As seen in the star evolution diagram on page 20-5, when gases and other matter of 50 or greater solar masses 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 (for the mass in our Sun, collapse into a Black Hole would yield a density of about 10²² grams per cubic meter; larger stars would produce densities several orders of magnitude greater) prevents all detectable radiation from within from escaping beyond its event horizon (sphere of influence). 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. A Black Hole generally is so small 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). Here is an HST view of the well-known galaxy, M87, in which its billions of stars are not resolved so as to appear as a yellow glow. The central "star" is actually light emitted from the exterior around a B.H., probably as a quasar (third paragraph below).

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 the radiation that extends 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:

Probably the best image to date of a 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:

Galaxies are thought to have multiple Black Holes, most of which are relatively small. 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.

Chandra has now established that the Milky Way galaxy has a moderate-sized Black Hole (located in the celestial hemisphere at a point close to Sagittarius A). The image below shows not the invisible hole itself, but the radiation emitted from excitation of gases and other matter drawn into the B.H. During this continuous exposure over 164 hours, the glowing gases underwent periodic flare-ups that combine in this composite pattern:

In principle, Black Holes should sometimes collide (but the consequences are not yet defined theoretically), 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.

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 perhaps the glow of radiation bursts ("hot spots" of gamma radiation and x-rays) from matter infalling into nuclei of active galaxies (probably supermassive Black Holes). The majority of quasars are located at or near a galaxy center, but 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 optical image of one (and possibly several) quasar(s) acquired by the HST:

And here is a very bright central core of a Seyfert Galaxy, NGC 3516, with a quasar producing a huge light emission (but probably being "intensified" by gravitational lensing) associated with infall to a massive Black Hole:

A quasar in M1000, as imaged from variations in x-radiation monitored by the Chandra X-ray Observatory, appears thusly:

A quasar with an associated Black Hole seems to be pulling in material from regions beyond. 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 infra-red 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:

But since Black Holes can still form in young cosmological time 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. HST has observed such events, which may be the case in this image of the elliptical galaxy NGC4261, in which the ring seemingly surrounds such a Black Hole.

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.

Black Holes can vary in dimensions, the smallest in the general class being much less than a kilometer in diameter but packing 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.) Humongous B.H.s can contain masses derived from billions of infalling stars and galactic matter, attaining sizes exceeding that of our Solar System. Massive to Supermassive Black Holes may be the customary state at the center of spiral and other galaxy types, having built up from millions stars and other matter converging inward as though moving to a drain. The HST view of NGC7742, a Seyfert type 2 active galaxy, shows a large glowing central region, within which a supermassive Black Hole is postulated. Its bright center probably represents a quiescent quasar state, 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.

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.

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. In some respects, 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 represent extreme densities concentrated in "points" as small as 10^-15 meters.

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.

Gamma Ray Bursts

Black holes almost certainly play a role in what are called Gamma Ray Bursts (GRB). These are the most intense and copious releases of energy observed in the Universe - less than that of the Big Bang itself but much more than given out by supernovae or quasars. GRBs can at their outset release enough energy to give them a luminosity calculated to be 10¹⁹ greater than that of the Sun. They are characterized by extreme outputs over very brief periods, measured in seconds to minutes at their peak. At least one GRB is observed each day somewhere in the Universe, so they are rather common events, albeit much less frequent by far than supernovae.

Despite being the largest rapid release high energy events in the Cosmos, GRBs were unknown (sometimes mistaken for ordinary supernovae) before 1967. The manner in which they were discovered is interesting: Nuclear explosions on Earth release large quantities of gamma ray energy. At that time, the U.S. was seeking ways to detect Soviet nuclear tests, so it built and orbited gamma ray detectors on military satellites. The gamma ray events thus found all proved to emanate from beyond Earth. Here is a plot of one of the first records:

Thus, the diagnostic signature of the GRB that separates it from supernovae is the predominance of high energy gamma rays over very short time periods. GBRs can be subdivided into two types: short burst (around 2 seconds) and long burst (more 2 seconds; initial emissions on the order of 20-30 seconds, with a few extending up to an hour). This time spike has been observed in GRBs detected by more sophisticated sensors that monitored such events. Thus, this example:

These GRBs puzzled astrophysicists. They were first thought to be in the Milky Way. And in fact some were actually located in our galaxy, where they occur on average about once in 10000 years. One such event was observed in the M.W. itself by an X-ray satellite called Beppo-SAX:

But, the frequency of occurrence suggested that the vast majority of GRBs were located in galaxies well beyond the Milky Way. As more observances accumulated, it became evident that GRBs are not concentrated in specific regions of the sky but are distributed at random (isotropic) over the entire sky. GRB's are also randomly distributed in time - occurring anywhere in the Universe (thus over the full extent of time since the first galaxies;(from thousands of light years to 12+ billion l.y.). A large number seem to be distant, near the outer part of the observed Universe, and hence were most common in the early history of the Universe. Here is a map of the sky showing many of the larger GRBs, as detected by the Compton Gamma Ray Observer and Beppo-SAX.

