Black Hole Overview

Typical Black Hole

A black hole is a spherical region of space that is so incredibly dense that not even light can escape from its surface, hence the name. The gravitational force of a black hole is extremely strong because all of its matter is concentrated in a core at its center. There are two major types of black holes plus a very few intermediate sized ones:

In the Observable Universe, there are an estimated 100 billion galaxies. Each galaxy has about 100 million stellar-mass black holes. So, somewhere out there a new stellar mass black hole is born in a supernova about every second. Since almost all galaxies (including the Milky Way) harbor a supermassive black hole at their center, the Observable Universe contains about 100 billion supermassive black holes.

History Of Black Holes

Karl Swarzschild

Eighteenth century scientists John Mitchell and Pierre-Simon Laplace surfaced the idea that if a star were sufficiently large, say 500 times the size of our sun, then its gravity would be so strong that even light could not escape. Nineteenth century scientists ignored this possibility because light had no mass and therefore was not affected by gravity according to Newton's Gravitational Law. However in 1915 Einstein announced his General Theory of Relativity which suggested that even though light is massless, light could be bent by large objects such as stars. In 1919 Sir Arthur Eddington's team proved that light indeed was bent by gravity. See Bending Of Light.

Subsequently in 1916 Karl Schwarzschild (photo at left) found a solution to Einstein's field equations. The solution had an interesting behavior where it became "singular", which in mathematics means that one or more terms in Einstein's equations become infinite. In this case it meant that a large dying star would collapse in on itself until it became infinitely heavy and formed a "central singularity". The "physical meaning" of these mathematical equations was not well understood at the time and many scientists, including Einstein, did not believe that a "real" massive body could collapse into an infinitely dense point.

Stevn Hawking

Einstein believed" singularities" were just mathematical curiosities. (Keep in mind that even today, "Black Holes" have not been positively confirmed, but there is overwhelming evidence that they exist.) However, in 1939, Robert Oppenheimer (future Father of the Atomic Bomb) and others predicted that large stars would in fact collapse into singularities and concluded that no law of physics was likely to stop at least some stars from total collapse.

The 1960's and '70's are considered the "Golden Years of Relativity" when Relativity became a mainstream subject of research. In 1967 pulsars were discovered and a few years later in 1969 they were found to be neutron stars. Like black holes were up until then, neutron stars were considered theoretical curiosities.

The term "black hole" was coined by Physicist John Archibald Wheeler during a lecture in 1967 at NASA Goddard and thereafter the name stuck. Between 1965 and 1970 Roger Penrose and Steven Hawking (photo at left) showed conclusively that there must be a singularity of infinite density and and infinite space-time curvature within a black hole. In 1969 Roger Penrose suggested that objects that fell into the outer borders of a rotating black hole could escape with more energy than they entered with. This is now know as the Penrose Process.

Perhaps the first object to be recognized as a black hole was the binary star Cygnus X-1. Its effect on its companion star suggested as early as 1971 that it must be a compact object with a mass too high for it to be a neutron star. (Today the standard way a black hole is detected is by its gravitational effect on other close by objects.) Cygnus X-1 is the brighter of the two stars in the lower center of the image to the left below.

Cygnus_X-1

Cygnus X-1 is about 6,100 light years away in the constellation Cygnus and is made up of a blue supergiant variable star orbiting its black hole. A pair of jets emanate perpendicular to the disk of the black hole. One of the jets has created a shock wave (semi-circle seen in the upper right) due to traveling greater than 100 km/s. It is probably less than 40,000 years old

In 1974 Steven Hawking combined theories of General Relativity and Quantum Mechanics to show that black holes radiate energy, (now known as Hawking Radiation) as particles are created in the black hole's vicinity. Although initially ridiculed, this is now generally accepted in the field of astrophysics. Therefore, back holes are not totally black after all!  Top

Stellar Mass Black Holes

Black Hole

If you try to envision a three dimensional picture of a black hole, it does not look like a drain hole, but a large black sphere (see the NASA sketch at the left) with an infinitely small, infinitely heavy core in the middle. They are called "black" because they absorb all the light that hits the spherical horizon just like a perfect "black body" in thermodynamics. Black holes do not "suck." things inside them. Suction is pulling something into a vacuum, which the a black hole definitely is not. Instead, objects fall into them because of their pull of gravity.

