Theory What Black Holes Are

illustration of a black hole. picture ESA/V. Beckmann (NASA-GSFC)

German astrophysicist Karl Schwarzschild derived the equations for black holes in 1916, but they remained rather a theoretical curiosity for several decades, until X-ray observations performed with space telescopes could finally probe the highly energetic emission from matter in the vicinity of these extreme objects. Black holes are the places of the most dense environment in our Universe. They are places where the matter has become condensed to the point that not even light, with its speed of 671 million miles per hour (300,000 km/s), may escape from them. Because no light can get out, one cannot see black holes and they are invisible. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. The main tendency now, emerging from the studies of black holes, is to think that there might be 3 kinds of them. Galactic, stellar, and intermediate mass ones. Some hypothetize that black holes, at their other end, would behave like a 'white hole' and allow to swiftly journey to other regions of the Universe. Internal turbulences inside a black hole however likely forbid such a possibility. Astronomers think supermassive black holes may form when a large cloud of gas, with a mass of about 10,000 to 100,000 times that of the Sun, collapses into a black hole as many of such black hole seeds then merge to form much larger supermassive ones. Alternately, a supermassive black hole seed could come from a giant star, about 100 times the Sun’s mass turning supernova. Space-time, inside a black hole, is moving faster than the speed of light. Black holes have been predicted by Albert Einstein but most scientists doubted that nature could actually make a object so gravitationally powerful that it would trap anything including light. Then, in the 1950s, astronomers discovered quasars and radio-galaxies the energy of which is too great to be explained by a thermal process and hypothetized black holes as the theory also came by 1971 for the radio waves emanating from our Milky Way Galaxy center. Sagittarius A* ('*' for a atom in its excited state in physics) was discovered and named by February 1974 and confirmed two years later but still not determined a black hole. That was done only in the early 1990's when the orbits of stars around could be calculated and revealed the mass and size of Sgr A*, mostly hinting to a black hole. Now astronomers are going to image the event horizon of it. The region also is affected by the 'paradox of youth,' the fact that unexpectedly young stars are seen in the black hole's vicinity which should be impossible as the gravitational pull should tear them apart. All which brings to that astronomers strongly suspect the presence of a galactic black hole but have not still the definitive evidence, stricto, of it. The first photograph ever of a galactic black hole was released by April 2019, in elliptical galaxy M87, confirming Einstein’s general relativity. Two properties only define a black hole, its mass and its spin. The 'no-hair theorem' indeed, states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in General Relativity are wholly -- and only -- characterized by three observable parameters: mass, electric charge, and angular momentum. All other information (for which 'hair' is a metaphor) disappear behind the black hole's event horizon. The strength of a black hole depends only upon its mass, like for example any stellar object it may swallow

It's now well-established that supermassive black holes, the orbit of Mercury in size and in the range of million to billions solar masses, are lurking in the center of any galaxy, our included. Supermassive galactic black holes at 10 billion times the mass of the Sun are called 'ultramassive black holes.' The current galactic black hole record holder tips the scale at 21 billion Suns and resides in the crowded Coma galaxy cluster that consists of over 1,000 galaxies. Galactic black holes are a common thing among the galaxies' world. Material streaming into a black hole is heated into a glowing plasma. The more massive a galaxy's central bulge, the larger the supermassive black hole as some galaxies are centered unto intermediate-mass black holes which likely feature specific interactions between the inner and outer parts of their accretion disk. The presence of a black hole in a distant galaxy can be inferred from its effect on the galaxy’s innermost stars; these stars experience a strong gravitational pull from the black hole and whizz around the galaxy’s center much faster than otherwise. The supermassive black hole at the center of a galaxy gives off intense radiation and ionises the surrounding gas so that it glows strongly with the glowing regions in typical active galaxies usually small, up to 10 percent of the diameter of the galaxy. The one in the Milky Way Galaxy is weighing 4 million solar masses! Those in the largest galaxies can be a thousand times larger. A monster black hole may also be lurking inside a dwarf galaxy, reaching to a 15 percent of the galaxy’s total mass, due to galactic collision leaving only a few stars around the black hole. The largest-ever found black hole was in giant galaxy NGC 4889, some 300 million light years away, at 21 billion solar masses and a event horizon of approximately 81 billion miles or 15 times the diameter of Neptune’s orbit as it was a quasar when younger. Supermassive black holes masses usually range between a few million and a few billion times that of our Sun. One thinks that the mass of the black hole sitting at a galaxy's center is linked with the larger-scale properties of the galaxy itself. How they formed there is still a mystery. Such black holes are active, that is that they are still swallowing matter -and in some cases, stars- as a ring of forming star might well be found around each of them. They may produce polar jets either side, which may reach up to about 1 million light-years of distance and shooting out mostly in the form of radio waves. Jets basically are chaotic braids of shooting twisted magnetic fields. Jets usually are how black holes react when confronted to a celestial object or material getting into its gravitational trap. Jets eventually are material falling towards black holes which are redirected outward at high speeds due to intense gravitational and magnetic fields. Jets, generally, however are not well understood and likely energetic plasma in a confined beam as shocks produced by collisions within the jet may further accelerate