As seen in the star evolution diagram on page 20-2, when gases and other matter in stars having solar masses much larger than the Sun gravitationally contract into small, compact bodies, the result is a Black Hole (B.H.), so called because the gravity associated with its extremely dense mass (if the mass in the Sun were to collapse into a Black Hole, this would yield a density of about 1022 grams per cubic meter; larger stars would produce densities several orders of magnitude greater). The center of a typical small B.H. is much smaller than a single atom. Consider this diagram:
The supermass characterizing a Black Hole prevents all detectable internal radiation originating from within from escaping beyond its event horizon (sphere of influence inside of which extreme gravitational forces preclude any mass or photon radiation from leaving). The distance from a B.H.'s center to the horizon is known as the Schwartzchild radius . Since the B.H. is itself invisible (black), its existence must usually be inferred from its gravitational effect on surrounding stars and interstellar matter. Before their observational discovery, Black Holes were predicated to exist from General Relativity considerations. Black Holes indeed have such strong gravitational influence that they notably warp the fabric of Einstein's spacetime dimensionality.
A terminology has evolved to describe Black Holes:
The hole itself is known as a singularity. This is the very center of the black hole, and is where the mass of the original star (and all acquired matter) lies. In a Kerr black hole (a black hole that assumes the star's core was spinning and had a magnetic field when it collapsed), the singularity is theorized to be ring-shaped. In a black hole that does not spin, the singularity is a dimensionless point of infinite density. Moving out from the center, the next part is the inner event horizon. Between the inner event horizon and the singularity, space is believed to be relatively normal - except for the fact that all objects are drawn towards the singularity and cannot escape. Next out is the outer event horizon. This marks the boundary at which the escape velocity is greater than the speed of light, and all known objects are drawn into the hole. This also marks the "outer edge" of the black hole; we cannot see into it, for no form of known radiation can escape the gravitational pull from this point inward. The ergosphere is a region of space where all particles are drawn in a circular path that match the hole's rotation. However, within the ergosphere, matter and energy can still escape the hole's grasp. The outer edge of the ergosphere is called the static limit. This is the distance that matter must maintain in order to keep a stable orbit and not be trapped by the hole's rotation. Other parts of a black hole are present only in "active" black holes. The accretion disk is matter that has been trapped in orbit around the black hole. It will gradually be pulled into the hole. As it gets closer, its speed increases, and it also gains energy and begins to emit light. The only physical part of a black hole is the singularity. The other terms refer to mathematical boundaries. There is no physical barrier called an event horizon, but it marks the boundaries between types of space under the influences of the singularity.
Black Holes cannot be directly imaged, since their supermass prevents light radiation from escaping. But they are usually surrounded by luminous matter excited as it spirals into the B.H. Dust also is concentrated in the region beyond the B.H. The galaxy M64 has such central luminous matter.
When entire galaxies are imaged, the central Supermassive B.H. usually appears as a large bright object (likened to a super star) surrounded by a glowing cloud (which is both gas and closely packed stars) as shown in this infrared image:
The Internet abounds with "artist's conceptions" of Black Holes. Here is one example, showing the bright central location of excited material in a galaxy:
Black Holes can vary in dimensions, the smallest in the general class being much less than a kilometer in diameter but packing dense mass equivalent to about 3 solar masses. (Theory indicates that mini-Black Holes can be as small as a few centimeters or even microscopic in size; so far, none this small have been detected; one school of thought holds that there are countless numbers of tiny B.H's distributed within galaxies and in intergalactic space that contribute to holding the Universe intact.) Black Holes whose masses are similar to that of most stars such as the Sun, and are the products of the final stages of a typical star's life, are known as Stellar Black Holes. The range of mass (with the Sun's mass [solar mass] being the unit of measure) of Black Holes is considerable, as shown in this diagram:
Humongous B.H.s can contain masses derived from billions of infalling stars and galactic matter, attaining sizes exceeding that of our Solar System. These are known as Supermassive Black Holes. Supermassive Black Holes are now thought to be the customary objects at the center of spiral and other galaxy types, having built up from millions of stars and other matter converging inward as though moving to a drain. Many cosmologists believe that at least one supermassive B.H. is present near the center of any galaxy. Its role is to keep the galaxy from flying apart (thus its gravitational forces are the stabilizer of the galactic structure). The presence of a Supermassive Black Hole, or any size Black Hole for that matter, cannot be confirmed directly, since Black Holes themselves are invisible insofar as being sources of detectable electromagnetic radiation. They are recognized mainly from their effect on stars and gases, causing these to be excited in distinctive ways (such as the Quasars soon to be discussed).
