellation	Virgo
Right ascension	12h 30m 49.42338
Declination	+12° 23′ 28.0439″
Redshift	0.00428 ± 0.00002
Heliocentric radial velocity	1,284 ± 5 km/s
Distance	16.4 ± 0.5 Mpc (53.5 ± 1.6 Mly)
Apparent magnitude (V)	8.6
Characteristics
Type	E+0-1 pec, NLRG Sy
Size	40.55 kpc (132,000 ly) (25.0 mag/arcsec2 B-band isophote)
Apparent size (V)	7.2 × 6.8 arcmin
Other designations
Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274, 3U 1228+12.

Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at a 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 cataloged it as a nebula. M87 is 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 black hole was imaged using data collected in 2017 by the Event Horizon Telescope (EHT), with a final, processed image released on 10 April 2019. In March 2021, the EHT Collaboration presented, for the first time, a polarized-based image of the black hole which may help better reveal the forces giving rise to quasars.

The galaxy is a strong source of multi-wavelength radiation, particularly radio waves. It has an isophotal diameter of 40.55 kiloparsecs (132,000 light-years), with a diffuse galactic envelope that extends to a radius of about 150 kiloparsecs (490,000 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.

Observation history

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, supernova SN 1919A within M87 reached a peak photographic magnitude of 11.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.

Identification as a galaxy

In Hubble's galaxy classification scheme, M87 is an E0 galaxy.

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 was at that time a synonym for the Milky Way. M87 continued to be labelled as an extragalactic nebula at least until 1954.

Modern research

In 1947, a prominent radio source, Virgo A, was identified with errors in its measured position that overlapped the location of M87. The 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 1969–1970, a strong component of the radio emission was found to closely align with the optical source of the jet. In 1966, the United States Naval Research Laboratory's Aerobee 150 rocket identified Virgo X-1, the first X-ray source in Virgo. The Aerobee rocket launched from White Sands Missile Range on 7 July 1967 yielded further evidence that the source of 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.

M87 has been an important testing ground for techniques that measure the masses of central supermassive black holes in galaxies. In 1978, stellar-dynamical modeling of the mass distribution in M87 gave evidence for a central mass of five billion M_☉ solar masses. After the installation of the COSTAR corrective-optics module in the Hubble Space Telescope in 1993, the Hubble Faint Object Spectrograph (FOS) was used to measure the rotation velocity of the ionized gas disk at the center of M87, as an "early release observation" designed to test the scientific performance of the post-repair Hubble instruments. The FOS data indicated a central black hole mass of 2.4 billion M_☉, with 30% uncertainty. Globular clusters within M87 have been used to calibrate metallicity relations as well.

M87 was observed by the Event Horizon Telescope (EHT) during much of 2017. The event horizon of the black hole at the center was directly imaged by the EHT, then revealed in a press conference on the issue date stated, filtering out from this the first image of a black hole's shadow.

Visibility

Area in constellation Virgo around M87

M87 is near a high declination limit of the Virgo constellation, abutting Coma Berenices. It lies along the line between the stars Epsilon Virginis and Denebola (Beta Leonis). The galaxy can be observed using a small telescope with a 6 cm (2.4 in) aperture, extending across an angular area of 7.2 × 6.8 arcminutes at a surface brightness of 12.9, with a very bright, 45 arcsecond core. Viewing the jet is a challenge without the aid of photography. Before 1991, the Ukrainian-American astronomer Otto Struve was the only person known to have seen the jet visually, using the 254 cm (100 in) Hooker telescope. In more recent years it has been observed in larger amateur telescopes under excellent conditions.

Properties

In the modified Hubble sequence galaxy morphological classification scheme of the French astronomer Gérard de Vaucouleurs, M87 is categorized as an E0p galaxy. "E0" designates an elliptical galaxy that displays no flattening—that is, it appears spherical. A "p" suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity is the presence of the jet emerging from the core. In the Yerkes (Morgan) scheme, M87 is classified as a type-cD galaxy. A D galaxy has an elliptical-like nucleus surrounded by an extensive, dustless, diffuse envelope. A D type supergiant is called a cD galaxy.

The distance to M87 has been estimated using several independent techniques. These include measurement of the luminosity of planetary nebulae, comparison with nearby galaxies whose distance is estimated using standard candles such as cepheid variables, the linear size distribution of globular clusters, and the tip of the red-giant branch method using individually resolved red giant stars. These measurements are consistent with each other, and their weighted average yields a distance estimate of 16.4 ± 0.5 megaparsecs (53.5 ± 1.63 million light-years).