particles. When those plasma jets are energized enough, they strongly radiate light. The radiation might be determined by the speed, temperature and other properties of particles at the jet’s base. The jet's X-ray light, generally, originates from material very close to the black hole and strong magnetic fields propel some of this material to high speeds along the jet, with particles colliding near light-speed, energizing the plasma until it shines in the visible. Electrons in black hole jets usually emit strongly at radio wavelengths, so typically these systems are found using radio observation. Jets seen at a black hole at 2.7 billion years after the Big Bang and a lenghth at least 300,000 light years are about 150 times brighter in X-rays than it would be in the nearby Universe because they move through the sea of the CMB radiation, which is much greater than today. As the jets collide with microwave photons, they boost the energy of the photons up into the X-ray band. Black holes' X-ray brightness thus is amplified by the CMB. The existence of a counterjet was eventually really evidenced as a radio lobe is where a jet is pushing into surrounding gas, or a hotspot where shock waves act near the jet's tip. X-ray emission of a jet and counterjet comes from electrons spiraling around magnetic field lines, a process called synchrotron emission, as the electrons must be continuously re-accelerated as they move out along the jet, a process not well understood. There is a unambiguous link between the presence of supermassive black holes with jets and a merger history of their host galaxies. They might originate at strong magnetic fields acting very close to the event horizon. The supermassive black holes are thought to rotate at highly excessive speeds, at about the limit of the light's speed and that's those high speeds which are a the origin of the jets, with the matter to be gobbled up to be accelerated at those speeds and further combining with powerfull magnetic fields. Jets are also powered by magnetic beams that spew out from the black hole as accretion disks may, or not, spin in the same direction as their black holes. The process to produce jets are the same for black holes, whatever their mass. Gas falling toward a black hole spirals inward and piles up into an accretion disk, where it becomes compressed and heated. A part of the black hole accretion disk is cool. Near the inner edge of the disk, on the threshold of the black hole's event horizon- the point of no return -some of the material becomes accelerated and races outward as a pair of jets flowing in opposite directions along the black hole's spin axis. Jets blasted by a black hole can flicker on and off as a result of gas-feeding events. These jets contain particles moving at nearly the speed of light, which produce gamma rays -the most extreme form of light- when they interact. Jets are also produced by such events like the tidal disruption of stars by supermassive black holes. Astronomers have found by 2012 that jets launched from active black holes possess fundamental similarities regardless of mass, age or environment -jets produce light by tapping into similar percentages, at between 3 and 15 percent, of the kinetic energy of particles moving along the jet- as that provides a tantalizing hint that common physical processes are at work, with the same fixed fraction of energy generating the gamma-ray light. Albeit theorecists expect gamma-ray outbursts occur only in close proximity to a galaxy's central black hole, the 2011 flares from a galaxy known as 4C +71.07 show the gamma-ray emission originated about 70 light-years away from the galaxy's central black hole. The gamma rays when produced when electrons moving near the speed of light within the jet collided with visible and infrared light originating outside of the jet, that can kick the light up to much higher energies, a process known as inverse-Compton scattering. As gas falls toward a black hole, it heats up and emits X-rays. Variations in X-ray brightness reflect changes occurring in the gas. X-ray blasts emitted from a black hole occur when a black hole pulls in matter from a normal star that is in orbit around it. As the matter spirals onto a spinning disk surrounding the black hole, an enormous amount of energy primarily in the form of X-rays is released. Black holes of extremely different masses, from stellar to galactic, produce similar kinds of X-ray activity, just at varying time scales proportional to their masses. Stellar-mass black holes undergo key changes in a matter of hours, while their supermassive cousins exhibit similar changes over years. The most rapid fluctuations happen near the brink of the black hole’s event horizon as such fluctuations are called 'quasi-periodic oscillations,' or QPOs. Such flashes are most likely caused by activity close to the black hole, where extreme gravity keeps all surrounding matter on a very tight leash. The larger the mass of a stellar or a medium-sized black hole, the slower the QPOs. QPOs exist in the slow and fast variety and they have a 3:2 rhythmic relationship. As astronomers thought that X-rays in the deepest regions of a black hole were warped by the clouds of gas there, recent studies have shown that they are indeed by the black hole's gravity. Astronomers had assumed that light at different energies came from regions at different distances from the black hole, as gamma rays, the highest-energy form of light, were thought to be produced closest to the black hole. That is not true. Einstein's theory predicts the faster a black hole spins, the closer the accretion disk lies to the black hole. One surprise from a study in 2016 was that high-energy X-rays arise from the inner part of the disk as astronomers had thought most of this emission originated from a narrow jet of particles accelerated to near the speed of light. In blazars, the most luminous galaxy class powered by supermassive black holes, jets produce most of the highest-energy emission. X-rays originating near a galactic black hole, generally, excite iron ions in the whirling gas, causing them to fluoresce with a distinctive high-energy glow called iron K-line emission. As an X-ray flare brightens and fades, the neighbouting gas follows in turn after a brief delay depending on its distance from the source. The closer the accretion disk is, the more gravity from the black hole will warp X-ray light streaming off the disk. Ultrafast winds, with a rapid variability, are created by disks of matter surrounding black holes. Latest