Overview of Black Hole
Blackholes are objects that have such high intense gravitational fields, they do not allow even light to escape from them. They also make it impossible for anything that falls into them to escape, because to do so, they would have to travel at speeds faster than light. No forms of matter or energy can travel faster than the speed of light, so that is why blackholes are so unusual!
Blackhole, is a cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole are formed by the death of a massive star. When such a star has exhausted the internal thermonuclear fuels in its core at the end of its life, the core becomes unstable and gravitationally collapses inward upon itself, and the star’s outer layers are blown away. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the Singularity.
In theory, blackholes are simple to describe. Just three parameters are sufficient to characterize them fully: mass, angular momentum (describing their rotation) and electric charge. Astrophysical blackholes are even simpler, as they have no charge: if they were charged, they would be quickly neutralized by the surrounding plasma.
It was once thought that blackholes do not emit anything. However, Stephen Hawking pointed out that if quantum effects are taken into account, they can radiate thermal energy and particles. This Hawking radiation carries energy away from the blackhole and reduces its mass, m. Therefore, a blackhole shrinks until it evaporates. This effect is important for very tiny blackholes, but astrophysical blackholes are very massive (about more than three times the mass of the Sun - MSun) and they would need a time much longer than the age of the Universe to evaporate, which will be approximately more than 13.8 billion years.
Schwarzschild’s solution to Einstein’s (E = mc2) equations, describes a blackhole that is not rotating, while Kerr’s solution is for rotating blackholes. It is reasonable to think that blackholes rotate: if they are formed from a rotating collapsing star or from the merger of two neutron stars, they must have preserved their rotational energy.
Blackholes can, in theory, come in any imaginable size. The size of a blackhole depends on the amount of mass it contains. It's a very simple formula, especially if the blackhole is not rotating. These 'non-rotating' blackholes are called Schwarzschild Blackholes.
The supermassive blackhole at the core of supergiant elliptical galaxy Messier 87, with a mass ~7 billion times the Sun's, as depicted in the first image released by the Event Horizon Telescope - 10 April 2019.
Image credit: By Event Horizon Telescope ESO Article, ESO TIF, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=77925953
The strange Fact About Blackhole
The paragraphs below was a quotes from “A General Introduction on a Blackhole article by Jean-Pierre Luminet, at Observatoire de Paris-Meudon, Departement d'Astrophysique Relativiste et de Cosmologie, CNRS UPR-176, F-92195 Meudon Cedex, France. The paragraphs tries to depict exactly how observations on Blackhole has been conducted, by using a story-telling like approach, which can also make the concept of a Blackhole more easy to understand.
History of Blackholes
Objects whose gravitational fields are so 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 blackhole 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. Blackholes were long considered a mathematical curiosity. It was not until the 1960s, that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967, sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Etymology of BlackHole
John Michell used the term dark star, and in the early 20th century, physicists used the term gravitationally collapsed object. Science writer Marcia Bartusiak traces the term blackhole to physicist Robert H. Dicke, who in the early 1960s reportedly compared the phenomenon to the Black Hole of Calcutta, notorious as a prison where people entered but never left alive.
The term blackhole was used in print by Life and Science News magazines in 1963, and by science journalist Ann Ewing in her article 'Blackholes' in Space", dated 18 January 1964, which was a report on a meeting of the American Association for the Advancement of Science held in Cleveland, Ohio.
In December 1967, a student reportedly suggested the phrase blackhole at a lecture by John Wheeler. Wheeler adopted the term for its brevity and "advertising value", and it quickly caught on, leading some to credit Wheeler with coining the phrase.
Formation of Blackholes
The traditional recipe to make a blackhole needs a single ingredient: a very massive star at the end of its life. More recently another mechanism has been found, the collision and merger of very dense objects, such as neutron stars.
Blackholes that form through these mechanisms usually have masses three to ten times greater than the Sun and they are called stellar-mass blackholes. In theory, blackholes of any size can exist. Supermassive blackholes of a million to a billion times the mass of our Sun are found at the center of (almost) all massive galaxies. How they form is still not understand, and a mystery to astronomers.