Enclosed mass

Radius kpc	Mass ×1012 M☉
32	2.4
44	3.0
47	5.7
50	6.0

Stellar velocity map of the central region of M87, showing the motion of stars relative to Earth:

  away

  

  

  

  

  towards

 

The image shows a slight rotation in the vertical plane (the lower right moving toward earth, the upper left moving away), showing that M87 is rotating slowly.

M87 is one of the most massive galaxies in the local Universe. Its diameter is estimated at 132,000 light-years, which is approximately 51% larger than that of the Milky Way. As an elliptical galaxy, the galaxy is a spheroid rather than a flattened disc, accounting for the substantially larger mass of M87. Within a radius of 32 kiloparsecs (100,000 light-years), the mass is (2.4±0.6)×10¹² times the mass of the Sun, which is double the mass of the Milky Way galaxy. As with other galaxies, only a fraction of this mass is in the form of stars: M87 has an estimated mass to luminosity ratio of 6.3 ± 0.8; that is, only about one part in six of the galaxy's mass is in the form of stars that radiate energy. This ratio varies from 5 to 30, approximately in proportion to r^1.7 in the region of 9–40 kiloparsecs (29,000–130,000 light-years) from the core. The total mass of M87 may be 200 times that of the Milky Way.

The galaxy experiences an infall of gas at the rate of two to three solar masses per year, most of which may be accreted onto the core region. The extended stellar envelope of this galaxy reaches a radius of about 150 kiloparsecs (490,000 light-years), compared with about 100 kiloparsecs (330,000 light-years) for the Milky Way. Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy. There is evidence of linear streams of stars to the northwest of the galaxy, which may have been created by tidal stripping of orbiting galaxies or by small satellite galaxies falling in toward M87. Moreover, a filament of hot, ionized gas in the northeastern outer part of the galaxy may be the remnant of a small, gas-rich galaxy that was disrupted by M87 and could be feeding its active nucleus. M87 is estimated to have at least 50 satellite galaxies, including NGC 4486B and NGC 4478.

The spectrum of the nuclear region of M87 shows the emission lines of various ions, including hydrogen (HI, HII), helium (HeI), oxygen (OI, OII, OIII), nitrogen (NI), magnesium (MgII), and sulfur (SII). The line intensities for weakly ionized atoms (such as neutral atomic oxygen, OI) are stronger than those of strongly ionized atoms (such as doubly ionized oxygen, OIII). A galactic nucleus with such spectral properties is termed a LINER, for "low-ionization nuclear emission-line region". The mechanism and source of weak-line-dominated ionization in LINERs and M87 are under debate. Possible causes include shock-induced excitation in the outer parts of the disk or photoionization in the inner region powered by the jet.

Elliptical galaxies such as M87 are believed to form as the result of one or more mergers of smaller galaxies. They generally contain relatively little cold interstellar gas (in comparison with spiral galaxies) and they are populated mostly by old stars, with little or no ongoing star formation. M87's elliptical shape is maintained by the random orbital motions of its constituent stars, in contrast to the more orderly rotational motions found in a spiral galaxy such as the Milky Way. Using the Very Large Telescope to study the motions of about 300 planetary nebulae, astronomers have determined that M87 absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in the addition of some younger, bluer stars to M87. The distinctive spectral properties of the planetary nebulae allowed astronomers to discover a chevron-like structure in M87's halo which was produced by the incomplete phase-space mixing of a disrupted galaxy.

Components

Supermassive black hole M87*

The Event Horizon Telescope image of the core of M87 using 1.3 mm microwaves. The central dark spot is the shadow of M87* and is larger than the black hole's event horizon.

 

A view of the M87* supermassive black hole released by the Event Horizon Telescope Collaboration with lines overlaid to mark the orientation of polarization of the magnetic field

 

A view of the jet and shadow of M87's black hole. Observations from the Global Millimetre VLBI Array (GMVA), the Atacama Large Millimeter/submillimeter Array (ALMA), and the Greenland Telescope.