The CGRO proved very effective in picking up GRBs. The BATSE (Burst and Transient Source Experiment instrument) was particularly suited to detecting GRBs). Here is one image of an event that occurred several billion light years away:

These GRB events should generate radiation at wavelengths longer than those of gamma rays. As studies of them expanded, traces of individual events were sought by other satellites that monitor at different wavelengths. The problem is that evidence of a GRB diminishes rapidly at shorter wavelengths. However, in time such events were picked up at various wavelengths when alerts were given and the sky locations established. Now, with experience this is the time frame for durations of GRBs over a range of wavelengths:

These signs of lower level energy at longer wavelengths persisting around a GRB are grouped under the general term "afterglow". X-rays proved useful as GRB signatures provided the searching satellite(s) could check out the source region within a few days. The x-ray emissions persist over periods of hours to days. This is one X-ray image of a GRB that was located in a galaxy nearby (some has classified this as a hypernova):

Images acquired by Beppo-Sax were especially helpful in the sky survey for GRBs. The top illustration consists of two intensity contoured images typical of X-ray renditions; note the reduction in intensity in just four days between February 28 and March 3. Below it is a pair of Beppo-SAX images taken first on December 15, showing the GRB as a bright dot and then on December 16 as the afterglow had faded away.

Special attention was given to finding GRBs at visible (optical) wavelengths, since these are capable of measuring red shifts by which approximate distance to the source can be estimated. About half the GRBs give off light in the visible for durations of a week or more. The HST and the Keck Observatory in Hawaii were pointed at targets reported by other observing satellites. Here is the HST image of event GRB000301c.

A ground telescope imaging of another GRB shows the burst as seen in visible light (here the print is a negative) at 21 hours (left) and 8 days (right) after first detection. The rapid fading of the galaxy-sized feature is evident (note arrows)

Although not used a lot for this purpose, radio telescopes have detected and imaged GRBs. Here is one made by the VLA group:

One very important GRB event led to some intriguing information that indicates that this phenomenon occurred early in cosmic time (and continues til the present) and helps to confirm the huge amounts of energy involved. Its magnitude is equivalent to 100 million billion solar radiances. On December 14, 1997 the CGRO registered this event. Word was sent to Beppo-SAX operators and to the HST and Keck telescopes to look for it as rapidly as possible. All succeeded. This is how the event was imaged by the HST:

The image on the right was taken on January 23, 1999 during its maximum. By February 8 (left) this burst (shown at a different scale, in the box) had faded to 1/4 millionth of the visible output.

When a redshift distance measurement was made on the GRB, it was found to be some 12 billion light years from Earth, proving the surmise that GRBs have probably been part of the Universe's history since soon after the Big Bang. It was also the brightest object yet found at that far distance from Earth.

Thus, the pattern found for most GRB events is rapid emission of gamma rays followed, as they fade, by the dominant radiation passing through x-ray, visible, and radio wavelengths, with the whole sequence being over in less than a few months.

The cause(s) of GRBs continues to be uncertain and tantalizing. The early idea of the explosion of material sucked in and around a neutron star (see top illustration on this page for a similar example) has been challenged. But, a variant postulates a role for a binary pair of neutron stars which, if they should collide, produce a huge release of energy. Another hypothesis, known as the Paczynski Model, starts with a supermassive (type O) rotating star that collapses to form a black hole that continues to draw more material around it until a critical state requires an intense explosion producing the GRB fireball. One version indicates that the energy release may be directed, something like the beam associated with a pulsar. Still others attribute the GRBs to some involvement with quasars. One school holds them to be the outcome of giant supernovae (hypernovae) which generate the most powerful short-time energy release levels known in the Universe. A recent hypothesis takes still a new tack - the GRBs are associated with large clusters of galaxies which together have such a strong gravitational pull that they accelerate matter both within and around the galaxies to high speeds that, upon colliding with intergalactic matter, release energy at the gamma-ray level.

