A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.
Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.
Objects whose gravity fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. These data can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of our Milky Way galaxy contains a supermassive black hole of about 4.3 million solar masses.
The concept of a body so massive that not even light could escape from it was put forward by the English geologist John Michell in a 1783 paper sent to the Royal Society. At that time, the Newtonian theory of gravity and the concept of escape velocity were well known. Michell computed that a body 500 times the radius of the Sun and of the same density would have at its surface an escape velocity equal to the speed of light, and therefore would be invisible. Although he thought it unlikely, Michell considered the possibility that many such objects that cannot be seen might be present in the cosmos.
In 1796, the French mathematician Pierre-Simon Laplace promoted the same idea in the first and second edition of his book Exposition du Systeme du Monde. It disappeared in later editions. The whole idea gained little attention in the 19th century, since light was thought to be a massless wave, not influenced by gravity.
In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties. This solution had a peculiar behavior at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time.
In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington–Finkelstein coordinates), although it took until 1933 for Georges LemaĒtre to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.
In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 solar masses) has no stable solutions. His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse. They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star, which is itself stable because of the Pauli exclusion principle.
But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes. Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars", because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. p the collapse. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse.
In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction". This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into a black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.
In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged. Through the work of Werner Israel, Brandon Carter,and David Robinson the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.
These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967, which, by 1969, were shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.
At first, it was suspected that the strange features of the black hole solutions were pathological artifacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late 1960s Roger Penrose and Stephen Hawking used global techniques to prove that singularities appear generically.
The first use of the term "black hole" in print was by journalist Ann Ewing in her article "'Black Holes' in Space", dated 18 January 1964, which was a report on a meeting of the American Association for the Advancement of Science.
John Wheeler used term "black hole" a lecture in 1967, leading some to credit him with coining the phrase. After Wheeler's use of the term, it was quickly adopted in general use.
Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics. These laws describe the behavior of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature.
The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.
No Black Holes Exist, Says Stephen Hawking - At Least Not Like We Think National Geographic - January 27, 2014
Black holes do not exist - at least, not as we know them, says renowned physicist Stephen Hawking, potentially provoking a rethink of one of space's most mysterious objects. A new study from Hawking also says that black holes may not possess "firewalls," destructive belts of radiation that some researchers have proposed would incinerate anything that passes through them but others scientists deem an impossibility. The conventional view of black holes posits that their gravitational pull is so powerful that nothing can escape from them - not even light, which is why they're called black holes. The boundary past which there is supposedly no return is known as the event horizon. In this conception, all information about anything that ventures past a black hole's event horizon is destroyed. On the other hand, quantum physics, the best description so far of how the universe behaves on a subatomic level, suggests that information cannot ever be destroyed, leading to a fundamental conflict in theory. Now Hawking is suggesting a resolution to the paradox: Black holes do not possess event horizons after all, so they do not destroy information.
Black Holes, Event Horizons, Stephen Hawking, Reality, Illusion
The defining feature of a black hole is the appearance of an event horizon - a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.
As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.
To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole. Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.
At the same time, all processes on this object slow down, for a fixed outside observer, causing emitted light to appear redder and dimmer, an effect known as gravitational redshift. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.
On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time without noting any singular behavior. In particular, he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.
The shape of the event horizon of a black hole is always approximately spherical. For non-rotating (static) black holes the geometry is precisely spherical, while for rotating black holes the sphere is somewhat oblate.
Black hole hums B flat BBC - September 10, 2003
Astronomers have detected sound waves from a super-massive black hole. The "note" is the deepest ever detected from an object in the Universe. The black hole lives in the Perseus cluster of galaxies, located 250 million light-years away. The tremendous amounts of energy carried from the black hole by these sound waves may solve a longstanding problem in astrophysics. The pitch of the sound can be determined. Although far too low to be heard, it is calculated to be B flat.
At the center of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of space-time becomes infinite and gravitational forces become infinitely strong. Space-time inside the event horizon is peculiar in that the singularity is in every observer's future, so all particles within the event horizon move inexorably towards it. This means that there is a conceptual inaccuracy in the non-relativistic concept of a black hole as originally proposed by John Michell in 1783.
It is expected that future refinements or generalizations of general relativity (in particular quantum gravity) will change what is thought about the nature of black hole interiors. Most theorists interpret the mathematical singularity of the equations as indicating that the current theory is not complete, and that new phenomena must come into play as one approaches the singularity. The question may be largely academic, as the cosmic censorship hypothesis asserts that there are no naked singularities in general relativity: Every singularity is hidden behind an event horizon and cannot be probed.
Another school of thought holds that no singularity occurs, because of a bubble-like local inflation in the interior of the collapsing star. Radii stop converging as they approach the event horizon, are parallel at the horizon, and begin diverging in the interior. The solution resembles a wormhole (from the exterior to the interior) in a neighborhood of the horizon, with the horizon as the neck.
In Michell's theory, the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope. General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon.
According to theory, the event horizon of a black hole that is not spinning is spherical, and its singularity is (informally speaking) a single point. If the black hole carries angular momentum (inherited from a star that is spinning at the time of its collapse), it begins to drag space-time surrounding the event horizon in an effect known as frame-dragging.
This spinning area surrounding the event horizon is called the ergosphere and has an ellipsoidal shape. Since the ergosphere is located outside the event horizon, objects can exist within the ergosphere without falling into the hole. However, because space-time itself is moving in the ergosphere, it is impossible for objects to remain in a fixed position. Objects grazing the ergosphere could in some circumstances be catapulted outwards at great speed, extracting energy (and angular momentum) from the hole, hence the name ergosphere ("sphere of work") because it is capable of doing work.