The core of the galaxy contains a supermassive black hole (SMBH), designated M87*, whose mass is billions of times that of the Earth's Sun; estimates had ranged from (3.5±0.8)×10⁹ M_☉ to (6.6±0.4)×10⁹ M_☉, surpassed by 7.22+0.34
−0.40×10⁹ M_☉ in 2016. In April 2019, the Event Horizon Telescope collaboration released measurements of the black hole's mass as (6.5 ± 0.2_stat ± 0.7_sys) × 10⁹ M_☉. This is one of the highest known masses for such an object. A rotating disk of ionized gas surrounds the black hole, and is roughly perpendicular to the relativistic jet. The disk rotates at velocities of up to roughly 1,000 km/s (2,200,000 mph) and spans a maximum diameter of 25,000 AU (3.7 trillion km; 2.3 trillion mi). By comparison, Pluto averages 39 AU (5.8 billion km; 3.6 billion mi) from the Sun. Gas accretes onto the black hole at an estimated rate of one solar mass every ten years (about 90 Earth masses per day). The Schwarzschild radius of the black hole is 120 AU (18 billion kilometres; 11 billion miles). The diameter of the accretion disk, as seen from Earth, is 42 μas (microarcsecond), and the diameter of the black hole itself is 15 μas. By comparison, the diameter of the core of M87 is 45" (as, arcsecond), and the size of M87 is 7.2' x 6.8' (am, arcminute).

A 2010 paper suggested that the black hole may be displaced from the galactic center by about seven parsecs (23 light-years). This was claimed to be in the opposite direction of the known jet, indicating acceleration of the black hole by it. Another suggestion was that the offset occurred during the merger of two supermassive black holes. However, a 2011 study did not find any statistically significant displacement, and a 2018 study of high-resolution images of M87 concluded that the apparent spatial offset was caused by temporal variations in the jet's brightness rather than a physical displacement of the black hole from the galaxy's center.

This black hole is the first to be imaged. Data to produce the image were taken in April 2017, the image was produced during 2018 and was published on 10 April 2019. The image shows the shadow of the black hole, surrounded by an asymmetric emission ring with a diameter of 690 AU (103 billion km; 64 billion mi). The shadow radius is 2.6 times that of the black hole's Schwarzschild radius. The asymmetry in the brightness of the ring is due to relativistic beaming, whereby material moving towards the observer at relativistic velocities appears brighter. The visible material around the black hole rotates mostly clockwise with respect to the observer, which due to the direction of the axis of rotation causes the bottom part of the emission region to have a component of velocity toward the observer. The rotation parameter was estimated at ${\displaystyle a=0.9\pm 0.1}$, corresponding to a rotation speed ≈ 0.4 c.

Composite image showing how the M87 system looked, across the entire electromagnetic spectrum, during the Event Horizon Telescope's April 2017 campaign to take the first image of a black hole. Requiring 19 different facilities on the Earth and in space, this image reveals the enormous scales spanned by the black hole and its forward-pointing jet. It shows the image of the larger-scale jet taken by ALMA (upper left), on the same scale as the visible image by the Hubble Space Telescope (center) and the X-ray image by Chandra (upper right).

After the black hole had been imaged, it was named Pōwehi, a Hawaiian word meaning "the adorned fathomless dark creation", taken from the ancient creation chant Kumulipo.

On 24 March 2021, the Event Horizon Telescope collaboration revealed a unprecedented unique view of the M87 black hole shadow: how it looks in polarized light. Polarization is a powerful tool which allows astronomers to probe physics behind the image in more detail. Light polarization informs us about the strength and orientation of magnetic fields in the ring of light around the black hole shadow. Knowing those is essential to understand how M87's supermassive black hole is launching jets of magnetized plasma, which expand at relativistic speeds beyond the M87 galaxy.

Sharpening of the original EHT imaging of the M87 black hole, using the PRIMO technique for interferometric modeling. The rightmost image adds back in some fuzzing to account for the limited resolving power of the underlying observations.

On 14 April 2021, astronomers further reported that the M87 black hole and its surroundings were studied during Event Horizon Telescope 2017 observing run also by many multi-wavelength observatories from around the world.

In April 2023, a team developed a new principal-component interferometric modeling (PRIMO) technique to produce sharper image reconstructions from EHT data. They applied this to the original EHT observations of the M87 black hole, yielding a crisper final image and allowing closer testing of the alignment of observations to theory.

Jet

The jet of matter is ejected from M87 at nearly the speed of light, and stretches 1.5 kpc (5 kly) from the galactic core.

 

In this X-ray (Chandra) and radio (VLA) composite image, hot matter (blue in X-ray) from the Virgo cluster falls toward the core of M87 and cools, where it is met by the relativistic jet (orange in radio), producing shock waves in the galaxy's interstellar medium.