Stellar mass black holes form when dying stars collapse from their own intense gravity to form a body with extreme density. The gravitational force of a black hole is extremely strong because almost all of its matter is concentrated in the region of its center. The intense gravitational field of a black hole prevents everything, including light and other electromagnetic radiation, from escaping. During a star's normal lifetime, nuclear fusion in its core generates electromagnetic radiation, including photons (particles of light) that we can see. This internal radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass, giving a star its outer shape.

As a star's nuclear fuel is consumed, the outward forces of radiation decrease and gravity forces cause the star to compress. The contraction of the core increases the temperature which allows the remaining nuclear materials to be consumed as fuel. Eventually, all the nuclear fuel is used up and the core collapses as part of a "supernova" explosion. How far it collapses is determined by the star's final mass and the remaining outward pressure that the nuclear residue (mostly iron) contributes. If a star has a mass between 5 to 30 times the mass of our sun, it will collapse into a black hole. If it is less massive, it will become a white dwarf. If it is more than about 30 times the mass of our sun, it will become a neutron star.

Black Hole Diagraam

Mathematical Representation. Around a black hole there is a mathematically defined surface called the "event horizon" that marks the point of no return. This is not a normal surface that you can see or touch. The event horizon is an imaginary border formed by light rays that will never escape or on the other hand, light rays that will never be drawn in. Inside a black hole the gravitational field is so intense that it bends space-time back into itself so that inside the event horizon there are literally no paths in space-time that lead outside the black hole. No matter what direction one would go, you would find that your path led back to the center of the black hole - the "singularity" (as it is called in mathematics). (An analogy might be a great circle here on earth. An airplane following any great circle around the globe eventually finds its way back to its original position.)

At the event horizon, the pull of gravity becomes infinitely strong. Behind this horizon, the inward pull of gravity is overwhelming and no information about the black hole's interior can ever escape to the outer universe, i.e. no events can escape the "event horizon", hence the name. When matter forms a black hole, it is transformed into a purely "gravitational entity". Astronomers use the radius of the event horizon to specify the size of a black hole. The radius of a black hole, measured in kilometers, equals three times the number of solar masses of material in the black hole.

The above description is based on black hole theory and the mathematics behind the theory. In the real world, is there a singularity and an event horizon? No one knows because as yet we do not have any good picture of the edge or insides of a black hole. Scientists are still trying to generate a detailed image of exactly what the edge and the rest of a black hole looks like. In all likelihood, the mathematical representation is probably a rough approximation of a real black hole.  Top

"Black Holes Have No Hair"

There are two possible solutions to Einstein's Field Equations (a set of ten equations in Einstein's General Theory of Relativity) depending upon the starting assumptions:

Schwarzschild Black Hole

These two types of black holes have become known as the Schwarzschild and Kerr black holes. Both types of black hole are "stationary" in that they are not traveling in time. They are among the simplest objects analyzed in the equations of General Relativity.

The resultant equations can be completely described in terms of just two variables - their mass "M" and their angular momentum "J" for rotating black holes. Theoretically, black holes may also possess electric charge, "Q", but they would quickly attract enough opposite charge from space dust and other material to become neutral. The net result is that any "realistic" black hole exhibits zero charge. In the equation in the chart at the left, "G" is the gravitational constant and "C" is the speed of light. "Rsch" is the Schwarzschild radius of the black hole.

The simplicity of black holes is summed up in the saying (also by John Archibald Wheeler) "black holes have no hair", meaning that apart from their mass and angular momentum, there are no other characteristics (or "hair") that a black hole can exhibit. All information (material make-up, shape, corona, etc.) is lost when a space object is consumed by a black hole. Except for "M" and "J", all black holes are exactly alike - i.e. they have no identifying hair.  Top

Rotating Black Holes - The Ergosphere

Ergosphere

Rotating black holes are formed in the gravitational collapse of a massive spinning star. The angular momentum of the rotating star must be conserved, which means that it is transferred to the newly formed black hole. Since most stars rotate it is probable that most black holes are rotating black holes. Rotating black holes are also known as Kerr black holes named after Roy Kerr who in 1964 solved Einstein's mathematical equations that describe revolving black holes. The fastest black hole in the Milky Way (GRS 1915+105) rotates about 1,150 times per second, approaching the theoretical upper limit of general relativity.