Most blackholes are formed from the remnants of a large star that dies in a supernova explosion. If the total mass of the star is large enough (more than three times the mass of the Sun, Msun - M☉), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity. However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the event horizon, time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.
Even bigger blackholes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as Gamma Ray Bursts (GMBs). Chandra and NASA's Hubble Space Telescope later collected data from the event's afterglow, and together the observations led astronomers to conclude that the powerful explosions can result when a blackhole and a neutron star collide, producing another blackhole.
Blackholes 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 blackholes, supermassive blackholes of millions of solar masses (M☉) may form. There is consensus that supermassive blackholes exist in the centers of most galaxies. Below, are some of the methods that Blackholes were formed from;
High-energy collisions:
One of the process that could create a Blackhole is called High-energy collisions. In principle, blackholes could be formed in high-energy collisions that achieve sufficient density. As of 2002, no such events have been detected, either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments. This suggests that there must be a lower limit for the mass of blackholes. Theoretically, this boundary is expected to lie around the Planck mass (mP=√ħ c/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to invalidate the predictions of general relativity. This would put the creation of blackholes firmly out of reach of any high-energy process occurring on or near the Earth. However, certain developments in quantum gravity suggest that the Planck mass could be much lower: some braneworld scenarios for example put the boundary as low as 1TeV/c2. This would make it conceivable for micro blackholes to be created in the high-energy collisions that occur when cosmic rays hit the Earth's atmosphere, or possibly in the Large Hadron Collider at CERN. These theories are very speculative, and the creation of blackholes in these processes is deemed unlikely by many specialists. Even if micro blackholes could be formed, it is expected that they would evaporate in about 10−25 seconds, posing no threat to the Earth.
Gravitational collapse:
Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars, this usually occurs either because a star has too little fuel left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case, the star's temperature is no longer high enough to prevent it from collapsing under its own weight. Remnants exceeding 5 times of Solar mass are produced by stars that were over 20 times Solar mass before the collapse.
The gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass blackholes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced blackholes of up to 103 M☉. These blackholes could be the seeds of the supermassive blackholes found in the centers of most galaxies. It was further been suggested that supermassive blackholes with typical masses of ~105 M☉ could have formed from the direct collapse of gas clouds in the young universe. Some candidates for such objects have been found in observations of the young universe.
Growth:
Once a blackhole has formed, it can continue to grow by absorbing additional matter. Any blackhole will continually absorb gas and interstellar dust from its surroundings. This is the primary process through which supermassive blackholes seem to have grown. A similar process has been suggested for the formation of intermediate-mass blackholes found in globular clusters. Blackholes can also merge with other objects such as stars or even other blackholes. This is thought to have been important, especially in the early growth of supermassive blackholes, which could have formed from the aggregation of many smaller objects. This process has also been proposed as the origin of some intermediate-mass blackholes.
Chandra X-Ray Observatory image of Cygnus X-1, the first strong black hole candidate discovered.
Image credit: By NASA's Chandra X-ray Observatory
Physical Properties of a BlackHole
Astronomers can't see blackholes the way they can see stars and other objects in space. Instead, astronomers can, however, infer the presence of blackholes and study them by detecting their effect on other matter nearby. If a blackhole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a blackhole. In this case, the blackhole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that blackholes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.
Event horizon:
The defining feature of a blackhole is the appearance of an event horizon - a boundary in spacetime through which matter and light can pass only inward towards the mass of the blackhole. 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 whether such an event occurred. It's the part that can be seen from the outside. It looks like a black, spherical surface with a very sharp edge in space.
Simulated view of a blackhole Event horizon in front of the Large Magellanic Cloud.
Image credit: By Wikimedia, CC BY-SA 2.5)
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 blackhole, this deformation becomes so strong that there are no paths that lead away from the black hole.