The effects of a black hole's gravity as described by the Theory of Relativity cause a number of peculiar effects. An object approaching simple Schwarzschild-type (non-rotating) black hole's center will appear to distant observers as having an increasingly slow descent as the object approaches the event horizon. This is because a photon takes an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate.
From the object's frame of reference, it will cross the event horizon and reach the singularity, or center of the black hole, all within a finite amount of time. Once the object crosses over the event horizon, light will no longer escape the black hole, and the object can no longer be observed outside of the black hole. As the object continues to approach the singularity, it will elongate, and the parts closest to the singularity will begin to red shift, until they finally become invisible. Nearing the singularity, the gradient of the gravitational field from head to foot will become considerable, will stretch and tear because of tidal forces: the parts closest to the singularity feel disproportionately stronger gravitational force than those parts farther away. This process is known as spaghetti-fication.
In 1971, Stephen Hawking showed that the total area of the event horizons of any collection of classical black holes can never decrease. This sounded remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. Classically, one could violate the second law of thermodynamics by material entering a black hole disappearing from our universe and resulting in a decrease of the total entropy of the universe.
Therefore, Jacob Bekenstein proposed that a black hole should have an entropy and that it should be proportional to its horizon area. Since black holes do not classically emit radiation, the thermodynamic viewpoint was simply an analogy. However, in 1974, Hawking applied quantum field theory to the curved space-time around the event horizon and discovered that black holes can emit thermal radiation, known as Hawking radiation.
Using the first law of black hole mechanics, it follows that the entropy of a black hole is one quarter of the area of the horizon. This is a universal result and can be extended to apply to cosmological horizons such as in de Sitter spacetime. It was later suggested that black holes are maximum-entropy objects, meaning that the maximum entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the holographic principle.
Hawking radiation originates just outside the event horizon and, so far as it is understood, does not carry information from its interior since it is thermal. However, this means that black holes are not completely black: the effect implies that the mass of a black hole slowly evaporates with time. Although these effects are negligible for astronomical black holes, they are significant for hypothetical very small black holes where quantum-mechanical effects dominate. Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation. Hence, every black hole that cannot consume new mass has a finite life that is directly related to its mass.
On 21 July 2004 Stephen Hawking presented a new argument that black holes do eventually emit information about what they swallow, reversing his previous position on information loss. He proposed that quantum perturbations of the event horizon could allow information to escape from a black hole, where it can influence subsequent Hawking radiation . The theory has not yet been reviewed by the scientific community and if it is accepted it is likely to resolve the black hole information paradox. In the meantime, the announcement has attracted a lot of attention in the media.
General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse. As the mass inside that region increases, its gravity becomes stronger - or, in the language of relativity, the space around it becomes increasingly deformed. When the escape velocity at a certain distance from the center reaches the speed of light, an event horizon is formed within which matter must inevitably collapse onto a single point, forming a singularity.
A quantitative analysis of this idea led to the prediction that a star remaining about three times the mass of the Sun at the end of its evolution (usually as a neutron star), will almost inevitably shrink to the critical size needed to undergo a gravitational collapse. Once it starts, the collapse cannot be stopped by any physical force, and a black hole is created.
Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.
Supermassive black holes containing millions to billions of solar masses could also form wherever a large number of stars are packed in a relatively small region of space, or by large amounts of mass falling into a "seed" black hole, or by repeated fusion of smaller black holes. The necessary conditions are believed to exist in the centers of some (if not most) galaxies, including our own Milky Way .
Theory says that we cannot detect black holes by light that is emitted or reflected by the matter inside them. However, those objects can be inductively detected from observation of phenomena near them, such as gravitational lensing and stars that appear to be in orbit around space where there is no visible matter.
The most conspicuous effects are believed to come from matter falling into a black hole, which (like water flowing into a drain) is predicted to collect into an extremely hot and fast-spinning accretion disk around the object before being swallowed by it. Friction between adjacent zones of the disk causes it to become extremely hot and emit large amounts of X-rays. This heating is extremely efficient and can convert about 50% of the mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other predicted effects are narrow jets of particles at relativistic speeds squirting off along the disk's axis.
However, accretion disks, jets, and orbiting objects are found not only around black holes, but also around other objects such as neutron stars; and the dynamics of bodies near these non-black hole attractors is largely similar to the dynamics of bodies around black holes, and is currently a very complex and active field of research involving magnetic fields and plasma physics.
Hence, for the most part, observations of accretion disks and orbital motions merely indicate that there is a compact object of a certain mass, and says very little about the nature of that object. The identification of an object as a black hole requires the further assumption that no other object (or bound system of objects) could be so massive and compact. Most astrophysicists accept that this is the case, since according to general relativity, any concentration of matter of sufficient density must necessarily collapse into a black hole.
One important observable difference between black holes and other compact massive objects is that any in falling matter will eventually collide with the latter, at relativistic speeds, leading to irregular intense flares of X-rays and other hard radiation. Thus the lack of such flare-ups around a compact concentration of mass is taken as evidence that the object is a black hole, with no surface onto which matter can be suddenly dumped.
The formation of micro black holes on Earth in particle accelerators have been tentatively reported, but not yet confirmed. So far there are no observed candidates for primordial black holes. Read more ...