studies are hinting to that supermassive black holes might be heaftier than thought, with masses in the order of 6.5 billion solar masses, which could interfere in the process by which star formation, the supermassive black hole, or dark matter are competing into the formation of a galaxy. The most heavy supermassive black hole found is weighing 6.4 billion times the mass of the Sun, as it sits in the giant M87 galaxy. Astronomers have found small supermassive black holes lying into spiral galaxies which lack a central bulge. That findings is showing that bulges which were thought necessary for a galactic black hole are not indeed. Such black holes could feed on the galactical disk, or the dark matter halo and not from the stellar bulge, as they have a mass of about 200,000 Suns. The supermassive black holes in the quasars, those much active, earlier-Universe galaxies, are weighing about 10 billion solar masses, as far as they are concerned. Rare 'ultramassive black holes' are weighing between 10 and 40 billion Solar masses as there might be more of them in the Universe than previously thought and their behavior looks like different from that of their less massive cousins. It might their presence be found in galaxies at the centers of massive galaxy clusters containing huge amounts of hot gas and outbursts powered by the central black holes, resulting from the absorption of the gas, are needed to prevent this hot gas from cooling and forming enormous numbers of stars. Ultramassive black holes likely are the largest black holes in the Universe. As far as stellar black holes are concerned, they mostly result from the collapse of one star, following a supernova event. Such black holes are small in size, about 7 to 25 solar masses. Stellar mass systems which often consists of one black hole, and a companion star, change much more quickly than supermassive black holes. There could be a much larger number of nearby black holes, a few times the mass of our Sun, in the Galaxy that have previously been unaccounted for. Black holes X-rays are brighter and also have a particular X-ray color. More black hole are lying closer to M31 center, for example, as it holds a larger bulge. Cygnus X-1, a stellar-mass black hole, in close orbit with a massive, blue companion star, gives a good example of how such a black hole is originating. It looks like such black holes already possess a large turn rate and mass at birth. Cygnus X-1 is spinning around more than 800 times a second! The youngest stellar black hole observed is a one which had been born by April 1979 from a supernova explosion. The 'No Hair' theorem postulates that all other information aside from mass, spin and charge is lost for eternity behind the event horizon

->The Black Hole Information Paradox
The black hole information paradox, which scientist Hawking identified more than 40 years ago comprises also that black holes are not truly black, and in fact emit some radiation. According to quantum physics, pairs of particles must appear out of quantum fluctuations just outside the event horizon. Some of these particles escape the pull of the black hole but take a portion of its mass with them, causing the black hole to slowly shrink and eventually disappear. Outflowing particles -now known as Hawking radiation- would have completely random properties. As a result, once the black hole was gone, the information carried by anything that had previously fallen into the hole would be lost to the Universe. But this result clashes with laws of physics that say that information, like energy, is conserved, creating the paradox. A recent study by 2016, by Hawking, is showing that in a zero-energy state, soft particles like low-energy versions of photons, hypothetical particles known as gravitons and other particles are extant, as they are usually mainly used to make calculations in particle physics. The vacuum in which a black hole sits thus need not be devoid of particles but only of energy. Anything falling into a black hole would leave an imprint on these particles. A mechanism for transferring that information to the black hole would have to happen for the paradox to be solved. Hawking does that by calculating how to encode the data in a quantum description of the event horizon, known whimsically as 'black hole hair'

The charge for an astronomical black hole is expected to be almost zero, so only the mass and spin are known. Some black holes may not have any rotation at all. X-rays are produced when a swirling accretion disk of gas and dust that surrounds the black hole creates a multimillion-degree cloud, or corona near the black hole. X-rays from this corona reflect off the inner edge of the accretion disk as corona, generally, are sources of extremely energetic particles generating X-rays. A black hole corona may vary according to the black hole's evolution. Coronas might 'shoot' themselves away from the black hole. Astronomers think coronas have one of two likely configurations. The lamppost model says they are compact sources of light, similar to light bulbs, that sit above and below the black hole, along its rotation axis. The other model proposes that the coronas are spread out more diffusely, either as a larger cloud around the black hole, or sandwiching the surrounding disk of material. In fact, it's possible that coronas switch between both the lamppost and sandwich configurations. Coronas can move very fast. A black hole's corona, which features particles moving near the speed of light, may collapse in toward the black hole, with the result that the black hole's intense gravity pulled all the light down onto its surrounding disk, where material is spiraling inward, blurring and stretching all of the X-ray light in the area. Around most massive and rapidly spinning supermassive black holes, space and time are dragged around. By measuring the spin of black holes researchers discover important clues about how these objects grow over time. If black holes grow mainly from collisions and mergers between galaxies, they should accumulate material in a stable disk, and the steady supply of new material from the disk should lead to rapidly spinning black holes. In contrast, if black holes grow through many small accretion episodes, they will accumulate material from random directions. Like a merry go round that is pushed both backwards and forwards, this would make the black hole spin more slowly. Intermediate-mass black holes (IMBHs), at last, might be, like the name, intermediate-sized black holes, in the order of the 