An example of a distinctive detection mode is afforded by the galaxy M84, at whose center a Supermassive Black Hole is postulated. EM radiation coming from its central region shows two opposing spectral wavelength shifts, one a redshift (towards longer wavelengths) and the other a blueshift (shorter wavelengths). This is attributed to the rapid velocities (400+ kilometers/sec) of whirling gases on either side of the central Supermassive Black Hole, as these rotate around the B.H. Thus infalling material is set to spiraling around the B.H. that captures it by intense gravitational attraction. This illustration shows the observed effects:
There can be other visible secondary indications. The HST view of NGC7742, a Seyfert type 2 active galaxy, shows a large glowing central region (an AGN), within which a supermassive Black Hole is postulated. Its bright center probably represents a quiescent Quasar state, there are periodic flare-ups resulting from energy release when stars spiral past the B.H. horizon into its interior; note the ring of bright hot, largely younger stars beyond and the faint spiral arms further out.
Many Black Holes are the end product of Supernovae explosions of Red Giant stars, as these burn up their fuel and reach the stage where Fe becomes the dominant element (which does not further ignite by fusion; see page 20-7). There are various models of how Black Holes form. These range from explanations of the mode of origin in a binary star system to the origin of central Black Holes in a galaxy to formation of Black Holes in intergalactic space. Illustrative of the origin of a B.H. in a binary system is this diagram; the B.H. results from the explosion of a Red Giant, followed by buildup of matter (mostly gas and dust) in a surrounding accretion disk, and continued supply of new material drawn from the companion star.
Smaller stars end up as Neutron stars which in principle can coalesce into B.H.'s. The larger B.H's have masses from millions to billions greater than the Sun. At the other extreme small B.H's may have only a fraction of a solar mass, perhaps up to a billion tons occupying a tiny volume such that the density of just a teaspoon-full of this compact matter is still enormous. This extraordinary density is possible because under the great pressure that formed the B.H. electrons and the atoms themselves become very closely compacted. Smaller B.H's may be ubiquitous - millions of remnants from earlier explosions within the Milky Way and galaxies in general; they may even exist within the Solar System but are too small to affect its spatial fabric and perturb planetary orbits.
A Black Hole generally is so small - yet so massive - that its spacetime expression produces a curvature so pronounced that all internal energy and radiation is seemingly trapped beneath the B.H. (within its horizon). An exception may be Hawking radiation (named after Stephen Hawking who devised the theory) consisting of particles created by quantum processes and driven by the gravitational energy within and around the Hole. The mechanism by which this process takes place is an excellent example of "quantum weirdness". In the 'empty' space just outside the B.H.'s event horizon, virtual particles and antiparticles are constantly created (as happens in general in this environment throughout the Universe). Under the strong gravity field around the B.H., one of these particles, the one with positive energy, is likely to be propelled away while the other is captured and dragged into the B.H. Antiparticles have negative energy and those brought into the B.H. react with B.H. particles to reduce the mass of the Hole and thereby lower its gravitational field. This B.H. gravitational field, in turn, loses the energy it provided to make the virtual pair. The escaping particles constitute the Hawking radiation, which is too "faint" to be detected from Earth but nevertheless causes the B.H to slowly "evaporate".
This escaping (emitted) radiation is most effective for tiny Black Holes and provides a means by which they can dissipate over extremely long times through this evaporation. While based on sound theoretical reasoning, Hawking radiation has not yet been directly detected. But if it is proved to exist, it provides a mechanism by which countless numbers of small primordial B.H.'s that formed at the outset of the Universe, because gravity was so intense then, have since vanished. At the present time, astrophysicists are learning more about B.H.'s by computer modeling and simulating their behavior.