Black holes heading for 'massive collision,' says astronomer CNN - September 22, 2015
A really, really big bang not due for a hundred millennia may help astronomers test one of the central theories of physics, according to one of the authors of a new paper. Two black holes in Virgo are heading for a massive collision. What astronomers are seeing in PG 1302-102 actually occurred when life had just emerged on Earth.
New support for converging black holes in Virgo constellation PhysOrg - September 18, 2015
Earlier this year, astronomers discovered what appeared to be a pair of supermassive black holes circling toward a collision so powerful it would send a burst of gravitational waves surging through the fabric of space-time itself.
Astronomers find teeny supermassive black hole Telegraph - August 12, 2015
In a dwarf, disc galaxy 340 million light years away, University of Michigan astronomers have found the smallest black hole ever observed in the centre of a galaxy. At just 50,000 times the mass of the sun, it's more than two times smaller than any other known object of its kind. It's a full 100,000 times less massive than the largest black holes at the heart of other galaxies.
Ancient black hole defied rules of galaxy formation PhysOrg - July 9, 2015
Black holes can be found at the centers of most galaxies. Most have little mass compared with their host galaxy. ETH researchers, however, have discovered a particularly massive black hole, which clearly grew so quickly that the host galaxy was not able to keep pace. This calls into question previous thinking on the co-evolution of galaxies and their central black holes.
Black hole awakens, erupts after 26 years CNN - July 7, 2015
Astronomers use cosmic gravity to create a 'black-hole-scope' PhysOrg - July 7, 2015
The Integral, Fermi and Swift space observatories have used the magnifying power of a cosmic lens to explore the inner regions of a supermassive black hole. Massive cosmic objects, from single stars to galaxy clusters, bend and focus the light that flows around them with their gravity, acting like giant magnifying glasses. Dr Neronov and colleagues used a star sitting between their target and Earth to 'zoom in' to the black hole and measure the size of the jet-emitting region - the first time this method has ever been used with gamma rays.
Black holes are not ruthless killers, but instead benign hologram generators Science Daily - June 17, 2015
New research in theoretical physics shows that black holes aren't the ruthless killers we've made them out to be, but instead benign - if imperfect - hologram generators. The world could have been captured by a black hole, and we wouldn't even notice, according to a new theoretical perspective. Mathur and his team have been expanding on their fuzzball theory, too, and they've come to a completely different conclusion. They see black holes not as killers, but rather as benign copy machines of a sort. They believe that when material touches the surface of a black hole, it becomes a hologram, a near-perfect copy of itself that continues to exist just as before.
Intense magnetic field close to supermassive black hole Science Daily - April 16, 2015
Astronomers have revealed an extremely powerful magnetic field, beyond anything previously detected in the core of a galaxy, very close to the event horizon of a supermassive black hole. This artist's impression shows the surroundings of a supermassive black hole, typical of that found at the heart of many galaxies. The black hole itself is surrounded by a brilliant accretion disc of very hot, infalling material and, further out, a dusty torus. There are also often high-speed jets of material ejected at the black hole's poles that can extend huge distances into space. Observations with ALMA have detected a very strong magnetic field close to the black hole at the base of the jets and this is probably involved in jet production and collimation.
Monster black hole discovered at cosmic dawn Science Daily - February 26, 2015
The discovery of the brightest quasar in the early universe, powered by the most massive black hole yet known at that time presents a puzzle to researchers: How could something so massive and luminous form so early in the universe, only 900 million years after the Big Bang?
Black holes do not exist where space and time do not exist, says new theory PhysOrg - January 31, 2015
The quintessential feature of a black hole is its "point of no return," or what is more technically called its event horizon. When anything - a star, a particle, or wayward human - crosses this horizon, the black hole's massive gravity pulls it in with such force that it is impossible to escape. At least, this is what happens in traditional black hole models based on general relativity. In general, the existence of the event horizon is responsible for most of the strange phenomena associated with black holes.
The black hole at the birth of the Universe PhysOrg - August 8, 2014
Our universe may have emerged from a black hole in a higher-dimensional universe, propose a trio of Perimeter Institute researchers. The big bang poses a big question: if it was indeed the cataclysm that blasted our universe into existence 13.7 billion years ago, what sparked it? In our three-dimensional universe, black holes have two-dimensional event horizons – that is, they are surrounded by a two-dimensional boundary that marks the "point of no return." In the case of a four-dimensional universe, a black hole would have a three-dimensional event horizon.
Surprisingly strong magnetic fields challenge black holes' pull PhysOrg - June 4, 2014
A new study of supermassive black holes at the centers of galaxies has found magnetic fields play an impressive role in the systems' dynamics. In fact, in dozens of black holes surveyed, the magnetic field strength matched the force produced by the black holes' powerful gravitational pull
Monster Black Hole Reveals its Pearly Bling Discovery - April 1, 2014
A massive black hole has, for the first time, revealed its bling - a string of star clusters arranged like a stellar String of Pearls. Using the infrared telescopes at the Keck Observatory atop Hawaii’s Mauna Kea, astronomers were able to cut through the light-blocking dust surrounding the supermassive black hole in the center of the galaxy NGC2110 in the constellation of Orion. NGC2110 is 120 million light-years away.
Black hole makes 'String of Pearls' clusters PhysOrg - April 1, 2014
Huge young star clusters resembling a string of pearls around a black hole in the centre of a galaxy 120 million light-years away have been discovered by researchers at Swinburne University of Technology. The galaxy, called NGC2110, is in the constellation of Orion. Supermassive black holes - condensations of matter so dense that not even light can escape from its gravity - are thought to be at the centre of all large galaxies.