The relativistic jet of matter emerging from the core extends at least 1.5 kiloparsecs (5,000 light-years) from the nucleus and consists of matter ejected from a supermassive black hole. The jet is highly collimated, appearing constrained to an angle of 60° within 0.8 pc (2.6 light-years) of the core, to about 16° at two parsecs (6.5 light-years), and to 6–7° at twelve parsecs (39 light-years). Its base has the diameter of 5.5 ± 0.4 Schwarzschild radii, and is probably powered by a prograde accretion disk around the spinning supermassive black hole. The German-American astronomer Walter Baade found that light from the jet was plane polarized, which suggests that the energy is generated by the acceleration of electrons moving at relativistic velocities in a magnetic field. The total energy of these electrons is estimated at 5.1 × 10⁵⁶ ergs (5.1 × 10⁴⁹ joules or 3.2 × 10⁶⁸ eV). This is roughly 10¹³ times the energy produced in the entire Milky Way in one second, which is estimated at 5 × 10³⁶ joules. The jet is surrounded by a lower-velocity non-relativistic component. There is evidence of a counter jet, but it remains unseen from the Earth due to relativistic beaming. The jet is precessing, causing the outflow to form a helical pattern out to 1.6 parsecs (5.2 light-years). Lobes of expelled matter extend out to 80 kiloparsecs (260,000 light-years).

In pictures taken by the Hubble Space Telescope in 1999, the motion of M87's jet was measured at four to six times the speed of light. This phenomenon, called superluminal motion, is an illusion caused by the relativistic velocity of the jet. The time interval between any two light pulses emitted by the jet is, as registered by the observer, less than the actual interval due to the relativistic speed of the jet moving in the direction of the observer. This results in perceived faster-than-light speeds, though the jet itself has a velocity of only 80–85% the speed of light. Detection of such motion is used to support the theory that quasars, BL Lacertae objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives. It is proposed that the nucleus of M87 is a BL Lacertae object (of lower luminosity than its surrounds) seen from a relatively large angle. Flux variations, characteristic of the BL Lacertae objects, have been observed in M87.

Radio wavelength image of M87 showing strong radio emission from the core

Observations indicate that the rate at which material is ejected from the supermassive black hole is variable. These variations produce pressure waves in the hot gas surrounding M87. The Chandra X-ray Observatory has detected loops and rings in the gas. Their distribution suggests that minor eruptions occur every few million years. One of the rings, caused by a major eruption, is a shock wave 26 kiloparsecs (85,000 light-years) in diameter around the black hole. Other features observed include narrow X-ray-emitting filaments up to 31 kiloparsecs (100,000 light-years) long, and a large cavity in the hot gas caused by a major eruption 70 million years ago. The regular eruptions prevent a huge reservoir of gas from cooling and forming stars, implying that M87's evolution may have been seriously affected, preventing it from becoming a large spiral galaxy.

M87 is a very strong source of gamma rays, the most energetic rays of the electromagnetic spectrum. Gamma rays emitted by M87 have been observed since the late 1990s. In 2006, using the High Energy Stereoscopic System Cherenkov telescopes, scientists measured the variations of the gamma ray flux coming from M87, and found that the flux changes over a matter of days. This short period indicates that the most likely source of the gamma rays is a supermassive black hole. In general, the smaller the diameter of the emission source, the faster the variation in flux.

M87 in infrared showing shocks produced by the jets

 

Spiral flow of the black hole-powered jet

A knot of matter in the jet (designated HST-1), about 65 parsecs (210 light-years) from the core, has been tracked by the Hubble Space Telescope and the Chandra X-ray Observatory. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period, while the X-ray emission has since been decaying in a variable manner.

The interaction of relativistic jets of plasma emanating from the core with the surrounding medium gives rise to radio lobes in active galaxies. The lobes occur in pairs and are often symmetrical. The two radio lobes of M87 together span about 80 kiloparsecs; the inner parts, extending up to 2 kiloparsecs, emit strongly at radio wavelengths. Two flows of material emerge from this region, one aligned with the jet itself and the other in the opposite direction. The flows are asymmetrical and deformed, implying that they encounter a dense intra-cluster medium. At greater distances, both flows diffuse into two lobes. The lobes are surrounded by a fainter halo of radio-emitting gas.

Interstellar medium

The space between the stars in M87 is filled with a diffuse interstellar medium of gas that has been chemically enriched by the elements ejected from stars as they passed beyond their main sequence lifetime. Carbon and nitrogen are continuously supplied by stars of intermediate mass as they pass through the asymptotic giant branch. The heavier elements from oxygen to iron are produced largely by supernova explosions within the galaxy. Of the heavy elements, about 60% were produced by core-collapse supernovae, while the remainder came from type Ia supernovae.