10,000 solar masses, resulting from the merger of several stellar class ones in the heart of a star cluster (like evidenced, for example, in April 2008), or being the central black holes of diminutive galaxies in the process of being absorbed by larger ones. Astronomers found by early 2017 one medium-sized black hole inside the 47 Tucanae globular cluster. IMBHs are important in that they could be the seeds from which supermassive black holes formed in the early Universe. Emitting jets they could clear the clouds where they live and shut off star formation. Late studies are showing that intermediate mass black holes are to be found too in the turbulent environment inside globular clusters, this pointing to that stellar black holes there accrete together and reach a mass sufficient to withdraw the powerful gravitational interaction in the cluster. As the existence of intermediate-mass black still was unconclusive, two mid-sized black holes found in the actively star-forming galaxy M82 in 2010 seems to close definitively the case. Such black holes which are close to the galaxy center, on the other hand, might well be the evidence that supermassive black holes are forming from those. Dwarf galaxies are a haven for these missing middleweight black holes. One leading explanation to the formation of galactic black holes is that they grow over time from smaller black holes seeds merging. Another explanation is that supermassive black holes form very quickly from the collapse of a giant cloud of gas with a mass equal to hundreds of thousands of times that of the Sun. Another possibility yet is that both mechanisms actually occur. One possible mechanism also for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole. A team of astronomers has observed by 2016 that cold dense clouds of gas can coalesce out of hot intergalactic gas and plunge into the heart of a galaxy to feed its central supermassive black hole adding to the idea that how accretion unfolds around a galactic black hole may also be swift and chaotic. Very hot gas can quickly cool, condense, and precipitate as massive clumps of such cold gas are swiftly careening toward the supermassive black hole in the galaxy’s core and containing as much material as a million Suns and tens of light-years across. 'Ultraluminous X-ray sources', or ULXs are a unusual, superbright class of objects, emitting more X-rays than any known stellar X-ray source, but less than the galactic supermassive black holes. They are found embedded in regions where stars are currently forming at a rapid rate. They were first observed in the 1980's and some of them may be stellar-mass black holes, containing up to a few tens of times the mass of the Sun. In 2014, observations showed that a few ULXs, which glow with X-ray light equal in luminosity to the total output at all wavelengths of millions of suns and were identified as neutron stars later. The ULX's exact nature is still a mystery as one suggestion is that some ULXs are black holes with masses between about a hundred and a thousand times that of the Sun, or that they might be mostly found in globular clusters. Another view is that some ULXs might be intermediate-mass black holes, with a few thousand times the mass of our Sun, with a unusually bright state. Recentest studies are still wondering whether ULXs are normal stellar-mass black holes gorging on unusually large amounts of gas, or long-sought intermediate mass black holes. ULXs might be volatile, binary star- black hole (or a neutron star) systems and would sort into two types, one containing young, persistently growing black holes and the other containing old black holes that grow erratically. Companion stars to ULXs are usually young, massive stars, implying their black holes are also young. Some ULXs however contain much older black holes and some observations may have been misidentified as young ones as observed during a jump in energy due to a sudden increase in the amount of material falling into the black hole. Recent studies are hinting to that ULXs are a binary system containing a black hole, or a neutron star, that is rapidly accreting gas from its stellar companion. However, to account for the brilliant high-energy output, gas must be flowing into the black hole at a rate very near a theoretical maximum, a feeding frenzy that astronomers do not yet fully understand. Only 4 ULXs has been observed in our Milky Way Galaxy. The Eddington limit, after Sir Arthur Eddington, the British astrophysicist who first recognized a similar cutoff to the maximum luminosity of a star, is the point when the X-ray emission becomes so intense that it pushes back on the inflowing gas, theoretically capping any further increase in the black hole's accretion rate. If a black hole breaks this limit, it could theoretically balloon in size at a breakneck pace as black holes have been observed breaking this limit. If a black hole spins slowly enough, it won't repel the incoming gas and allow itself to gobble more matter. Powerful radio-emitting jets that move near the speed of light usually come with ULX as radio emission is quite variable, decreasing in one instance, by a factor of two in just half an hour, hinting to that the region producing radio waves is extremely small in size, like no farther across than the distance between Jupiter and the Sun. Astronomers have debated whether many ULXs represent hard-to-find "middleweight" black hole versions, and likely the remains of stars of about 10 solar masses. Intermediate-mass black holes, at about 20,000 solar masses, may also be found in a galaxy like a result from a galactic collision and likely originating from a dwarf galaxy. One of those has been found lying far above the galactic plane of its home galaxy. It is surrounded further by a cluster of young, blue stars at 250 light-years across which comforts the collision theory when that triggered star formation around the black hole, as the irradiation of the accretion disk might be a alternate explanation indeed, or star formation might have also been triggered by compression around the black hole. Such black holes might eventually aggregate and participate to the formation of supermassive ones inside galaxies. Supermassive galactic black holes also affect the central bulge of a galaxy where forming star are orbiting at speeds larger than more outwards in the spiral arms. The supermassive black holes in active galaxies can produce famed narrow, polar particle jets and less-known wider, spiral streams of gas known as ultra-fast outflows (or