Quantum Supergravity Could Explain Weirdness of Black Holes Wired - March 19, 2014
Physicists have searched for a theory of quantum gravity for 80 years. Though gravitons are individually too weak to detect, most physicists believe the particles roam the quantum realm in droves, and that their behavior somehow collectively gives rise to the macroscopic force of gravity, just as light is a macroscopic effect of particles called photons. But every proposed theory of how gravity particles might behave faces the same problem: upon close inspection, it doesn’t make mathematical sense. Calculations of graviton interactions might seem to work at first, but when physicists attempt to make them more exact, they yield gibberish - an answer of infinity...
First Black Hole Orbiting a 'Spinning' Star Science Daily - January 16, 2014
Scientists have discovered the first binary system ever known to consist of a black hole and a 'spinning' star -- or more accurately, a Be-type star. Although predicted by theory, none had previously been found. Be-type stars are quite common across the Universe. In our Galaxy alone more than 80 of them are known in binary systems together with neutron stars. 'Their distinctive property is their strong centrifugal force: they rotate very fast, close to their break-up speed. It's like they were cosmic spinning tops.
Do Black Holes Come in Size Medium? Science Daily - November 29, 2013
Black holes can be petite, with masses only about 10 times that of our sun -- or monstrous, boasting the equivalent in mass up to 10 billion suns. Do black holes also come in size medium? NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, is busy scrutinizing a class of black holes that may fall into the proposed medium-sized category.
Do Black Holes Have 'Hair'? New Hypothesis Challenges 'Clean' Model Science Daily - September 30, 2013
A black hole. A simple and clear concept, at least according to the hypothesis by Roy Kerr, who in 1963 proposed a "clean" black hole model, which is the current theoretical paradigm. From theory to reality things may be quite different. According to the traditional model, black holes are defined by only two quantities: mass and angular momentum (a black hole rotation velocity).
The Dragon Awakens: Colossal Explosion from Supermassive Black Hole at Centre of Galaxy Revealed Science Daily - September 24, 2013
Two million years ago a supermassive black hole at the heart of our galaxy erupted in an explosion so immensely powerful that it lit up a cloud 200,000 light years away. The finding is an exciting confirmation that black holes can 'flicker', moving from maximum power to switching off over, in cosmic terms, short periods of time. The region around the galaxy's supermassive black hole and the black hole is called Sagittarius A* (pronounced Sagittarius A-star). It emits radio, infrared, ultraviolet, x-ray and gamma ray emissions. Flickers of radiation rise up when small clouds of gas fall onto the hot disk of matter that swirls around the black hole.
How Giant Black Holes Spin: New Twist Revealed Live Science - July 31, 2013
Astronomers watched as a black hole that sits at the core of a spiral galaxy 500 million light-years from Earth gobbled up gas and dust from its surrounding accretion disk. They were able to measure the distance between the inner edge of the disk and the black hole, which, in turn, allowed them to estimate the black hole's spin.
Space-time loops may help explain black holes MSNBC - July 12, 2013
Black holes can't fully be described by general relativity, but physicists hope to understand the inner workings of these odd objects by applying a theory called loop quantum gravity. Physics cannot describe what happens inside a black hole. There, current theories break down, and general relativity collides with quantum mechanics, creating what's called a singularity, or a point at which the equations spit out infinities.
Spin up of a Supermassive Black Hole NASA - March 12, 2013
How fast can a black hole spin? If any object made of regular matter spins too fast -- it breaks apart. But a black hole might not be able to break apart -- and its maximum spin rate is really unknown. Theorists usual model rapidly rotating black holes with the Kerr solution to Einstein's General Theory of Relativity, which predicts several amazing and unusual things. Perhaps its most easily testable prediction, though, is that matter entering a maximally rotating black hole should be last seen orbiting at near the speed of light, as seen from far away. This prediction was tested recently by NASA's NuSTAR and ESA's XMM satellites by observing the supermassive black hole at the center of spiral galaxy NGC 1365. The near light-speed limit was confirmed by measuring the heating and spectral line broadening of nuclear emissions at the inner edge of the surrounding accretion disk. Pictured above is an artist's illustration depicting an accretion disk of normal matter swirling around a black hole, with a jet emanating from the top. Since matter randomly falling into the black hole should not spin up a black hole this much, the NuSTAR and XMM measurements also validate the existence of the surrounding accretion disk.
How supermassive black holes came into existence shortly after the Big Bang PhysOrg - December 13, 2011
Researchers at Carnegie Mellon University's Bruce and Astrid McWilliams Center for Cosmology have discovered what caused the rapid growth of early supermassive black holes - a steady diet of cold, fast food. Computer simulations, completed using supercomputers at the National Institute for Computational Sciences and the Pittsburgh Supercomputing Center, and viewed using CMU's GigaPan technology, show that thin streams of cold gas flow uncontrolled into the center of the first black holes, causing them to grow faster than anything else in the universe.
Astronomers discover biggest black holes ever PhysOrg - December 5, 2011
University of California, Berkeley, astronomers have discovered the largest black holes to date two monsters with masses equivalent to 10 billion suns that are threatening to consume anything, even light, within a region five times the size of our solar system. These black holes are at the centers of two galaxies more than 300 million light years from Earth, and may be the dark remnants of some of
Astronomers Find Largest, Oldest Mass of Water in Universe Live Science - July 23, 2011
Astronomers have discovered the largest and oldest mass of water ever detected in the universe - a gigantic, 12-billion-year-old cloud harboring 140 trillion times more water than all of Earth's oceans combined. "This discovery pushes the detection of water one billion years closer to the Big Bang than any previous find.