Rotating black holes have an additional theoretical element called the "Ergosphere", shown at the left. The outer surface of the Ergosphere, called the "static limit", has an elliptical shape like a flattened sphere. The inner surface is the "radius of no return" or the "event horizon". The north and south poles of the Ergosphere touch the top and bottom of the event horizon. The Ergosphere's equator stretches out depending on the mass of the black hole and its angular momentum. The faster the black hole rotates, the narrower is the Ergosphere up to the limit of rotation where it becomes the same size as the event horizon.

Objects and radiation can escape from the Ergosphere, i.e. they are not bound forever like objects that fall into the event horizon. Objects can even emerge from the Ergosphere with more energy than they entered, known as the Penrose Process. An object can gain energy by entering the black hole’s Ergosphere and then escape from it taking some of the energy with it. This energy is taken from the rotational energy of the black hole complex causing it to slow down. On the other hand, objects that fall into the Ergosphere will increase its energy causing the black hole complex to speed up.

A word about the rate of rotation, spin rate, of a black hole. What does a spinning black hole mean? When we think about celestial spin, we think about something like the earth spinning about its axis on a daily basis. But wait, if you were to watch a spinning black hole, you would see nothing spinning as there is no light, or any turbulence, because nothing can escape. All you would see is darkness. So you have to imagine something spinning, like maybe a spinning dust cloud with no dust - a mental imprint of a spinning vacuum. But since the gravity of the black hole carries things with it (see the frame dragging next section) there would be a lot of "real" visible gas and material spinning very close by in the accretion disk that can be measured. This is how we estimate the spin of the black hole itself.

Almost all astrophysicists believe black holes do exist, however they are not as positive about ergo spheres. Ergospheres at this time remain a theoretical entity until there is some hard evidence for them. To date there has been no hard evidence.  Top

Frame Dragging

Frame Dragging

The black hole has another unusual attribute which is called "frame dragging". Frame dragging is a phenomenon where matter and/or electromagnetic radiation close to a rotating black hole will be forced to participate in its rotation, not because of any torque, but because of the curvature of space-time accompanying rotating bodies. Kerr mathematics for rotating black holes infer frame dragging as a result of Einstein's equations and most astrophysicists believe it is a realistic phenomenon. The artist's sketch, to the left, illustrates how space-time is frame dragged around a black hole.

According to General Relativity, at close distances, everything, even light, must rotate along with the black hole. There is a narrow region outside but close to a black hole where photons of light are forced to travel in a circular orbit. This region is known as the "photon sphere". Light inside the photon sphere but outside the event horizon is free to travel. It either shoots off into outer space or gets absorbed by the black hole. Note the red and blue photon ring in the center of the simulated accretion disk image at the left below.

However stepping back a bit, keep in mind that all of the above information is theory which is the result of mathematical and computer modeling. To date we have only inferred black holes as no one has yet been able to "positively confirm" a black hole. Black holes are inferred by the very strong x-ray and gamma-ray radiation they emit and are being intensely studied. There are very few black hole doubters in the astrophysicist community.  Top

Accretion Disks

Black Hole Accretion Disk

Accretion disks are mostly gas (with some dust and other debris) found around black holes from burned out stars to massive black holes at the center of galaxies. These disks transport gas to the black hole at their centers. The gas is pulled into the accretion disk from interstellar space or from another star. Accretion disks are formed when outer space gas and other materials fall together because of a strong gravity pull. The gas normally has latent angular momentum from its previous position. Because "angular momentum must be conserved", the gas is forced into orbit "around" the black hole and increases its rotational speed.