UFOs), which are powerful enough to regulate both star formation in the wider galaxy and the growth of the black hole. Active black holes acquire their power by gradually accreting million-degree gas stored in a vast surrounding disk. This hot disk lies within a corona of energetic particles. Near the inner edge of the disk, a fraction of the matter orbiting a black hole often is redirected into an outward particle jet. Although these jets can hurl matter at half the speed of light, computer simulations show that they remain narrow and deposit most of their energy far beyond the galaxy's star-forming regions. Over the last decade, evidence for a new type of black-hole-driven outflow has emerged, closer to the black hole and with a average velocity of about 14 percent the speed of light. UFOs have a larger potential to transmit feedback effects from a black hole into the galaxy at large as they remove mass that would otherwise fall into the supermassive black hole and they may put the brakes on its growth. At the same time, UFOs may strip gas from star-forming regions in the galaxy's bulge, slowing or even shutting down star formation. Ejections from a galactic black hole may be found well outside the region where rapid outflow, or winds occur, yet inside the much larger cavities and filaments observed in the hot gas around many massive galaxies

Supermassive black holes exhibit a wide variety of activity levels, from dormant to just lethargic to practically hyper. As galactic black holes grow at the same rate that new stars form, until blasting radiation from the black holes ultimately shuts down star formation, giant black holes may aslo be asleep in the midst of tremendous star-forming activity around. ULXs, or stellar-mass black holes feeding off material from a partner star could be bumping along near the center such galaxies and adding to observation. The most lively supermassive black holes produce what are called 'active galactic nuclei,' or AGN, by pulling in large quantities of gas. Dust surrounding active, ravenous black holes known like "active galactic nuclei" is much more compact than previously thought, with tori 30 percent smaller than anticipated. AGNs are galaxies with a strong rate of star formation. Binary AGNs are rare and likely resulting from a galaxy merger, as two only have been found close to use, with a example like Markarian 739 or NGC 3758, at 425 million light-years in constellation Leo possesses two galactic black holes instead of one, separated by some 11,000 light-years. Galaxy mergers generally tend to be considered the source of emergence of AGNs. Gas is heated as it falls in and glows brightly in X-ray light. Where the motion of gas first becomes dominated by the supermassive black hole's gravity and falls inwards, that distance from the black hole where this occurs is known as the 'Bondi radius' as it is helping to determine a black hole mass. Astronomers also found that only 1 percent of galaxies with masses similar to our Milky Way Galaxy contain supermassive black holes in their most active phase. A striking correlation between the mass of the giant black holes and the mass of the central regions of their host galaxy suggests that the growth of supermassive black holes and their host galaxies are strongly linked. The fraction of galaxies containing AGN depends on the mass of the galaxy. The most massive galaxies are the most likely to host AGN, whereas galaxies that are only about a tenth as massive as the Milky Way have about a ten times smaller chance of containing an AGN. Another result is that a gradual decrease in the AGN fraction is seen with cosmic time since the Big Bang implying that either the fuel supply or the fueling mechanism for the black holes is changing with time. AGNs may exist in dense galaxy clusters or with isolated 'field' galaxies as both might have evolved wit time in both environments as the AGNs in clusters might have started higher in clusters and then decreased more rapidly. If the Milky Way follows such a trend, our supermassive, Sgr A* black hole should be about a billion times brighter in X-rays for roughly 1% of the remaining lifetime of the Sun. Such activity is likely to have been much more common in the distant past. Any planet close, or directly in the line of fire, would receive large and potentially damaging amounts of radiation. More generally, a ring of neutral hydrogen, part of a structure near the center of most usual galaxy, and distorted by gravitational interactions with the rest of that galaxy, as it includes material falling towards the supermassive black hole might be a evidence for feedback from active black holes to the surrounding gas on galaxy scales. This would resemble the larger scale feedback, observed on galaxy cluster scales, from active black holes interacting with the surrounding gas, as seen in objects like the Perseus Cluster. The merger of galaxies on a other hand usually result into the formation at their center of a dual supermassive black hole, whatever the respective sizes of the galaxies involved likely pointing that such mergers are the most usual process by which galactic black holes formed in the past. A galactic black hole 'recoiling' is a one which was kicked and moving inside a galaxy as a result of a galactic collision. When the two supermassive black holes collided and merged, that generated gravitational waves that emitted more strongly in one direction than others, which the newly form black hole followed. Two black holes merging may trigger a titanic blast with the power of 100 million supernovae. Intermediate mass black holes with masses ranging between about 100 and 100,000 times that of the Sun might also be of the supermassive type, found anywhere in a galaxy, and be a massive black hole from a other galaxy during a merger. Such black holes often are of the 'hyper-luminous X-ray source' (HLX) type as 10,000 to 100,000 times more luminous in X-rays than stellar black holes, and 10 to 100 times more powerful than ultraluminous X-ray sources, or ULXs. Such a brightness comes from material falling into. The 'final parsec problem' refers to the failure of theoretical models to predict what the final stages of a black hole merger look like, or even how long the process might take. A black hole in a pair of two due to a galaxy merger may have one gorging on gas, while its partner is either dormant or hidden under gas and dust. After some billion years both black holes eventually merge. Numerous galactic black holes, on a other hand might roam