Black Holes are also capable of ejecting matter in jets or streams of particles moving in beams almost at the speed of light. (Jets also occur during star formation and during late stages of star death). This next image, made by the Swift telescope, shows the star, around which a Black Hole is orbiting, and two distinct jets.
On a galactic scale, perhaps the best known image to date that shows distinct jets of luminous material emanating from a central Black Hole in two opposing directions perpendicular to the galactic plane is this artist's sketch based on a Chandra image of NGC1365; the galaxy itself appears here as a dark dust/gas cloud obscuring its many stars.:
A single jet of high speed luminous particles from a central Black Hole in the M87 galaxy. Here is a Fermi gamma ray view of the well-known galaxy, M87, in which its billions of stars are not resolved so as to appear as a yellowish-red glow. The central "star" is actually light emitted from the exterior around a B.H., probably as a Quasar.
Below are three more views of the jet streamer from M87; the top is imaged by Chandra in the X-ray region; the center is visible light; and the bottom from Radio waves. The origin of such streamers, found also associated with other galaxies, is still imperfectly known. But, the Black Hole(s) causing this ejection of gas and particles are the source of strong, directional electromagnetic fields. The gases may be excited by synchrotron radiation, causing photons whose energy levels extend over most of the Electromagnetic Spectrum.
Still another example of a jet associated with the presumed central Black Hole in a galaxy is Centaurus A (NGC5128) located some 11 million l.y. from Earth. On one side the jet is obvious but it has a faint companion on the other side. This jet pair lines up with the axis of rotation of the galaxy. The image, made by Chandra, is converted to a visible view using data sensed in the X-ray region of the spectrum:
This jet, which follows magnetic lines, is even more splendidly displayed in an HST image that is combined with a Chandra image, as shown below, with the strongest X-ray signals shown in blue:
Probably the best image to date of a paired jet associated with a supermassive Black Hole is that recently captured by HST as it trained on Quasar 3C120; the jet, composed of X-rays and electrons, follows strong magnetic lines:
A jet emanating from Quasar 3C273, which is powered by a Super Black Hole, extends out 100000 light years, as seen in this color composite made from a Chandra X-ray image (blue), a HST Visible image (green), and a Spitzer IR image (red):
These jets may be the same phenomena commonly detected by Radio astronomy, as illustrated near the bottom of page 20-3.
Galaxies are thought to have multiple Black Holes, most of which are relatively small. Evidence is growing that most spiral galaxies, at least, have one or more large Black Holes in their central regions. Such B.H.'s eem to serve as a stabilizing influence on the maintenance and evolution of a galaxy, causing stars and stellar gas and dust to migrate inward and be dragged into the Hole. Matter is constantly being attracted into the B.H. such that over time all of the galaxy will converge into the B.H. and thus be wiped out.
The first case in which two supermassive B.H.'s occur in the central core of a galaxy has been found in NGC6240. This irregular galaxy is shown in visible light in the left HST image below; the Chandra image on the right indicates a pair of Black Holes, which create a strong X-ray signal (in blue; weaker X-rays in red and yellow) as infalling material is heated to very high temperatures. Astronomers predict that these B.H.'s will eventually merge by collision.
The amount of gas and dust surrounding a central Black Hole can be much greater than farther out in a galaxy. In NGC 1068 (also known as Messier 77), the HST image on the left of this next figure shows reflected light from the dust in blue, ionized Oxygen gas in yellow, and ionized Hydrogen gas in red; on the right is a Chandra image of its center in which the orange-red corresponds to highly energized material around the immediate B.H. that is emitting X-rays.