To a distant observer, clocks near a blackhole would appear to tick more slowly than those farther away from the blackhole. Due to this effect, known as gravitational time dilation, an object falling into a blackhole appears to slow as it approaches the event horizon, taking an infinite time to reach it. At the same time, all processes on this object slow down, from the view point of a fixed outside observer, causing any light emitted by the object to appear redder and dimmer, an effect known as gravitational redshift. Eventually, the falling object fades away until it can no longer be seen. Typically, this process happens very rapidly with an object disappearing from view within less than a second.
Sngulariity:
At the center of a blackhole, as described by general relativity, may lie a gravitational singularity, a region where the spacetime curvature becomes infinite. For a non-rotating blackhole, this region takes the shape of a single point and for a rotating blackhole, it is smeared out to form a ring singularity that lies in the plane of rotation. In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the blackhole solution.
It's the place that matter goes when it falls through the event horizon. It's located at the center of the blackhole, and it has an enormous density. The singular region can thus be thought of as having infinite density.
Blackhole Artist's rendering of matter swirling around a blackhole.
Image credit: By Britannica media.
Photon sphere:
The photon sphere is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the blackhole. For non-rotating blackholes, the photon sphere has a radius 1.5 times the Schwarzschild radius. Their orbits would be dynamically unstable, hence any small perturbation, such as a particle of infalling matter, would cause an instability that would grow over time, either setting the photon on an outward trajectory causing it to escape the blackhole, or on an inward spiral where it would eventually cross the event horizon.
While light can still escape from the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the blackhole. Hence any light that reaches an outside observer from the photon sphere must have been emitted by objects between the photon sphere and the event horizon.
Ergosphere:
Rotating blackholes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging. General relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating blackhole, this effect is so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.
The ergosphere of a blackhole is a volume whose inner boundary is the blackhole's oblate spheroid event horizon and a pumpkin-shaped outer boundary, which coincides with the event horizon at the poles but noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.
Innermost stable circular orbit (ISCO):
In Newtonian gravity, test particles can stably orbit at arbitrary distances from a central object. In general relativity, however, there exists an innermost stable circular orbit (often called the ISCO), inside of which, any infinitesimal perturbations to a circular orbit will lead to inspiral into the blackhole. The location of the ISCO depends on the spin of the blackhole, in the case of a Schwarzschild blackhole (spin zero) is:
and decreases with increasing blackhole spin for particles orbiting in the same direction as the spin.
Astrophysicist, Sheperd Doeleman, head of the Event Horizon Telescope collaboration, speaks with TED's Chris Anderson about the iconic, first-ever image of a blackhole -- and the epic, worldwide effort involved in capturing it.
Video credit: TED's Talk!
Classification of Blackholes
Blackholes are classified based on their masses, using the mass of the Earth’s Sun MSun (M☉), as the unit for the measurement. Blackholes were basically groups into three (3) groups:
Stellar blackholes:
When a star burns through the last of its fuel, the object may collapse, or fall into itself. For smaller stars (those up to about three times the sun's mass), the new core will become a neutron star or a white dwarf. But when a larger star collapses, it continues to compress and creates a stellar blackhole.
Blackholes formed by the collapse of individual stars are relatively small, but incredibly dense. One of these object packs more than three times the mass of the sun into the diameter of a city. This leads to a crazy amount of gravitational force pulling on objects around the object. Stellar blackholes then consume the dust and gas from their surrounding galaxies, which keeps them growing in size.
According to the Harvard-Smithsonian Center for Astrophysics, the Milky Way contains a few hundred million stellar blackholes.
Intermediate blackholes:
Scientists once thought that blackholes came in only small and large sizes, but recent research has revealed the possibility that midsize, or intermediate, blackholes (IMBHs) could exist. Such bodies could form when stars in a cluster collide in a chain reaction. Several of these IMBHs forming in the same region could then eventually fall together in the center of a galaxy and create a supermassive blackhole.
In 2014, astronomers found what appeared to be an intermediate-mass blackhole in the arm of a spiral galaxy. Astronomers have been looking very hard for these medium-sized blackholes, study co-author Tim Roberts, of the University of Durham in the United Kingdom, said in a statement. "There have been hints that they exist, but IMBHs have been acting like a long-lost relative that isn't interested in being found".
