Black holes have such strange characteristics that scientists doubted their existence for years. Since the late 1960’s, however, most of these doubts have been dispelled, and astronomers, armed with the latest technology, have set out to find these elusive objects.
In May 1994, the findings of the Hubble Space Telescope gave scientists the most conclusive observation of black holes yet—a swirling concentration of matter at the center of a galaxy known as M87. According to the reports, this phenomenon could be explained only by the presence of a huge black hole with a mass of up to 3 billion times the mass of the sun.
How black holes are formed
Today, scientists believe that black holes come in various sizes. Many are formed from the collapsed remnants of very large stars. Others, like the one in M87, are supermassive objects formed at the heart of galaxies. And mini black holes, much smaller than a pinhead, may have been created during the big bang, the explosive birth of the universe some 15 billion years ago.
While a star burns fuel, it produces an outward push that counters the inward pull of gravity. When no more fuel remains, its core cools and shrinks, the internal pressure drops, and the star can no longer support its enormous weight. What happens next varies, depending on the mass of the star and its remaining core.
A star with a mass of about four to eight times the mass of the sun usually ends its life as a supernova, collapsing under its own weight. As it collapses, atomic particles in its core fuse, causing a shattering explosion that rips apart the outer avers of the original star. When a core mass of between .4 and 3 times the mass of the sun remains, it becomes a neutron star.
A star with a mass 8 to 25 times the mass of the sun may also collapse and explode but may not stop at the neutronrar stage. If the remnant is more than three times the mass of the sun, it may shrink to a tiny point so small and so dense that its gravitational attraction is strong enough to prevent even light itself from escaping. This point then becomes the enter of a black hole.
A star that contains more than 25 times the mass of the sun may collapse without creating an explosion. If its remain-ing mass is at least three times that of the sun, it will shrink to form a black hole.
Some scientists believe that supermassive black holes may be at the center of many galaxies. In such a galaxy, huge gas clouds and swarms of stars rotate in a swirling mass. At the center of the galaxy, stars and gas clouds may become so closely packed that they are drawn together by gravity, eventually forming a single large mass. Continued gravitational contraction produces a black hole with the mass of millions or billions of suns.
Innumerable mini black holes may have formed during the big bang. The tremendous pressures within the explosion could have squeezed pockets of subatomic particles to incredibly high densities, converting them to mini black hoIes.
Searching for black holes
A black hole is invisible, but its incredibly powerful gravitational field wreaks havoc on surrounding matter. And these effects can be detected in the form of X rays.
Scientists have the best chance of finding a black hole in binary star systems. In such systems, a black hole can strip gas from its partner. If this happens, the gas falls violently toward the black hole in a tightening spiral called an accretion disk. As the gas gets closer to the black hole, it accelerates to nearly the speed of light. Compression and friction cause the gas to become progressively hotter, raising its temperature up to tens of millions of degrees. The hot gases emit enormous amounts of radiation, which can be detected by scientific instruments.
Scientists believe this process is generating the powerful X rays being emitted from the first black hole to be identified—Cygnus X-l in the constellation Cygnus. Once its position was known, astronomers discovered that this X-ray source had a companion star—a massive blue supergiant To find the supergiant’s dark companion, scientists used a process of elimination.
First, the orbital motion of the supergiant told scientists that the mass of the unseen object must be 6 to 15 times the mass of our sun. Second, it could not be a normal star, because a star so massive would almost certainly be visible. And third, it could not be a neutron star, because a neutron star cannot have a mass more than three times that of the sun. That left only one other possibility—a black hole.
Today, scientists have identified a few other possible black holes formed from dead stars: V404 Cygni, another suspicious object in Cygnus; LMC X-3, an erratic X-ray source in a nearby galaxy called the Large Magellanic Cloud; and the dark companion of A0620-00, an orange sunlike star that whips around in the Monoceros constellation.
Although just one star in millions is massive enough to end its life as a black hole, most galaxies contain at least 100 billion stars, so black holes resulting from collapsed stars would hardly be rarities. There may be millions of black holes in the Milky Way alone.