undetected out in the vast spaces between galaxies as when two galaxies collide, both their central black holes also collide. When merging, that can yield gravitational waves -with are ripples in the fabric of space- and can exert an extremely powerful force. Such gravitational waves are able to kick the newly formed black hole from the resulting galaxy, with the smaller one kicked off! Galactic black holes may be propelled out of the center of galaxy only -- and remaining inside the galaxy -- following a galaxy merger, with two black holes merging. As the gravitational waves generated between two black holes of different size eventually produce thrust in a preferential direction, the resulting black hole is expelled from the new galactic center. Before two black holes merge, they circle each other until reaching a point known as the 'lowest stable orbit,' after which the merger occurs. The mass of a supermassive black hole typically is a tiny fraction -about 0.2 percent- of the mass contained in the bulge surrounding it. It might that some atypical supermassive black holes, which grow faster than their galaxy's bulge, and their evolution are tied to their galaxy's dark matter halos and not with the bulges

The supermassive black hole of our own Milky Way Galaxy, also known as 'Sagittarius A*' or 'Sgr A*' for short, as it weights a four-million-solar-mass, might well feed from about 0.1 percent only of the material brought there by the stellar winds of the neighbouring stars. Those winds might well be contained by a conduction process by which collisions between particles close to the hot, inner region -or 'event horizon'- of the black hole transfer energy to particles lying in a cooler, outer region, which host stars serving like a fuel for the black hole. Such a process is generating more outward pressure. The question might be whether those processes are at work too in other galaxies similar to ours. The supermassive black hole of the Milky Way further is sided by two lobes of hot gas extending for a dozen light years either side, likely evidence of powerful eruptions of the black hole which occurred several times over the last ten thousand years. Mysterious X-ray filaments around the black hole might be huge magnetic structures interacting with streams of energetic electrons produced by rapidly spinning neutron stars close to Sgr A* and known like pulsar wind nebulas. Massive stars also exist in the central, 300 light-years-wide region of our Galaxy, as they are mostly condensed into three clusters, the Central, Arches and Quintuplet clusters as some also formed in isolation or found themselves isolated when their cluster where disrupted by strong gravitational tidal forces. Astronomers suspect that the Milky Way's black hole might awaken in a undetermined future and unleash, after a flurry of star formation about it, two polar jets. A study by early 2012 is hypothetizing that the Milky Way supermassive black hole's flares to be due to the vaporization of asteroids which lie in a cloud which contains trillions of asteroids and comets stripped from their parent stars. Even exoplanets might be also, albeit less frequently devoured by the black hole. Such flares last a few hours with brightness ranging from a few times to nearly one hundred times that of the black hole's regular output

->A recent, 2005 study by NASA's Chandra Telescope is showing that a mass between 25 and 40 Sun might be the condition for a massive star to eventually turn into a black hole. More massive stars than that would turn into a neutron star due to that they blow off mass more effectively during their lives. Other factors like the star’s chemical composition, its rotation rate, or whether it is part of a double star system, may play a role in determining whether a massive star leaves behind a neutron star or a black hole

All black holes are characterized by an accretion disk, surrounding them, which is where the infalling matter is orbiting before being definitively swallowed. As hot gas in the innermost disk spirals toward a black hole, it reaches a point astronomers refer to as the innermost stable circular orbit (ISCO). Any closer to the black hole and gas rapidly plunges into the event horizon, the point of no return. The inward spiraling gas tends to pile up around the ISCO, where it becomes tremendously heated and radiates a flood of X-rays. The brightness of these X-rays varies in a pattern that repeats at a nearly regular interval, creating a QPO, or 'quasi-periodic oscillation' signal, which is allowing to pinpoint the distance to the black hole's center. The 'singularity'lying at the center of a black hole is seen like a point of infinite density as the true nature of it will eventually be disclosed only when a theory of quantum gravity, reconciling both General Relativity and quantum mechanics will have been achieved. Such singularities might be indeed a lot smaller than the size of a quark but without any infinite density. The swiftier a black hole rotate, the closer the dust disk. Usually, a supermassive black hole is surrounded by an accretion disk, which itself is surrounded by a dark doughnut-like dusty structure called a dust torus. A growing supermassive black hole in a normal galaxy gets a donut-shaped structure of gas and dust around it as, in a merging galaxy, a sphere of material obscures the black hole. But for the primitive black holes, in the primitive quasars for example, the dust tori are missing and only gas disks are observed. This is because the early universe was more clean. Enough time had not passed for molecules to clump together into dust particles. Some black holes forming during this era thus started out lacking dust. As they grew, gobbling up more and more mass, they are thought to have accumulated dusty rings. Doughnut-shaped disks of gas and dust around supermassive black holes were first proposed in the mid-1980s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to Earth affects the way we perceive a black hole and its intense radiation. This idea is referred to as the unified model because it neatly joins together the different black hole types, based solely upon orientation. Tori, on a other hand, might not be rounded but defective and clumpy instead as that keeps ill-explained and for now multiple causes could provide for. Supermassive black holes, with their immense gravitational pull, are notoriously good at clearing out their immediate surroundings by eating nearby objects. When a black hole attracts and destroys a star, a event astronomers call 'stellar tidal disruption' or 'tidal disruption event' (TDE), it releases an enormous amount of energy, brightening the surroundings in an event called a flare. TDE spectra fall into three classes, dominated by hydrogen, helium, or a mixture of gases each signaling large, young stars, or older stars whose hydrogen shells were stripped away -- perhaps by an earlier brush with the black hole -- respectively. Above a mass of 100 million Suns, black holes should swallow stars whole rather than tearing them apart. Grazing encounters between a star and a black hole, where the star is not swallowed by, should be more common than direct collisions given the statistics of cosmic traffic patterns, but they could easily be missed. 