Supermassive blackholes:
Small blackholes populate the universe, but their cousins, supermassive blackholes, dominate. These enormous blackholes are millions or even billions of times as massive as the Sun, but are about the same size in diameter. Such blackholes are thought to lie at the center of pretty much every galaxy, including the Milky Way.
Scientists aren't certain how such large blackholes spawn. Once these giants have formed, they gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to even more enormous sizes.
Supermassive blackholes may be the result of hundreds or thousands of tiny blackholes that merge together. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. Another option a Supermassive Blackhole that can be form is the collapse of a stellar cluster, a group of stars all falling together. Fourth way a supermassive blackholes could arise was from large clusters of dark matter. This is a substance that Astronomers can observe through its gravitational effect on other objects. However, Scientists don't know yet what dark matter is composed of, because it does not emit light and cannot be directly observed.
Sagittarius A*, one such supermassive black hole, exists at the centre of the Milky Way Galaxy. Observations of stars orbiting the position of Sagittarius A* demonstrate the presence of a blackhole with a mass equivalent to 4,154,000 Suns. Supermassive blackholes have been detected in other galaxies as well. In 2017, the Event Horizon Telescope obtained an image of the supermassive blackhole at the center of the M87 galaxy. That blackhole has a mass equal to six and a half billion of Suns but is only 38 billion km (24 billion miles) across. It was the first blackhole to be imaged directly. The existence of even larger blackholes, each with a mass equal to 10 billion Suns, can be inferred from the energetic effects on gas swirling at extremely high velocities around the centre of NGC 3842 and NGC 4889, galaxies near the Milky Way.
| Class | Approx. mass | Approx. radius |
|---|---|---|
| Supermassive blackhole | 105–1010MSun | 0.001–400AU |
| Intermediate-mass blackhole | 103 MSun | 103km ≈ REarth |
| Stellar blackhole | 10MSun | 30km |
Classification of BlackHole with their exact weight and radius.
Mysterious facts about blackholes
- If you fell into a blackhole, theory has long suggested that gravity would stretch you out like spaghetti, though your death would come before you reached the singularity. But a 2012 study published in the journal Nature suggested that quantum effects would cause the event horizon to act much like a wall of fire, which would instantly burn you to death.
- Blackholes don't suck. Suction is caused by pulling something into a vacuum, which the massive blackhole definitely is not. Instead, objects fall into them just as they fall toward anything that exerts gravity, like the Earth.
- The first object considered to be a blackhole is Cygnus X-1. Cygnus X-1 was the subject of a 1974 friendly wager between Stephen Hawking and fellow physicist Kip Thorne, with Hawking betting that the source was not a blackhole. In 1990, Hawking conceded defeat.
- Miniature blackholes may have formed immediately after the Big Bang. Rapidly expanding space may have squeezed some regions into tiny, dense blackholes less massive than the sun.
- If a low-mass star passes too close to a blackhole, the star can be torn apart.
- Astronomers estimates that the Milky Way has anywhere from 10 million to 1 billion stellar blackholes, with masses roughly three times that of the sun.
- Blackholes remain terrific fodder for science fiction books and movies. You can check out the movie Interstellar, which relied heavily on Thorne to incorporate science. Thorne's work with the movie's special effects team led to scientists' improved understanding of how distant stars might appear when seen near a fast-spinning blackhole.
Observations with the Hubble Space Telescope have shown dramatic evidence for the existence of blackholes in the centers of many other galaxies. These blackholes can contain more than a billion solar masses. The feeding frenzy of such supermassive blackholes may be responsible for some of the most energetic phenomena in the universe. And evidence from more recent X-ray observations is also indicates the existence of middle-weight blackholes, whose masses are dozens to thousands of times the mass of the Sun.
Blackhole acts like a graveyard to a collapsing star, but with a mysterious process that astronomers were not able to fully explain the details of what is happening once the star/objects reached the surface of event horizon.
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