Black holes are among the most mysterious and fascinating objects or the celestial things investigated by modern scientists and astrophysicists. The concepts of the black holes on investigation helped us to illustrate the effects of the theory of relativity in a most spectacular way. The strong curvature of space-time around them prevents not only all their light from reaching us, but has an equally striking effect on time. The study of black holes reveals many interesting and mysterious things regarding the universe.
As the advancements took in the study of the black holes, now a new visualization and observation of a black hole illustrate how its gravity distorts our view (it is assumed that nearby black hole the gravitational pull will be extremely high), wrapping its surrounding regions as if seen in a carnival mirror. The visualization simulates the appearance of a black hole where the infalling matter has collected into a thin, hot structure known as an accretion disk.
The black hole’s extreme gravity skews all the photons emitted by different regions of the disk, producing the misshapen appearance. Due to the extreme gravitational force, the black hole attracts all the materials, such as the dust, gas, and other stellar debris that has arrived closer to the regions of the black hole but not yet got into the black hole will form a flattened spinning band of the material or the matter around the event horizon, is called an accretion disk.
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An accretion disc is a crucial tool for the investigation of black holes. Almost everything we know about black holes and has learned about black holes is with the help of the study of accretion disc or accretion disks. Why the accretion disks are of this important? One of the most important thing that had changed the view of astronomy from the ground was when the scientists realized that there was more to see and analyze about the universe than what optical telescopes had until then allowed them to observe.
After a period of time astrophysicists have discovered that visible light was only a small fraction of the whole electromagnetic spectrum and that information travel through the universe on various wavelengths from short-range to long-range, i.e., from radio, through microwave, infrared, optical, ultraviolet, X-ray, to gamma rays. Since then, everything we learned and witnessed about the universe is with the aid of electromagnetic radiations and many mysterious aspects of the universe have learned from electromagnetic radiation.
As a matter of fact and study, the black holes do not emit any kind of radiation and that would make them impossible to study if they did not have accretion discs around them. Those black hole accretion disks are what we actually study, observe, and from what we collect and synthesize properties of their central gravitating objects.
Black Hole Accretion Disk
A black hole is a region where the gravity is found to be so strong that any light or the photons that try to escape gets dragged back. Because nothing can travel faster than light, everything else will definitely get dragged back too, if it is too near the region of the black hole. So, if any matter falls into the black hole, it would never get out of it. A black hole has always been thought of as the ultimate prison from which there is no escape!!
Let us understand, what a black hole is made of or what it consists of. According to the study of the black holes till today, it is found that it is an edge and it is called a horizon. We can say this horizon is analogous to the edge of a waterfall. A black hole is also appeared to have a thin band around it made of all the stellar debris, dust and matter that was passing through the event horizon and this band of matter which is at the edge of the horizon and has not fallen into the black hole is known as an accretion disk.
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What do we mean by the black hole accretion disk? Basically, an accretion is nothing but just a process of growth of a massive object by gravitationally attracting (using extreme gravitational forces) and collecting all the additional material or matter. Typically, this ends up forming a disk-like structure of diffused matter or the material or gas that is in orbital motion around the central accreting object. Accretion disks are the most important features in the universe and can be found even around smaller stars or stellar remnants, in close binary stars, in the centres of spiral galaxies, in quasars, and also they can be formed in gamma-ray bursts.
The accretion disk can have many forms. It can be either spherical or planar. It can also be either persistent or episodic. The usual observation for accretion is that the material flows from one celestial object to another. Then there exists a preferred direction given by the particular orbital plane of the two bodies. The flow of the matter also keeps that plane but does not extend straight from one object to the other since it has some angular momentum generated from the orbital motion of the two. It is pushed a little bit aside by Coriolis force and forms a disk around the target object and that disk is named an accretion disk.
In an identical way, the matter piles up in a dense spinning accretion disk orbiting a black hole, star or other gravitating objects. Friction between adjoining layers will cause the gas in the disk to heat up as its potential energy will slowly dissipate into heat. The gas also loses its angular momentum which allows it to get closer to the central object and orbit faster than desired time. But, the faster motion will result in more friction and as the gas gets very hot it radiates out energy (usually in the form of light). Ultimately, It depends on the mass of the central object what temperature the disk can reach, the more massive it is the lower temperature the disk has.