10 tidal disruptions so far have been observed. Tidal forces outstrip the star's own gravity, converting the star into a stream of debris as that matter falls toward the black hole's spinning accretion disk, eventually spilling over the black hole's event horizon. A giant black hole gorges on a star it ripped during one year. Longer tidal disruption events may hint to a large star or a star completely disrupted as the larger the delay, the more the black hole accretes more matter. Flares from black holes eating stars contain high-energy radiation, including ultraviolet and X-ray light. Such flares destroy any dust that hangs out around a black hole. But at a certain distance from a black hole, dust can survive because the flare's radiation that reaches it is not as intense. At half a light-year from a black hole, one may found a patchy, spherical web of dust. The activity of a black hole is generating too two polar jets. It's mostly magnetic fields which are involved into creating those jets of matter and anti-matter emanating from a black hole (or any similar object). From a stellar-mass black hole lying in a binary system, with a Sun-like star orbiting the black hole and pouring material into there, astronomers have observed 20 million-mph winds blowing off the black hole's accretion disk, either direction up or down from the plane. Such a speed is usually seen only with giant black holes as the study found that winds may be carrying away more material than the black hole is capturing! Such rotating black holes drag along nearby space-time and the inner accretion disk with it. Also production of winds, which turn on and off over time, are seen stifling jets usually found at the black hole. Astronomers believe that magnetic fields in the disks of black holes are responsible for producing both winds and jets. The geometry of the magnetic fields and rate at which material falls towards the black hole must influence whether jets or winds are produced as magnetic fields impact how a galactic black hole is active. As far as galactic, supermassive black holes are concerned, the jets, as their tips, they're releasing blobs of gas, are easing the black hole -and the host galaxy- from ballooning to mega sizes. When a star wanders too close a galactic black hole, it is simply torn apart as the event creates a temporary accretion disk which can flare in the X-rays or last about a year. Such a case also generate two jets driving matter at velocities greater than 90 percent the speed of light form along the black hole's spin axis. Jets are due to rapid motion in the inner disk and magnetism. When a star is torn apart by the gravitational forces of a galactic black hole, some part of the star's remains falls into the black hole, while the rest is ejected at high speeds. Long before the end, for example, a red giant hydrogen-filled envelope may belifted off by the same black hole letting a sole helium core as orbits of star around a supermassive black hole may be of the elliptical sort. When small black holes are accreting material, it means a lot for them and some material thus is ejected in the form of high winds. Late studies, by 2009, have shown how the process of jets might be similar in supermassive black holes as well in stellar ones. It's well seen at some stellar-mass black holes called 'micro-quasars'. Those jets seem to regulate themselves when a wind coming from their accretion disk is circling the black hole, ejecting away as much matter as the jets are doing as the process is similar to the one of any supermassive black hole and is regulating the growth of the black holes (times are just much longer for the supermassive variety, like a hour at a micro-quasar meaning 10,000 years at the center of a galaxy!). Astronomers also have been able to study processes occurring at the base of stellar black holes jets as they undergo huge and erratic fluctuations on timescales ranging from 11 seconds to a few hours and a variability of the base's width. A jet's base width may be of the order of 15,000 miles (24,140 kilometers) and vary with changes as large as a factor of 10 or more. As the densest chunks of matter approach the black hole, they give off especially bright X-rays and flares of visible light lag an average of 0.1 seconds behind, the time that it takes for a jet to start glowing. Huge bullets of gas may be driven away from the gravitation of a feeding galactic black hole where turbulences exist and a corona of very hot gas is seen hovering above the disk of matter that is falling into the black hole. Magnetic fields could be helping power galactic black holes by confining dust in the torus at a distance allowing for the latter to feed black hole. The emission of black holes, generally, changes in behavior from strong radiation to strong outflow both in stellar-mass, or galactic ones but on different timescale. Or the emission from a quasar passes from a broad beam to a more concentrated output in the form of collimated jets of particles. Generally, black holes are spotted by dedicated tools, like the Chandra X-Ray Observatory, this space telescope operating in the X-ray, as their activity is mostly yielding in this wavelengths range. Galactic black holes, which are usually 'dormant' and quiet, may -about once every 10,000 years or even 100,000 years- swallow a passing star. All types of radiations then increase as the temperature is decreasing on a timescale of some days. Such a event emits X-rays flares until that the star has been eventually swallowed. The destruction of a star as it plunges into the central black hole of its galaxy has the star ripped apart by the intense tidal forces, and its gas continue to stream inward as the move may last months. A delay of 2–3 months may occur between the object being disrupted and the heating