Gravity causes the material in the disc to spiral towards its center and also compresses the material. This compression heats up the materials to extremely high temperatures and causes it to emit electromagnetic radiation. The amount of energy released by the gas increases as the gas comes closer to the central body. Thus, most of the energy released by an accretion disk comes from the disk's inner edge. However, the inner edge does not touch the black hole. The amount of space between the edge and the event horizon depends upon the spin rate of the disk. The faster the spin rate, the closer the edge is to the event horizon. When some of the gas falls into a black hole, its energy and angular momentum is absorbed by the black hole.

The frequency range of the electromagnetic radiation given off by a black hole depends on the individual black hole's size (mass) and spin rate. Discs around single black holes radiate x-rays. Disks around black holes that are in "binary systems" radiate massive amounts of energy at ultraviolet and x-ray frequencies. Some of these binary systems are among the brightest x-ray sources in the sky.   Top

Black Hole "Jets"

Galaxy Centaurus A

There are two types of jets in interstellar space - "polar" jets and "relativistic" jets. A polar jet is a phenomenon where streams of matter are emitted along the axis of rotation of a compact object. They are thought to be caused by the dynamic rotating magnetic fields in the center of the accretion disk of a star. When matter is ejected at speeds approaching the speed of light, these jets are called "relativistic" jets. Relativistic jets are extremely powerful jets of plasma which emerge mainly from supermassive black holes. About 10% of giant black holes have jet streams. Relativistic jet stream "lengths" can reach hundreds of thousands of light years.

Shown at the left is a composite image of Centaurus A (NGC 5128) made up of three individual images superimposed. The visual picture with the star background is the base, while x-ray jet streams are shown in light blue and the radio wave jet lobes are shown in orange. The lobes are filled with ejected matter a million light years from the galaxy center! This NASA picture was released in May of 2011.

Accretion discs around different stellar objects are believed to be the source of jets. However, the jets around black holes are the fastest and most active. This is because the speed of the jet is about the same speed as the escape velocity from the central object. This makes the speed of a jet from the accretion disk of a black hole near the speed of light, while jets from newly born stars are much slower.  Top

Supermassive Black Holes

NGC 4258

Astronomers now believe (almost) all galaxies contain supermassive black holes at their centers. See NGC 4258 at the left, a supermassive black hole galaxy with pink gas emanating from its center.

Supermassive black holes have properties which distinguish them from lower mass black holes:

The Milky Way's Supermassive Black Hole

Milky Way Black Hole

Astronomers are confident that our own Milky Way Galaxy has a supermassive black hole (SMBH) at its center, 26,000 light-years from our Solar System, in a region called Sagittarius A Star (A*). In the image (to the left) of the center of our Milky Way, lobes of hot gas (red) surround the Milky Way center. The black hole is believed to be inside the white region of intense x-ray emissions. While the black hole can not be "seen", there is very good evidence that it is there.

From the orbit of Star S2 in Sagittarius A*, the center object's mass is calculated to be 4.1 million solar masses. Only a black hole is dense enough to contain 4.1 million solar masses in this volume of space. It is now almost completely accepted that the center of every galaxy contains a supermassive black hole.

For years astronomers have been puzzled as to why our Milky Way galaxy's SMBH is almost dormant. "If we had been around to see it two million years ago, the situation would have been very different," says Philip Maloney of the University of Colorado in Boulder. "The Milky Way's black hole was maybe ten million times brighter then."

Magellanic Stream

Maloney believes powerful beams of energy erupting from the SMBH two million years ago hit the Magellanic Stream. This caused its hydrogen gas to be ionized and light up, much like the glow of northern lights we see here on earth. The ionization of the Magellanic Stream has puzzled scientists since its discovery 40 years ago.

The team now suspects that this glowing stream of galactic gas is the imprint of the SMBH erupting two million years ago. The orientation and energy of the outburst fit very well with the proposed model.

Astronomers believe gas clouds orbiting the SMBH today will trigger a future outburst. The question is not if there will be another SMBH eruption, but when they say.