Radio telescopes capture best-ever snapshot of black hole jets PhysOrg - May 21, 2011
An international team, including NASA-funded researchers, using radio telescopes located throughout the Southern Hemisphere has produced the most detailed image of particle jets erupting from a supermassive black hole in a nearby galaxy.
Mini black holes that look like atoms could pass through Earth daily PhysOrg - May 13, 2011
In a new study, scientists have proposed that mini black holes may interact with matter very differently than previously thought. If the proposal is correct, it would mean that the time it would take for a mini black hole to swallow the Earth would be many orders of magnitude longer than the age of the Universe.
The impact of double black holes and radio galaxies in the Milky Way PhysOrg - January 4, 2011
Radio galaxies beam as much as one trillion solar-luminosities of radiation into space at radio wavelengths. They are therefore cosmic beacons, and the light from the most distant ones known was emitted back when the universe was only a few billions of years old (compared with its age today of about 13.7 billion years). The origin of this intense emission is thought to lie in the hot environment of a massive black hole at the galaxy's nucleus, with the radio emission being produced by electrons moving rapidly in strong magnetic fields. Astronomers seeking to better understand galaxies in general, and the context of the Milky Way's origins, want to know when and how radio galaxies formed, how they evolved, and how they impact their environments.
The music of gravitational waves PhysOrg - November 24, 2010
This artist's concept shows the proposed LISA mission, which would consist of three distinct spacecraft, each connected by laser beams. It would be the first space-based mission to attempt the detection of gravitational waves -- ripples in space-time that are emitted by exotic objects such as black holes. A team of scientists and engineers at NASA's Jet Propulsion Laboratory has brought the world one step closer to "hearing" gravitational waves -- ripples in space and time predicted by Albert Einstein in the early 20th century. Studies of these cosmic waves began in earnest decades ago when, in 1974, researchers discovered a pair of orbiting dead stars -- a type called pulsars -- that were spiraling closer and closer together due to an unexplainable loss of energy. That energy was later shown to be in the form of gravitational waves. This was the first indirect proof of the waves, and ultimately earned the 1993 Nobel Prize in Physics.
Black Hole Blasts Superheated Early Universe National Geographic - October 9, 2010
Monster galaxies with supermassive black hole hearts released fierce blasts that superheated the early universe, new Hubble observations suggest. The scorching conditions also stunted the growth of smaller dwarf galaxies, the new research shows. Between 11.7 to 11.3 billion years ago, ultraviolet (UV) light emitted by quasars - enormous galaxies with supermassive black holes at their centers - stripped electrons of cosmic helium, according to observations made with the Cosmic Origins Spectrograph (COS) on NASA's Hubble Space Telescope. The big bang that created our universe occurred around 13.7 billion years ago. The electron-stripping process, known as ionization, heated the helium gas from 18,000 to nearly 40,000 degrees Fahrenheit (10,000 to 22,000 degrees Celsius).
Black hole mystery unveiled by magnetic star discovery BBC - August 18, 2010
Black hole blows huge gas bubble BBC - July 8, 2010
A small black hole has been observed blowing a vast bubble of hot gas 1,000 light-years across.The gas is expanding because it is being heated by powerful particle "jets" being released by the black hole.
Scientists Create Artificial Mini 'Black Hole' Science Daily - June 3, 2010
Chinese researchers have successfully built an electromagnetic absorbing device for microwave frequencies. The device, made of a thin cylinder comprising 60 concentric rings of metamaterials, is capable of absorbing microwave radiation, and has been compared to an astrophysical black hole (which, in space, soaks up matter and light).
Our Universe Was Born in a Black Hole, Theory Says Space.com - April 28, 2010
Our universe might have originated from a black hole that lies within another universe. The idea centers on how matter and energy falling into a black hole could in theory come out a "white hole" in another universe. In such a situation, both the black hole and the white hole are mouths of an Einstein-Rosen bridge, popularly known as a wormhole.
How do supermassive black holes get so big? PhysOrg - April 26, 2010
At the center of most galaxies lie supermassive black holes that can grow to become more than a billion times larger than our Sun. However, astrophysicists don't fully understand the formation and evolution of supermassive black holes - specifically, how swirling gas from the galaxy loses its large angular momentum to allow it to be consumed by the black hole. Eventually, the stellar disk grows in size to stretch over a distance of dozens of light years from the center of the galaxy. Once it becomes large enough, its eccentric shape pulls unevenly on the incoming gas. This torque causes different gas streams to collide, reducing the gas' momentum and allowing it to flow close enough to the black hole (less than one light year) to allow the black hole's gravity to dominate and swallow the gas. The researchers' simulations showed that this process could enable black holes to consume several solar masses of gas each year, which could have helped Andromeda's black hole to gain much of its mass.
Immaculate Black Holes Found Near Universe's Conception National Geographic - March 19, 2010
A newfound pair of ancient, supermassive black holes may lift the veil on how stars and galaxies form, a new study says. That's because the black holes, which belong to a special group known as quasars, are what astronomers are calling the first "immaculate" - and thus unobscured by dust clouds - quasars ever found. Quasars lie at the hearts of galaxies, and are thousands to millions of times more massive than stellar black holes, which are created when huge stars die.