Chandra and Radio telescopy have now established that the central region of the Milky Way galaxy has a supermassive Black Hole (located in the celestial hemisphere at a point close to Sagittarius A); perhaps there is more than one in this inner part. Proof of the presence of a large B.H. in the M.W. was hard to come by, because the central region is shrouded by dust. As Infrared images of this region accrued, the stars within the dust region were imaged. This allowed determination of their orbits as time lapse views permitted calculation of their movements. Many stars showed just the pathways expected from theory that would occur if a B.H. were sucking in these bodies. Later surveys of the central region using Gamma ray and X-ray radiation to image the behavior of the B.H.-seeking stars confirmed the presence of a very large B.H. at the centerpoint of the Milky Way. However, as this Chandra image discloses, the strongly radiating dust and gas at X-ray wavelengths does not single out Sagittarius A or any other manifestation of the B.H.
The size of the M.W.'s central Black Hole has been hard to determine because of this masking matter. It mass has been estimated to be about 2.6 to 4 million solar masses. Early estimates placed the diameter of a sphere to its event horizon at about 1.5 to 23 million kilometers (1 to 14 million miles). Recently, studies done by penetrating Radio waves, using Radio telescopes, has shed light on its dimensions, so that the upper value is considered close. This Radio wave image shows Sagittarius A, a bright object that may be the glow of excited radiation around the B.H.:
In principle, Black Holes should sometimes collide (but the consequences are not yet defined explicitly), especially when two galaxies collide with the B.H's at their centers then interacting. Evidence for this is sparse. However, such an event is postulated for the observations by the Wide Field Camera on HST of NGC326. In the main view below is the pattern of jet lobes from that galaxy seen a few years ago. In the offset second image is a more recent observation in which the orientation of the principal jets has now shifted more than 90°. The favored explanation is that two Black Holes have now interacted causing the spin axis of one to shift notably.
As implied above, Black Holes play a large role in the life of a galaxy. Recent UV observations by the Galaxy Evolution Explorer (Galex) finds a broad relationship between B.H. size and galaxy size (the larger the first, the bigger the second; the study was confined to elliptical galaxies but probably holds true for spiral ones as well). It was also found that for large central B.H. galaxies, the production of new/young stars in the inner regions became distinctly sparse. This has been attributed to B.H.-controlled heating of Hydrogen gas to temperatures too high to form stars and/or to expulsion of the gas from the inner galaxy.
A Black Hole's incredible gravity pulls in particles from outside the event horizon until their velocities are accelerated to nearly the speed of light. Matter is literally torn apart upon entering the Black Hole. As these particles close in, monstrous energy releases produce continuous bursts of energy outside the horizon, a process believed responsible for most Quasars (a contractive term for "quasi-stellar" to describe a star-like appearance even though the observed feature is not a single star). Quasars are extremely bright objects (very high luminosity, comparable or even exceeding that of an entire typical galaxy), being considered by most astronomers to be the glow of radiation bursts ("hot spots" of Gamma radiation, X-rays and Visible light) from both stellar and interstellar matter continuously infalling into the central regions of active galaxies, whose cores are probably supermassive Black Holes. While the majority of Quasars are located at or near a galaxy center, some occur in the spiral galaxy arms or in the regions beyond an elliptical galaxy's core. They were initially discovered as intense Radio wave sources detected by Radio telescopes. Now it is known that most Quasars are not accompanied by Radio waves (less than 2% are dominantly Radio sources, in which that wavelength region marks energy developed by synchrotron radiation) but are instead sources of more intense, shorter wavelength radiation. Here is an HST optical image of one (and possibly several) Quasar(s):
Quasar HE 1013-2136 at a distance of 10 billion l.y., imaged by an ESO telescope on a Chilean mountaintop, seems to be drawing gases from a galaxy to the left:
This pair of images shows a Quasar in Visible (bright in the blue) and Infrared light.
The powerful Quasar qso 1 Zw 1, as seen in the Infrared, is also a strong Radio source (contours superimposed).