The distribution of oxygen is roughly uniform throughout, at about half of the solar value (i.e., oxygen abundance in the Sun), while iron distribution peaks near the center where it approaches the solar iron value. Since oxygen is produced mainly by core-collapse supernovae, which occur during the early stages of galaxies, and mostly in outer star-forming regions, the distribution of these elements suggests an early enrichment of the interstellar medium from core-collapse supernovae and a continuous contribution from type Ia supernovae throughout the history of M87. The contribution of elements from these sources was much lower than in the Milky Way.

Selected elemental abundances in the M87 core

Element	Abundance (solar values)
C	0.63 ± 0.16
N	1.64 ± 0.24
O	0.58 ± 0.03
Ne	1.41 ± 0.12
Mg	0.67 ± 0.05
Fe	0.95 ± 0.03

Examination of M87 at far infrared wavelengths shows an excess emission at wavelengths longer than 25 μm. Normally, this may be an indication of thermal emission by warm dust. In the case of M87, the emission can be fully explained by synchrotron radiation from the jet; within the galaxy, silicate grains are expected to survive for no more than 46 million years because of the X-ray emission from the core. This dust may be destroyed by the hostile environment or expelled from the galaxy. The combined mass of dust in M87 is no more than 70,000 times the mass of the Sun. By comparison, the Milky Way's dust equals about a hundred million (10⁸) solar masses.

Although M87 is an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in it, which arise from gas falling towards the core. Emission probably comes from shock-induced excitation as the falling gas streams encounter X-rays from the core region. These filaments have an estimated mass of about 10,000 M_☉. Surrounding the galaxy is an extended corona with hot, low-density gas.

Globular clusters

M87 has an abnormally large population of globular clusters. A 2006 survey out to an angular distance of 25′ from the core estimates that there are 12,000 ± 800 globular clusters in orbit around M87, compared with 150–200 in and around the Milky Way. The clusters are similar in size distribution to those of the Milky Way, most having an effective radius of 1 to 6 parsecs. The size of the M87 clusters gradually increases with distance from the galactic center. Within a four-kiloparsec (13,000-light-year) radius of the core, the cluster metallicity—the abundance of elements other than hydrogen and helium—is about half the abundance in the Sun. Outside this radius, metallicity steadily declines as the cluster distance from the core increases. Clusters with low metallicity are somewhat larger than metal-rich clusters. In 2014, HVGC-1, the first hypervelocity globular cluster, was discovered escaping from M87 at 2,300 km/s. The escape of the cluster with such a high velocity was speculated to have been the result of a close encounter with, and subsequent gravitational kick from, a supermassive black hole binary.

Almost a hundred ultra-compact dwarfs have been identified in M87. They resemble globular clusters but have a diameter of ten parsecs (33 light-years) or more, much larger than the three-parsec (9.8-light-year) maximum of globular clusters. It is unclear whether they are dwarf galaxies captured by M87 or a new class of massive globular cluster.

Environment

Main article: Virgo Cluster

Photograph of the Virgo Cluster (European Southern Observatory 2009). M87 is visible in the lower left, the upper half of the image is taken up by Markarian's Chain. The dark spots mark the locations of bright foreground stars that were removed from the image.

M87 is near (or at) the center of the Virgo Cluster, a closely compacted structure of about 2,000 galaxies. This forms the core of the larger Virgo Supercluster, of which the Local Group (including the Milky Way) is an outlying member. It is organized into at least three distinct subsystems associated with the three large galaxies—M87, M49 and M86—with the core subgroup including M87 (Virgo A) and M49 (Virgo B). There is a preponderance of elliptical and S0 galaxies around M87. A chain of elliptical galaxies roughly aligns with the jet. In terms of mass, M87 is likely to be the largest, and coupled with centrality appears to be moving very little relative to the cluster as a whole. It is defined in one study as the cluster center. The cluster has a sparse gaseous medium that emits X-rays, lower in temperature toward the middle. The combined mass of the cluster is estimated to be 0.15 to 1.5 × 10¹⁵ M_☉.

Measurements of the motion of those intracluster starburst ("planetary") nebulae between M87 and M86 suggest that the two galaxies are moving toward each other and that this may be their first encounter. M87 may have interacted with M84, as evidenced by the truncation of M87's outer halo by tidal interactions. The truncated halo may also have been caused by contraction due to an unseen mass falling into M87 from the rest of the cluster, which may be the hypothesized dark matter. A third possibility is that the halo's formation was truncated by early feedback from the active galactic nucleus.