Black Holes in Star Clusters Stir Up Time and Space Science Daily - December 21, 2009
By modeling the behavior of stars in clusters, the Bonn team finds that they are ideal environments for black holes to coalesce. These merger events produce ripples in time and space (gravitational waves) that could be detected by instruments from as early as 2015. Clusters of stars are found throughout our own and other galaxies and most stars are thought to have formed in them. The smallest looser 'open clusters' have only a few stellar members, whilst the largest tightly bound 'globular clusters' have as many as several million stars. The highest mass stars in clusters use up their hydrogen fuel relatively quickly (in just a few million years). The cores of these stars collapse, leading to a violent supernova explosion where the outer layers of the star are expelled into space. The explosion leaves behind a stellar remnant with gravitational field so strong that not even light can escape -- a black hole.
Black Hole Found to Be Much Closer to Earth Than Previously Thought Science Daily - December 14, 2009
An international team of astronomers has accurately measured the distance from Earth to a black hole for the first time. Without needing to rely on mathematical models the astronomers came up with a distance of 7800 light years, much closer than had been assumed until now. The researchers achieved this breakthrough by measuring the radio emissions from the black hole and its associated dying star. Due to the much lower error margin), astronomers can now gain a better picture of how black holes evolve. Moreover, an exact distance is important for measurements of the black hole's spin.
Suzaku catches retreat of a black hole's disk PhysOrg - December 10, 2009
Studies of one of the galaxy's most active black-hole binaries reveal a dramatic change that will help scientists better understand how these systems expel fast-moving particle jets. Binary systems where a normal star is paired with a black hole often produce large swings in X-ray emission and blast jets of gas at speeds exceeding one-third that of light. What fuels this activity is gas pulled from the normal star, which spirals toward the black hole and piles up in a dense accretion disk.
Galaxy Collision Switches on Black Hole PhysOrg - December 10, 2009
This composite image of data from three different telescopes shows an ongoing collision between two galaxies, NGC 6872 and IC 4970. This composite image of data from three different telescopes shows an ongoing collision between two galaxies, NGC 6872 and IC 4970.
Double Nucleus Galaxies: Ravenous Black Holes And Ripples In Space-Time Continuum Science Daily - September 15, 2009
It may sound like science fiction, but freakish galactic events such as ravenous black holes and ripples in the space-time continuum could be happening all around us. according to new research from Swinburne University of Technology. In a study published in the Monthly Notices of the Royal Astronomical Society, Swinburne researchers examined 50 regular galaxies to determine their composition and structure. The researchers, Associate Professor Alister Graham and Dr Lee Spitler, found that 12 of these galaxies contained a double nucleus - that is, they had both a super massive black hole and a dense star cluster containing up to ten million stars at their centre.
First black holes born starving PhysOrg - August 11, 2009
The first black holes in the universe had dramatic effects on their surroundings despite the fact that they were small and grew very slowly, according to recent supercomputer simulations.
NASA's Spitzer Images Out-of-This-World Galaxy Science Daily - August 5, 2009
NASA's Spitzer Space Telescope has imaged a wild creature of the dark - a coiled galaxy with an eye-like object at its center. The galaxy, called NGC 1097, is located 50 million light-years away. It is spiral-shaped like our Milky Way, with long, spindly arms of stars. The "eye" at the center of the galaxy is actually a monstrous black hole surrounded by a ring of stars. In this color-coded infrared view from Spitzer, the area around the invisible black hole is blue and the ring of stars, white. The black hole is huge, about 100 million times the mass of our sun, and is feeding off gas and dust along with the occasional unlucky star. Our Milky Way's central black hole is tame by comparison, with a mass of a few million suns.
New Class of Black Hole Found National Geographic - July 1, 2009
An extraordinarily bright object in a galaxy 290 million light-years away could be a new type of black hole - one that Goldilocks would approve of. Scientists have been trying to confirm whether intermediate-mass black holes, those that are not too light and not too heavy, really exist. The newfound object, dubbed HLX-1, is "the best candidate yet" for this proposed black hole class. Smaller, so-called stellar-mass black holes are created by dying stars and only reach 20 to 30 times the mass of our sun. Supermassive black holes thought to sit at the centers of most galaxies, meanwhile, clock in at millions to billions of solar masses.
Physicists create 'black hole for sound' New Scientist - June 17, 2009
An artificial black hole that traps sound instead of light has been created in an attempt to detect theoretical Hawking radiation. The radiation, proposed by physicist Stephen Hawking more than 30 years ago, causes black holes to evaporate over time. Astrophysical black holes are created when matter becomes so dense that it collapses to a point called a singularity. The black hole's gravity is so great that nothing - not even light - can escape from a boundary around it called an event horizon. But physicists have also been developing 'black holes' for sound. They do this by coaxing a material to move faster than the speed of sound in that medium, so that sound waves traveling within it cannot keep up, like fish swimming in a fast-moving stream. The sound is effectively trapped in the stream-like event horizon.
Black hole spews water vapor BBC - April 22, 2009
Astronomers have found the most distant evidence of water in the Universe, a major conference has been told. The vapor is thought to be present in a jet ejected from a supermassive black hole at the centre of a galaxy that is billions of light-years away. The discovery, by a US-European team, was announced at the European Week of Astronomy and Space Science meeting. The water was emitted from the black hole when the Universe was only about 2.5 billion years old. This is about one fifth of the Universe's current age, astronomers say. The water's signature, seen at radio wavelengths, is only now being detected because of the huge distance in space between the black hole and Earth.