Luminous Black Hole Disks
The most effective collectors of matter are the most compact objects present in the cosmos, and they are definitely the black holes. The black holes are the perfect spacetime traps, it means that nothing that falls into a black hole can ever escape from it, not even light!! Therefore, black holes are indeed as black as their name indicates (i.e., the complete absence of light), and the study of black holes are very difficult for astronomers to detect. At the same time, one of the fascinating pieces of information about the black holes is, the entire situation will change dramatically once a black hole is fed with enough matter from its vicinity, then, black holes can transform their surroundings into the brightest and most spectacular regions of the cosmos!!!
There are several different ways for black holes to light up their entire cosmic neighbourhood. Some require very special and controlled circumstances, but one is universal, wherever matter falls into a black hole, then it will result in the production of thermal radiation (emitting light radiation). Matter falling towards a central object under the influence of high gravity gets accelerated to higher and higher speeds, gaining more and more kinetic energy. But once a particle of infalling matter is attracted to an accretion disk and then the particle’s motion will be disturbed.
Due to frequent collisions between all the different particles of the stellar matter, there are no well-defined simple orbits. Instead, the whole ensemble of particles is in random motion. Such random motion with eddies and instabilities is just like in a turbulent fluid and is commonplace within accretion disks. We know that according to the definition, disordered microscopic particle motion is thermal motion, and as such directly related to temperature. As the motion of the infalling particles becomes more random or chaotic, matter in the accretion disk is heated to very high temperatures, which further end up emitting the photon radiations.
The accretion disks around stellar-mass black holes i.e., black hole accretion disks have temperatures around millions of Kelvins and radiate in the form of X-rays, at the same time the accretion disks around supermassive black holes have temperatures around thousands of Kelvins and radiate in optical or ultraviolet light.
The study of black holes is not as easy as the theory suggests, it is so hard to detect a black hole and in fact, till the date, no astronomer has managed to take detailed images of the accretion flow onto a central black hole, that might require a higher resolution than current advanced telescopes can provide.
Nowadays the astrophysicists have indirect ways of testing their assumptions about what happens to the matter that is near the regions of the black holes. Using advanced technologies, such as using computer simulations, astrophysicists can predict the spectra of accretion disks, i.e., accretion disk emits electromagnetic spectrum and the way the radiation energy is distributed among the different frequencies can be studied.
The spectra emitted by the accretion disks carry a clear imprint of the local conditions, such as a strong gravitational redshift tells of the central object’s compactness, and a systematic Doppler shifts record how matter transit at nearly the speed of light (to be noted not equal to the speed of light) in the surrounding of an accretion disk. Whenever observations show the mass concentrated in the innermost region to be high enough, where there is a complete absence of light or with no luminous object visible at that particular spot, at such locations, there is a strong likelihood for the central object to be a black hole.
From this list of salient features and the characteristic properties of black holes, astronomers have a clear notion of what to look for, and, as it turns out, there are indeed objects in the night sky with exactly the required properties. In fact, for a number of required objects, the match between prediction and observation is quite impressive, many space stations are working on this rigorously. Thus, it appears that our universe does actually contain the required amount of black holes accreting matter.
Also due to the presence of high gravity around a black hole, an object in its gravitational field experiences a slowing down of time, known as gravitational time dilation, relative to observers outside the field. From the viewpoint of a distant observer, an object falling into a black hole appears to slow down and fade away after some time, eventually, it is all based on the theories that have been studied and till date, the observations are going on to understand the process of black holes. And the particles that are attracted to the black hole disk or black hole accretion disk will appear to be approaching but never quite reaching the event horizon. Ultimately, at a particular point of time just before it reaches the event horizon, it becomes so dim that it can no longer be seen (due to the time dilation effect).
The accretion disks have been a gift for many astronomers, as a recent advancement in the study of the black holes is identified with the help of the accretion disk that gave evidential information regarding the presence of gravitational waves.
Did You Know
There is a huge misconception about black holes, many think that the black holes are like cosmic vacuums that suck everything around their neighbourhood in space, but in fact, black holes are like all other objects in space, albeit with a really strong gravitational field. For example, if you replace the Sun with a black hole of equal mass, Earth would not get sucked in, it would rather continue orbiting the black hole as it orbits the Sun, today.
Black holes appear like they're sucking in matter from their surrounding, but that's a common misconception. The fact is that many companion stars shed some of their mass in the form of stellar wind, and the matter in that wind then falls into the grip of its hungry neighbour, a black hole.
Though the blackholes will not suck the entire matter present around them, one of the most interesting aspects of the black holes is that they have this incredible ability to literally stretch everything into a long spaghetti-like strand. Appropriately, this phenomenon is known as spaghettification!!