Scientists have been monitoring a nearby gas cloud and predict that it will fall into the SMBH in the very near future. However, the amount of gas material is far less than the event that illuminated the Magellanic Stream. For a good site to learn about black holes visit the Hubble Black Hole Site.  Top

A Spectacular Supermassive Black Hole

Spectacular Black Hole

The dazzling composite light show at the left, in the nearby spiral galaxy Messier 106, was captured by NASA's Spitzer Space Telescope, the Chandra X-ray Observatory and the Herschel Space Observatory. At its heart is a supermassive black hole, but this one is unusually active. Unlike the black hole at the center of our Milky Way, which only occasionally pulls in wisps of gas, Messier 106’s black hole is actively gobbling up material.

As the gas spirals towards the black hole, it heats up and emits powerful radiation. Messier 106 has two swirling arms that glow in X-ray light. This suggests that the gas inside the galaxy is heated to millions of degrees and then rides along shock waves streaming from the black hole to its outer regions.

Instead of a normal spiral galaxy with two arms, Messier 106 has four arms. Although the second pair of arms can be seen in visible light as ghostly wisps of gas, they are much more prominent using X-ray and radio waves. Unlike normal arms, these two extra arms are made up of only hot gases rather than stars. Their origin remained unexplained until very recently. Astronomers now think these gas arms are initiated by the black hole at Messier 106’s center. This means they are a totally different phenomenon from a spiral galaxy’s normal star-filled arms.

Scientists believe energetic jets are heating up the material in Messier 106's center which power shock waves that are driving the gases out of the galaxy's interior. These gases are normally responsible for creating new stars. A new study estimates that the shock waves have currently pushed out two-thirds of the gas from the center of Messier 106. "Jets from the supermassive black hole at the center of Messier 106 are having a profound influence on the available gas for making stars in this galaxy," said lead author Patrick Ogle, an astrophysicist at the California Institute of Technology.

"This process may eventually transform the spiral galaxy Messier 106 into a lenticular galaxy, depriving it of the raw material to form stars. Our results demonstrate that these black hole jets can have a significant impact on the evolution of their host galaxies, eventually sterilizing them and making them bereft of the gas needed to form new stars," Ogle added. Lenticular galaxies are flat disks that are full of old, red stars. Since Messier 106 appears to be losing its star-producing gases, researchers speculate that it is transitioning into a lenticular galaxy.  Top

Some Unusual Supermassive Black Holes

Galaxy IC 342

Spiral Galaxy IC 342, shown at the left and also known as Caldwell 5, lies 7 million light years away in the constellation Camelopardalis (the Giraffe). The two magenta X-ray blobs are huge black holes, called ultra-luminous X-ray sources (ULXs) while the galaxy itself is shown in visible light. How ULXs can shine so brilliantly is still a mystery in astrophysics.

While these ULX black holes are not as powerful as supermassive black holes at the heart of galaxies, they are more than 10 times brighter than stellar mass (burned out star) black holes. Astronomers speculate that ULXs might be intermediate sized black holes with masses between a hundred and a thousand times that of our sun.

The NuSTAR satellite, launched in June of 2012, is the newest orbiting telescope and has the ability to focus on high energy X-ray light. "High energy X-rays hold the key to unlocking the mystery surrounding some of these objects," said Fiona Harrison, NuSTAR principal investigator at CalTech. "Whether they are massive black holes, or there is new physics in how they feed, the answers are going to be fascinating."

Colossal Black Hole

Colossal Black Hole In Galaxy NGC 1277. The Hubble image to the left shows a relatively small galaxy, NGC 1277, which contains a large central supermassive black hole. This is one of the most massive black holes found to date. With a mass of about 17 billion suns, the black hole contains 14% of the total mass of Galaxy NGC 1277. The usual percentage of a total galaxy's mass is about 0.1%. A mass of 14% is much greater than current models predict. Astronomers would have expected a black hole of this size to be located inside a galaxy ten times larger. Instead, this black hole sits inside a fairly small disk galaxy.

Is this massive black hole a freak accident? Analysis of additional data suggests not. So far, the search has uncovered five additional galaxies that are small, yet harbor unusually large black holes. As of February, 2015 there are 10 humongous black holes with masses above a billion suns that have been confirmed by NASA's NuSTAR, an x-ray space telescope designed to study black holes.