Some of the above information has been extracted from an article in the December 2002 edition of Scientific American, entitled "The Brightest Explosions in the Universe", by N. Gehrels, L Piro, and P. Leonard. The article contains this illustration that summarizes the authors' ideas on the formation of GRBs:

From Scientific American, December 2002

In their model, similar to some others proposed, GRBs are definitely associated explosive processes that will end forming black holes. In one common mechanism, a massive star collapses and explodes as a hypernova, leading to a disk of matter/energy surrounding a black hole; this is a fast process in the sense that at a critical time, the hypernova ensues without anything discernible obviously leading up to it. Alternatively, over a long span of time (millions of years, the same end result occurs as two neutron stars mutually orbiting finally crash into each other. The wedge to the right of the 'Central Engine' conforms to a jet that carries the photons released in the GRB outward at near light-speed. This material moves outward as "blobs" that catch-up and coalesce forming internal shock waves that generate the gamma bursts. With expansion over time, the high energy photons are replaced by those of progressively lower energies represented by x-rays, light, and radio waves as the emissions encounter the galactic/intergalactic medium. The final result is an afterglow that fades over time.

Needless to say, GRBs continue to fascinate cosmologists since they represent the largest and fastest explosive events beyond that of the Big Bang itself. As they are better understood, they may reveal the action of physical processes only now being speculated upon, and suggested by particle physics experiments.

Colliding Stars

At least some of the GRBs and X-Ray bursts may stem from collisions of (usually two) stars (check back at the bottom of page 20-3 for the earlier review of galaxy collisions). As recently as the 1970s astronomers considered collisions to be rare stellar events. Although an actual collision has as yet not be observed by HST or other astronomical satellites and ground telescopes, phenomena associated with certain stellar configurations have now been postulated (attributed) to either head-on or glancing encounters between stars.

As we have seen, stars in the arms of spiral galaxies or the fringes of elliptical galaxies are very widely spaced and hence the probability of collision is low. But star distributions in central cores of these two types show much closer spacing (denser). Even higher densities are found in globular clusters, such as 47 Tucanae:

To appreciate the significant increase in density, if one counts the stars that are about 25 light years from our Sun, the number would be about 100 but if the same 25 l.y. volume is set around the center of a globular cluster, that star number rises to an order of about 1,000,000. This crowding means that those stars are very closely packed and hence capable of numerous collisions. Three processes make collisions much more likely: 1) a process called "evaporation", in which stars approach other and some are then flinged out of the grouping which contracts to such densities as to make collisions inevitable; 2) gravitational focusing, in which approaching stars have their pathways deflected so that two stars now follow a collision course; 3) tidal capture, in which neutron stars or black holes latch onto nearby stars and in time draw these into the high gravity

Theoreticians have developed computer models to simulate pictorially different modes of collision. Shown here is the sequence of change as two Sunlike stars are merged:

The end result of a collision depends on several factors: 1) whether there is a direct hit or a glancing encounter; 2) the relative size (mass) difference between colliding bodies; 3) the terminal speed of each body. The process can be as brief as an hour; or as long as days to years (this rapid time for completion is one reason while such events have yet to be observed in "real time"). Any two of the 7 density types shown near the top of page 20-5 can experience a collision. In some combinations, such as a white dwarf striking a red giant, the end result is two White Dwarfs (one being the incoming member; the other [Red Giant] dispersing and losing so much of its gas by the interaction that only its core remain which quickly evolves into the new White Dwarf, Or, one star remains relatively intact as the second star is incorporated within it.

In this last case, the result is that the now coalesced star pair has gained considerable mass. This means that it now appears to be a bigger star, and since the total mass determines the rate of hydrogen fuel consumption, the new, brighter star would appear as though it will burn its hydrogen mass much faster and thus appears to have a shorter lifetime - hence seems younger. A specific case: if two stars, each with a mass of the Sun (5 billion years old) that has a total burn out time of 10 billion years, collide and form a single star with twice the mass, the now more luminous composite star would have a life expectancy of 800 million years. This seems to be the best explanation of "Blue Straggler" stars - much brighter than the majority of stars in a globular cluster. This is evident in this HST image of NGC 6397:

An excellent summary of collision processes and consequences is offered in the November 2002 edition of Scientific American under the title "When Stars Collide", by Michael Sharma.