of the debris in the vicinity of the black hole. During tidal disruptions some of the stellar debris are flung outward at high speeds, while the rest falls toward the black hole. This causes a distinct X-ray flare that can last for a few years. Soon after this surge of X-rays, the amount of light decreases as the material falls beyond the black hole's event horizon, the point beyond which no light can escape. Gas often falls toward black holes by spiraling inward in a disk but how this process starts remains a mystery. The X-rays being produced come from material that is either very close to or is actually in the smallest possible stable orbit around the black hole. The X-ray data also suggest the presence of a wind moving away from the black hole as it is not moving fast enough to escape the black hole’s gravitational grasp. A explanation for that relatively low speed is that gas from the disrupted star is following an elliptical orbit around the black hole and is at the greatest distance from the black hole where it is traveling the slowest. The object, when of a large planetary mass, may be swallowed in terms of its external layers only as its denser core is left orbiting the black hole. Feeding events may be detected more frequently, every few years in neighbouring galaxies. As far as our Milky Way Galaxy's black hole is concerned, studies by the NASA in spring 2008 have shown that the last episode of activity of it occurred about 300 years ago, with a giant star exploding nearby and sweeping gas into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber. The supermassive black hole at the center of our Milky Way Galaxy, which is known as Sagittarius A* (or Sgr A*, for short) is a relatively calm one as that might be due to that it parts between two regions in terms of energy. A hot, inner region provides for collisions between particles and a transfer of energy towards a cooler, outer region (which includes the black hole's fuel source) via a process called conduction. Such a additional outward pressure makes nearly all of the gas of the outer region to flow away from the black hole. That further is provided a good explanation for the extended shape of hot gas detected around Sgr A*. The famed so-called 'event horizon' of a black hole may be defined like the closest a object can get to a black hole before it becomes impossible to escape. The larger and more massive a black hole is, the wider its event horizon

For long, astronomers thought that supermassive black holes were mostly powered in relation to the rotation speed of them as new studies are showing that most ferocious jets-featuring black hole are the supermassive black holes that spin backwards. The black holes can spin either in the same direction than their accretion disks, called prograde black holes, or against the flow - the retrograde black holes. The supermassive black holes might too evolve over time from a retrograde to a prograde state. How fast a black hole spins is thought to reflect the history of its formation. A black hole that grows steadily, fed by a uniform flow of matter spiralling in, should end up spinning rapidly. Rapid rotation could also be the result of two smaller black holes merging. On the other hand, a black hole buffeted by small clumps of material hitting from all directions will end up rotating relatively slowly. Some black holes are spinning almost as fast as Einstein's theory of gravity will allow. Backward black holes shoot more powerful jets because there's more space between the black hole and the inner edge of the orbiting disk. This gap provides more room for the build-up of magnetic fields, which fuel the jets, an idea known as the Reynold's conjecture. Due to jets and winds playing key roles in shaping the fate of galaxies, like slowing or even preventing the formation of stars not just in a host galaxy itself, but also in other nearby galaxies, or transporting huge amounts of energy to the outskirts of their host galaxy, displace large volumes of the intergalactic gas, etc., such remarks could be of importance in the understanding of how galaxies evolve over time

When it comes to assumptions, black holes are believed to be the place of interconnection with places where the laws of physics might be completely different, or where it might be possible to travel in time. One thing is certain however. Should you venture for a long space journey, never approach a black hole of any kind. You would never be back! General Relativity equations are not relevant to describe the physics of a black hole as it can only provide a solution only for how a black hole ends by the end of its evolution. That state is called a 'Kerr spacetime.' Time at that stage is just becoming still as nothing then changes over time and no further process occurs with the black hole

->Less Medium-Sized Black Holes Than Thought?
From an observation inside a globular cluster, it seems like they don't possess any medium-sized black hole as this category might in fact to be found either in the outskirts of galaxies, in the surrounding 'dwarf' galaxies, or in the remnants of dwarf galaxies being swallowed by a larger galaxy, being faint and difficult to find. Only about half a dozen objects have been really found in the middle ground

->Merging Galactic Black Holes and Gravitational Waves
In the case of two black holes merging, gravitational waves are produced according to Einstein's Relativity. Study of them will give astrophysicists unprecedented new insights into the fundamental laws of physics, the death of stars, the birth of black holes and, perhaps, the earliest moments of the Universe. Such waves are due to space and time become repeatedly flexed and warped by the gigantic masses involved. During the last few orbits around each other, both black holes can be moving at more than half the speed of light with their torus disks of hot, magnetized gas around them. Magnetized gas, or a plasma endures complex electrical and magnetic interactions known as magnetohydrodynamics. On a other hand, the disks' initial magnetic field are rapidly intensified by about 100 times as they are twisted and compressed. A cleared-out zone that extends up out of the accretion disk near the merged black hole is also created, which likely is driving particle jets. Two merging black holes at last -which occur mostly when two galaxies merge- may induce a energy which can kick the resulting black hole from the resulting galaxy, and leaving it racing along at 5,900,000 mph (9,500,000 km/s) in the void of the Universe