Black Hole Eats Star

Black Hole Eats A Star. In March 2011, NASA's Swift Satellite alerted astronomers to unusual high energy flares from a new source in the constellation Draco. They soon realized that the source, which is now known as Swift J1644+57, was the result of a truly extraordinary event - the awakening of a distant galaxy's dormant black hole as it shredded and consumed a nearby star. See a NASA artist's conception at the left and a photo of the "real" black hole eating the star immediately below it.

The galaxy is so far away that the radiation from the blast has traveled 3.9 billion years before reaching earth. The black hole in the galaxy hosting Swift J1644+57 is twice the mass of the four million solar mass black hole at the center of our own Milky Way.

As the star falls towards the black hole being pulled by gravity, it

Black Hole Eats Star

is ripped apart by huge gravitational forces. The gas is forced into the accretion disk that swirls around the black hole and becomes heated to temperatures of millions of degrees.

The innermost gas of the disk spirals toward the black hole, where the rapid motion and magnetism creates dual "jets", through which some particles escape. Particle jets, driving matter at velocities greater than 80 to 90 percent of the speed of light, form along the event horizon and line up with the black hole's spin axis. In the case of Swift J1644+57, one of these jets just happened to point straight at the earth.

This gamma-ray explosion was observed by the Swift Satellite, whose mission is to quickly detect gamma ray bursts from supernovas. Flaring from a supernova usually lasts a couple of hours. But, scientists say this blast was very unusual because the effects were long lasting. More than a week later, they continued to see high energy radiation spiking and fading at the source as the star gradually succumbed to the Swift J1644+57 black hole. An event like this had never before been seen by astrophysicists.

OJ 287 - The Largest Black Hole?

Black Hole OJ 287

About 3.5 billion light-years away, galaxy OJ 287 is estimated to contain the largest "confirmed" black hole at 18 billion solar masses. (Several other black holes have been suggested to be larger, but their estimates are questionable because of the techniques used to estimate their masses. Also, some have very large plus/minus error limits with the mid-point chosen as the final estimate.)

The reason we are able to learn so much about OJ 287 black hole's size and central region   is because of the 100 million solar mass black hole that is orbiting the larger one. See the small yellow circle in the blue in the photo to the left. The smaller one's orbit reveals a lot of information about the central black hole, which in conjunction with Einstein's Relativity equations, allows scientists to calculate the larger one's mass quite accurately.

The orbiting black hole is by no means a small black hole compared to the rest of the pack. It is 25 times larger than the one at our Milky Way’s core. Maximum brightness occurs when the smaller black hole is in sync in front of its supermassive bigger partner. The smaller black hole is edging ever closer to the larger one and will spiral into the larger one in about 10,000 years making this black hole even bigger.

OJ 287 has been seen on photographic plates since at least 1887. It belongs to a special class of celestial objects known as blazars, which are a compact celestial radio source that are some of the most energetic objects in the universe. (Check out the Blazar Section.) It is thought that one of OJ 287's powerful jets is pointed right at the earth.

This object’s brightness varies periodically ,   and emits outbursts with a narrow, double peak at its maximum brightness. (Blazars have wavelength intensities that vary dramatically over time.) It emits wavelengths in both the radio and x-ray frequencies. The double peak indicates that not only is there a central supermassive black hole of extraordinary magnitude, but also another smaller black hole in close orbit around the larger one making it a binary black hole system.

How Big Can A Black Hole Grow? Professor Andrew King, from the University of Leicester's Department of Physics and Astronomy, has calculated how big black holes can be before expending the gas accretion discs they require to fuel themselves. At a certain large size, stars begin to form in the accretion disk due to the self gravity of the disk. The disc becomes unstable and it begins to crumble into a set of new stars. Professor King has come up with a figure of roughly 50 billion solar masses. The study suggests that without a disc, the black hole would stop growing, meaning 50 billion suns is the approximate upper limit of a black hole. The only way one could grow larger is if a star happened to fall straight in or another black hole merged with it. OJ 287, at 18 billion solar masses, is well within the 50 billion solar mass limit.

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