Most Quasars are so far away (but some more recent ones are nearby) that light arriving at Earth left the Quasar source when the young Universe was only about 1/4 to 1/6 its present size. Thus, most (estimates in excess of 75%) Quasars formed early in Universe history and many, particularly the larger ones, have since become either greatly diminished ("dormant", with occasional flare-ups) or are now extinguished in today's time frame. This generalized (smooth) plot of Quasar history, both in terms of time since the Big Bang and when the numbers of galaxies relative to the expansion size of the Universe are normalized to 1 (maximum), illustrates these points:
This distribution of quasars in time is also evident in this Sloan Sky map portrayal, in which the outer blue dots are Quasars (at that distance they are young in terms of Universe age), the red dots are Luminous Red Galaxies (clusters of elliptical galaxies), and the black dots are closer to the Milky Way and are relatively old:
Since Black Holes can still form in young cosmological time, i.e., recently, throughout the Universe, conceivably they are giving rise (usually after only millions of years) to new Quasars. Quasars are made visible because of emission of light resulting from energy conversion as stars and interstellar gases are gravitationally sucked into supermassive Black Holes.
Perhaps as much as 50% of the EM radiation in the Universe is related to Quasars around Black Holes. The Quasars result from material being pulled off nearby star(s), transferred as stellar winds along magnetic lines from the stars, and accumulating in a disk around the Black Hole. A study of Chandra data for J1655 leads to this pictorial interpretation:
This may be the most common mechanism for Quasar production.
Very energetic quasars emit their radiation primarily in the Gamma Ray segment of the EM spectrum. Such quasars are called blazars. Here is an artist's conception of a blazar, based on telescope observations, in which there is a jet moving out of each end; in this view, the direction of look is straight down parallel with the central jet.
Black Holes that occur outside galaxies, or in a star-sparse region within a galaxy, do not attract enough material to become readily visible by virtue of the excitation of incoming matter. But their presence is often suspected where an X-ray or gammma-ray source is observed without a corresponding visible body. Recently, Black Holes have been detected in Globular Clusters by analyzing the patterns of movement and velocities of stars that can be resolved in the assemblages making up the clusters. These B.H.'s have estimated masses intermediate between the small isolated ones mentioned above and the Supermassive ones described in the previous paragraph. Although the numbers of points in the following plot relating B.H. mass to stellar assemblage mass are still few, a general trend that fits size to a straight line is evident:
In the early years after first postulated and then discovered, Black Holes were treated almost as a curiosity, without any special importance in the initial phases of the Universe's history. But, with the discovery that most (if not all) galaxies have B.H's in their core, there is a growing belief among astronomers that they are the necessary starting point in the formation of a galaxy, serving as the nucleus or core that attracts the matter that eventually organizes into a galaxy. Recent reports of both observational and theoretical studies now offer two important ideas: 1) both Black Holes and Neutron stars are more abundant in the inner or central part of a galaxy - a fact related to the idea that massive stars tend to form more readily in the core region; and 2) in early cosmological time Black Holes had a definite symbiotic relation to the processes that form and develop galaxies, i.e., massive B.H.'s can serve either as a nucleus for a growing galaxy or at the least aid in gathering matter into organized gas clumps that evolve into primitive galaxies.
In some respects, the smallest Black Holes are an approximation to the supersingularity postulated as the starting point of the Big Bang except that they have finite dimensions of meters to several kilometers and even much larger for those in galactic centers depending on their amounts of mass (can be equivalent to the cumulate mass of hundreds of millions to billions of Suns). One theoretical class of Black Holes consists of concentrations of extreme densities collected in "points" as small as 10-15 meters.
Some Black Holes are thought to be the sole surviving remnants of galaxies that have been completely swept into them. Other Black Holes may have formed during the first seconds of the Big Bang. There are increasing indications that supermassive Black Holes were in existence within the first billion years of the Universe. Many of these are either relics of the B.B. or remnants of early Supernovae.
Speculatively, one future outcome for the Universe (depending on the ultimate mode of expansion [see page 20-8]), after 50 b.y. or so, could be a collection of billions of Black Holes that eventually converge upon themselves to coalesce into a single ultra-dense Black Hole that ultimately would become the singularity for the next Universe (in this model, any number of successive Universes, exploding and contracting cyclically, is feasible). Such a concept of repeating Universes (treated in more detail on page 20-10) is referred to as the "Big Crunch", or even more colloquially, as the "Bounce" in reference to the repetition of an explosion after total collapse to the B.H. singularity.