The View Near a Black Hole NASA - April 19, 2009
In the center of a swirling whirlpool of hot gas is likely a beast that has never been seen directly: a black hole. Studies of the bright light emitted by the swirling gas frequently indicate not only that a black hole is present, but also likely attributes. The gas surrounding GRO J1655-40, for example, has been found to display an unusual flickering at a rate of 450 times a second. Given a previous mass estimate for the central object of seven times the mass of our Sun, the rate of the fast flickering can be explained by a black hole that is rotating very rapidly. What physical mechanisms actually cause the flickering -- and a slower quasi-periodic oscillation (QPO) -- in accretion disks surrounding black holes and neutron stars remains a topic of much research.
Dancing black hole twins spotted BBC - March 4, 2009
Researchers have seen the best evidence yet for a pair of black holes orbiting each other within the same galaxy. While such "binary systems" have been postulated before, none has ever been conclusively shown to exist. The new black hole pair is dancing significantly closer than the prior best binary system candidate. The work, published in the journal Nature, is in line with the theory of the growth of galaxies, each with a black hole at their centre. The theory has it that as galaxies near one another, their central black holes should orbit each other until merging together. But evidence for black holes nearing and orbiting has so far been scant.
Monster Black Holes Spawned Early Galaxies National Geographic - January 8, 2009
Monster black holes served as seeds from which early galaxies sprouted, new research suggests. The discovery could solve the cosmic chicken-and-egg riddle of which came first - galaxies or the supermassive black holes nestled in their cores. Supermassive black holes have masses equal to a billion suns or more, and they have been detected at the center of many large galaxies, including our own Milky Way. Previous studies show an intriguing link between the masses of black holes and the central "bulges" of stars and gas in their resident galaxies. Regardless of their sizes or ages, the bulges of large galaxies appear to be about 700 times more massive than their central black holes.
There is a giant black hole at the center of our galaxy, a study has confirmed. BBC - December 10, 2008
There is a giant black hole at the centre of our galaxy, a 16-year study by German astronomers has confirmed. They tracked the movement of 28 stars circling the centre of the Milky Way, using two telescopes in Chile. The black hole, said to be 27,000 light years from Earth, is four million times bigger than the Sun, according to the paper in The Astrophysical Journal. Black holes are objects whose gravity is so great that nothing - including light - can escape them.
Supermassive black hole at the Center of the Milky Way NASA - December 11, 2008
At the center of our Milky Way Galaxy lies a supermassive black hole. Once a controversial claim, this conclusion is now solidly based on 16 years of observations that map the orbits of 28 stars very near the galactic center. Using European Southern Observatory telescopes and sophisticated near infrared cameras, astronomers patiently measured the positions of the stars over time, following one star, designated S2, through a complete orbit as it came within about 1 light-day of the center of the Milky Way. Their results convincingly show that S2 is moving under the influence of the enormous gravity of a compact, unseen object -- a black hole with 4 million times the mass of the Sun. Their ability to track stars so close to the galactic center accurately measures the black hole's mass and also determines the distance to the center to be 27,000 light-years. This deep, near-infrared image shows the crowded inner 3 light-years of the central Milky Way. Spectacular time-lapse animations of the stars orbiting within light-days of the galactic center can be found here.
Scientists find black hole 'missing link' PhysOrg - September 17, 2008
For the first time the researchers have discovered that a strong X-ray pulse is emitting from a giant black hole in a galaxy 500 million light years from Earth. The pulse has been created by gas being sucked by gravity on to the black hole at the centre of the REJ1034+396 galaxy. X-ray pulses are common among smaller black holes, but the Durham research is the first to identify this activity in a super-massive black hole. Most galaxies, including the Milky Way, are believed to contain super-massive black holes at their centres.
Closest Look Ever at the Edge of a Black Hole PhysOrg - September 3, 2008
Astronomers have taken the closest look ever at the giant black hole in the center of the Milky Way. By combining telescopes in Hawaii, Arizona, and California, they detected structure at a tiny angular scale of 37 micro-arcseconds - the equivalent of a baseball seen on the surface of the moon, 240,000 miles distant. These observations are among the highest resolution ever done in astronomy.
Closest Look Yet at Milky Way's Black Hole Live Science - September 3, 2008
Photo: Star Portrait Reveals "Family Tree" National Geographic - August 22, 2008
Several generations of stars "pose" for a family portrait amid curtains of clouds in the star-forming region called W5, about 6,500 light-years away. The composite image was released today to mark the upcoming five-year anniversary of the Spitzer Space Telescope, which lifted off from Cape Canaveral Air Force Station in Florida on August 25, 2003. Since its launch, Spitzer's infrared images have been helping astronomers peer through gases and dust that can block visible light, revealing distant cosmic objects in high detail.
How stars form amid black hole chaos MSNBC - August 21, 2008
Deep in the center of our galaxy, circling suspiciously close to the giant black hole lurking there, is a group of massive stars. Now scientists have designed a model that shows for the first time how these stars might have formed in such an extreme environment. Astronomers have long puzzled how these massive stars came to be in the vicinity of a huge black hole. They couldn't have formed as most stars do, from a tenuous cloud of gas, because this cloud would have been ripped apart by the savage gravitational forces from the black hole nearby. One guess was that the stars originally formed elsewhere as a cluster and later spiraled inward. But no trace has been found of the trail of stars this process would have left behind.
