Black Hole

I-INTRODUCTION

Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.

II-PROPERTIES

The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.

III-FORMATION

Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form "wormholes," connections between otherwise distant parts of the universe. He concludes that an unknown form of "exotic matter" would be necessary for such wormholes to survive.

Reviewed By: Jay M. Pasachoff

black hole

cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole can be formed by the death of a massive star. When such a star has exhausted its internal thermonuclear fuels at the end of its life, it becomes unstable and gravitationally collapses inward upon itself. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity. Details of the structure of a black hole are calculated from Albert Einstein's general theory of relativity. The singularity constitutes the centre of a black hole and is hidden by the object's "surface," the event horizon. Inside the event horizon the escape velocity (i.e., the velocity required for matter to escape from the gravitational field of a cosmic object) exceeds the speed of light, so that not even rays of light can escape into space. The radius of the event horizon is called the Schwarzschild radius, after the German astronomer Karl Schwarzschild, who in 1916 predicted the existence of collapsed stellar bodies that emit no radiation. The size of the Schwarzschild radius is thought to be proportional to the mass of the collapsing star. For a black hole with a mass 10 times as great as that of the Sun, the radius would be 30 km (18.6 miles).

Only the most massive stars--those of more than three solar masses--become black holes at the end of their lives. Stars with a smaller amount of mass evolve into less compressed bodies, either white dwarfs or neutron stars.

Black holes are difficult to observe on account of both their small size and the fact that they emit no light. They can be "observed," however, by the effects of their enormous gravitational fields on nearby matter. For example, if a black hole is a member of a binary star system, matter flowing into it from its companion becomes intensely heated and then radiates X rays copiously before entering the event horizon of the black hole and disappearing forever. Many investigators believe that one of the component stars of the binary X-ray system Cygnus X-1 is a black hole. Discovered in 1971 in the constellation Cygnus, this binary consists of a blue supergiant and an invisible companion star that revolve about one another in a period of 5.6 days.

Some black holes apparently have nonstellar origins. Various astronomers have speculated that large volumes of interstellar gas collect and collapse into supermassive black holes at the centres of quasars and galaxies. A mass of gas falling rapidly into a black hole is estimated to give off more than 100 times as much energy as is released by the identical amount of mass through nuclear fusion. Accordingly, the collapse of millions or billions of solar masses of interstellar gas under gravitational force into a large black hole would account for the enormous energy output of quasars and certain galactic systems. In 1994 the Hubble Space Telescope provided conclusive evidence for the existence of a supermassive black hole at the centre of the M87 galaxy. It has a mass equal to two to three billion Suns but is no larger than the solar system. The black hole's existence can be strongly inferred from its energetic effects on an envelope of gas swirling around it at extremely high velocities. Similar evidence suggests that a massive black hole with a mass of about 2.6 million Suns lies at the centre of our own Milky Way Galaxy.

The existence of another kind of nonstellar black hole has been proposed by the British astrophysicist Stephen Hawking. According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equal to that of an asteroid or less, might have been created during the big bang, a state of extremely high temperatures and density in which the universe is thought to have originated roughly 10 billion years ago. These so-called mini black holes, unlike the more massive variety, lose mass over time and disappear. Subatomic particles such as protons and their antiparticles (i.e., antiprotons) may be created very near a mini black hole. If a proton and an antiproton escape its gravitational attraction, they annihilate each other and in so doing generate energy--energy that they in effect drain from the black hole. If this process is repeated again and again, the black hole evaporates, having lost all of its energy and thereby its mass, since these are equivalent.

Black-hole model for active galactic nuclei

The fact that the total output from the nucleus of an active galaxy can vary by substantial factors supports the argument that the central machine is a single coherent body. A competing theory, however, holds that the less powerful sources may be understood in terms of multiple supernova explosions in a confined space near the centres of starburst galaxies. Nevertheless, for the most powerful cases, the theoretical candidate of choice is a supermassive black hole that releases energy by the accretion of matter through a viscous disk. The idea is that the rubbing of gas in the shearing layers of a differentially rotating disk would frictionally generate heat, liberating photons as the mass moves inward and the angular momentum is transported outward. Scaled-down versions of the process have been invoked to model the primitive solar nebula and the disks that develop in interacting binary stars.

The black hole has to be supermassive for its gravitational attraction to overwhelm the strong radiation forces that attempt to push the accreting matter back out. For a luminosity of 10⁴⁶ erg/sec, which is a typical inferred X-ray value for quasars, the black hole must exceed 10⁸ solar masses. The event horizon of a 10⁸ solar-mass black hole, from inside which even photons would not be able to escape, has a circumference of about two light-hours. Matter orbiting in a circle somewhat outside of the event horizon would be hot enough to emit X rays and have an orbital period of several hours; if this material is lumpy or has a nonaxisymmetric distribution as it disappears into the event horizon, variations of the X-ray output on a time scale of a few hours might naturally be expected.

To produce 10⁴⁶ erg/sec, the black hole has to swallow about two solar masses per year if the process is assumed to have an efficiency of about 10 percent for producing energy from accreted mass. The rough estimate that 10 percent of the rest energy of the matter in an accretion disk would be eventually liberated as photons, in accordance with Einstein's formula E = mc², should be contrasted with a total efficiency of about 1 percent in nuclear reactions if a mass of hydrogen were to be converted entirely into iron. If the large-scale annihilation of matter and antimatter is excluded from consideration, the release of gravitational binding energy when matter settles onto compact objects is the most powerful mechanism for generating energy in the known universe. (Even supernovas use this mechanism, for most of the energy released in the explosion comes from the gravitational binding energy or mass deficit of the remnant neutron star.)

Interacting and merging galaxies provide the currently preferred routes to supply the matter swirling into the black hole. The direct ingestion of a gas-rich galaxy yields an obvious external source of matter, but the enhanced accretion of the parent galaxy's internal gas through tidal interactions (or bar formation) may suffice in most cases. At lower luminosities, other contributing factors may come from the tidal breakup of stars passing too close to the central black hole or from the mass loss from stars in the central regions of the galaxy. Gathering matter at a rate of two solar masses per year (90 percent of which ends up as the gravitating mass of the black hole) will build up a black hole of 10⁸ solar masses in several tens of millions of years. This estimate for the lifetime of an active galactic nucleus is in approximate accord with the statistics of such objects. This does not imply that supermassive black holes at the centres of galaxies necessarily accumulate from a seed of very small mass by steady accretion. There remain many viable routes for their formation, the study of such processes being in a state of infancy.

Black holes

If the core remnant of a supernova exceeds about two solar masses, it continues to contract. The gravitational field of the collapsing star is predicted to be so powerful that neither matter nor light can escape it. The "star" then collapses to a black hole--a singularity, or point of zero volume and infinite mass, hidden by an event horizon at a distance called the Schwarzschild radius. Bodies crossing the event horizon, or a beam of light directed at such an object, would seemingly just disappear--pulled into a "bottomless pit."

Black holes remain hypothetical, but observations suggest that such phenomena may possibly exist in the star system Cygnus X-1 and at the centre of the Galaxy. (For further information on the subject, see Cosmos: Black-hole model for active galactic nuclei .)