Astronomy & Astrology

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 German American 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 may form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their heat is no longer available to resist contraction of the core to ever higher densities. Two new types of 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 core mass exceeds about 1.7 solar masses, 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 measured the acceleration of gases around the center of the galaxy M 87 and found that an object of 2.5 billion to 3.5 billion solar masses must be present. A second potential black hole at the center of the galaxy NGC 4258 was discovered in 1995 by astronomers using the Very Long Baseline Array (VBLA), an array of radio telescopes that spans a large geographical area. Also in 1995, astronomers used the HST to discover a third black hole near, but slightly displaced from, the center of the galaxy NGC 4261. In both of the 1995 discoveries, astronomers detected accretion disks, or disks of hot, gaseous material, circling the centers of the galaxies with accelerations that indicated the presence of a very massive object. In 1997 astronomers using the HST and ground-based telescopes in Hawaii announced that a census of nearby, ordinary galaxies shows that almost every galaxy may contain a massive black hole in its center. Knowing more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

The results of two studies announced in early November 1997 provide unprecedented support for "frame-dragging," a concept predicted by physicist Albert Einstein's general theory of relativity. Frame-dragging describes how massive objects actually distort space and time around themselves as they rotate. One of the studies examined frame-dragging around black holes, an example of which is shown here in an artist's conception.

Joe Bergeron

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. In reaction to the concept of singularities, Hawking has also proposed that black holes do not collapse in such a manner but instead form so-called "worm holes" to other universes besides our own.

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. 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.