Most Massive Stellar Black Hole Found in Binary System National Geographic - October 17, 2007
The binary system M33 X-7 contains a massive black hole (right) and a huge star locked in tight orbit, as seen in this artist's conception. At roughly 16 times the mass of the sun, the black hole is the most massive known to have formed from the collapse of a star's core, astronomers recently announced. Its companion star is about 70 times the mass of the sun, making it the most massive star in a binary system containing a black hole.
Black Hole Eclipse NASA - April 13, 2007
According to these measurements, the source of X-rays is about 2 billion times smaller than the host galaxy NGC 1365 and only about 10 times larger than the estimated size of the black hole's event horizon. This is consistent with theoretical predictions. n addition to measuring the size of this disk of material, Risaliti and his colleagues were also able to estimate the location of the dense gas cloud that eclipsed the X-ray source and central black hole. The Chandra data show that this cloud is one hundredth of a light year from the black hole's event horizon much closer than anyone expected. So this is a bit of a puzzle.
Black hole found in ancient lair BBC - January 5, 2007
A black hole has been found inside a compact group of ancient stars known as a globular cluster. Astronomers say the discovery is interesting because many doubted black holes could exist in such locations. Some computer simulations had suggested a newly formed black hole would simply be ejected from the cluster as a result of gravitational interactions. An international team says its work provides the first convincing evidence that some black holes might not only survive but grow and flourish in globular clusters.
Magnetism, Not Just Gravity, Makes Black Holes Suck, Study Says National Geographic - June 21, 2006
n artist's representation of GRO J1655-40, a nearby star and black hole locked in mutual orbit in our galaxy. Popular theory states that black holes suck in light and matter thanks to their strong gravitational fields. But new research suggests a mighty magnetic field helps matter fall into black holes. Black holes have a reputation for being incredibly dense, sucking in all the light and matter around them thanks to their strong gravitational fields. But new research suggests it's actually magnetic wind, not just gravity, that makes these massive collapsed stars so impossible to resist.
Chandra finds black holes are 'green' National Geographic - April 24, 2006
Supermassive black holes are actually "green," scientists announced today as they described a new study on the energy efficiency of black holes. If cars were as fuel efficient as these black holes, researchers say, the vehicles could theoretically travel over a billion miles (1.6 billion kilometers) on a gallon of gasoline. According to the study, most of the energy released by matter falling toward these supermassive black holes is in the form of high-energy jets traveling near the speed of light away from the black hole. These jets create bubbles thousands of light-years across in the hot gas in galaxies. The energy supplied to these bubbles keeps the hot gas from cooling, thereby preventing billions of new stars from forming.
Black hole mergers modelled in 3D BBC - April 19, 2006
Simulations on a supercomputer have allowed Nasa scientists to understand finally the pattern of gravitational waves produced by merging black holes. The work should help the worldwide effort that is currently underway to make the first detection of these "ripples" in the fabric of space-time. Ultra-sensitive equipment set up in the US and Europe is expected to achieve the breakthrough observation very soon. The new research will make it easier to recognize the correct signals.
Black Holes Bound to Merge Space.com - April 7, 2006
Two supermassive black holes have been found to be spiraling toward a merger, astronomers said today. The collision will create a single super-supermassive black hole capable of swallowing material equal to billions of stars, the researchers said. Mergers between black holes are thought to be one way they grow. A handful of similar setups have been observed in which black holes appear inevitably on a merger course. This pair, at the center of a galaxy cluster called Abell 400, was known to be close but their fate hadn't been determined.
Supermassive Black Hole at Center of Milky Way "Sagittarius A Star" National Geographic - November 2, 2005
Astronomers are closing in on proof that a supermassive black hole is the source of mysterious radio waves at the center of our galaxy, the Milky Way. Black holes are objects whose gravitational pull is so strong that nothing, not even light, can escape. Supermassive black holes contain the mass of millions, if not billions, of suns. Astronomers have long suspected that supermassive black holes sit at the heart of most galaxies and may be closely related to galaxy growth. But concrete proof of the existence of these black holes has remained elusive.
Scientists Watch Black Hole Born In Split-Second Light Flash Science Daily - May 17, 2005
After 30 years, they finally caught one. Scientists on Monday have for the first time detected and pinned down the location of a so-called "short" gamma-ray burst, lasting only 50 milliseconds. The burst marks the birth of a black hole. The astronomy community is buzzing with speculation on what could have caused the burst, perhaps a collision of two older black holes or two neutron stars. A multitude of follow-up observations are planned; the answer might come in a few more days. Gamma-ray bursts are the most powerful explosions known in the universe. Recently, the longer ones -- lasting more than two seconds -- have become easy prey for NASA satellites such as Swift, built to detect and quickly locate the flashes. Short bursts had remained elusive until Monday, when Swift detected one, autonomously locked onto a location, and focused its onboard telescope.
Black hole 'on a diet' BBC - January 2003
It seems the supermassive black hole that sits at the centre of our galaxy, the Milky Way, is famished. The latest data from the orbiting Chandra observatory shows the object's X-ray emissions are quite weak, suggesting relatively little gas is falling into the hole at times. Scientists think this is because previous violent explosions may have cleared matter away from the immediate vicinity of the hole. Known as Sagittarius A* or Sgr A*, because of its position in the southern sky, the black hole at our galaxy's core is calculated to have a mass 2.6 million times that of the Sun. Its presence has been established by measuring the tremendous speeds - 5,000 kilometres (3,000 miles) per second - of stars skirting the hole's edge, or event horizon.
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