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The following material was downloaded from the NASA SpaceLink
BBS at the National Aeronautics and Space Administration, George C. Marshall Space Flight Center, Marshall Space Flight Center, Alabama 35812 on 11/16/88.
B L A C K H O L E S I N S P A C E
There is much more to black holes than meets the eye. In fact, your eyes, even with the aid of the most advanced telescope, will never see a black hole in space. The reason is that the matter within a black hole is so dense and has so great a gravitational pull that it prevents even light from escaping.
Like other electromagnetic radiation (radio waves, infrared rays, ultraviolet radiation, X-rays, and gamma radiation), light is the fastest traveler in the Universe. It moves at nearly 300,000 kilometers (about 186,000 miles) per second. At such a speed, you could circle the Earth seven times between heartbeats.
If light can't escape a black hole, it follows that nothing else can. Consequently, there is no direct way to detect a black hole.
In fact, the principal evidence of the existence of black holes comes not from observation but from solutions to complex equations based on Einstein's Theory of General Relativity. Among other things, the calculations indicate that black holes may occur in a variety of sizes and be more abundant than most of us realize.
MINI BLACK HOLES
Some black holes are theorized to be nearly as old as the Big Bang, which is hypothesized to have started our Universe 10 to 20 billion years ago. The rapid early expansion of some parts of the dense hot matter in this nascent Universe is said to have so compressed less rapidly moving parts that the latter became superdense and collapsed further, forming black holes. Among the holes so created may be the submicroscopic mini-black holes.
A mini-black hole may be as small as an atomic particle but contain as much mass (material) as Mount Everest. Never underestimate the power of a mini-black hole. If some event caused it to decompress, it would be as if millions of hydrogen bombs were simultaneously detonated.
HOW STARS DIE
The most widespread support is given to the theory that a black hole is the natural end product of a giant star's death. According to this theory, a star like our Sun and others we see in the sky lives as long as thermal energy and radiation from nuclear reactions in its core provide sufficient outward pressure to counteract the inward pressure of gravity caused by the star's own great mass.
When the star exhausts its nuclear fuels, it succumbs to the forces of its own gravity and literally collapses inward. According to equations derived from quantum mechanics and Einstein's Theory of General Relativity, the star's remaining mass determines whether it becomes a white dwarf, a neutron star, or black hole.
Stars are usually measured in comparison with our Sun's mass. A star whose remaining mass is about that of our Sun condenses to approximately the size of Earth. The star's contraction is halted by the collective resistance of electrons pressed against each other and their atomic nuclei. Matter in this collapsed star is so tightly packed that a piece the size of a sugar cube would weigh thousands of kilograms. Gravitational contraction would also have made the star white hot. It is appropriately called a white dwarf.
Astronomers have detected white dwarfs in space. The first discovery was a planet-sized object that seemed to exert a disproportionately high gravitational effect upon a celestial companion, the so call dog star Sirius, which is about 2.28 times our Sun's mass. It appeared that this planet-sized object would have to be about as massive as our Sun to affect Sirius as it did. Moreover, spectral analysis indicated the star's color was white.
Based upon these and other studies, astronomers concluded that they had found a white dwarf. However, it took many years after the discovery in 1914 before most scientists accepted the fact that an object thousands of times denser than anything possible on Earth could exist.
NEUTRON STARS AND SUPERNOVAS
Giant stars usually lose most of their mass during their normal lifetimes. If such a star still retains 1 1/2 to 3 solar masses after exhaustion of its nuclear fuels, it would collapse to even greater density and smaller size than the white dwarf. The reason is that there is a limit on the amount of compression electrons can resist in the presence of atomic nuclei.
In this instance, the limit is breached. Electrons are literally driven into atomic nuclei, mating with protons to form neutrons and thus transmuting nuclei into neutrons. The resulting object is aptly called a neutron star. It may be only a few kilometers in diameter. A sugar-cube size piece of this star would weigh about one-half a trillion kilograms.
Sometimes, as electrons are driven into protons in atomic nuclei, neutrinos are blown outward so forcefully that they blast off the star's outer layer. This creates a supernova that may temporarily outshine all of the other stars in a galaxy.
The most prominent object believed to be a neutron star is the Crab Nebula, the remnant of a supernova observed and reported by Chinese astronomers in 1504. A star-like object in the nebula blinks, or pulses, about 30 times per second in visible light, radio waves, and X and gamma rays. The radio pulses are believed to result from interaction between a point on the spinning star and the star's magnetic field. As the star rotates, this point is theorized alternately to face and be turned away from Earth. The fast rotation rate implied by the interval between pulses indicates the star is no more than a few kilometers in diameter because if it were larger, it would be torn apart by centrifugal force.
Radio telescopes have detected a large number of other objects which send out naturally pulsed radio signals. They were named pulsars. Like the object in the Crab Nebula, they are presumed to be rotating neutron stars.
Of these pulsars, only the Vela pulsar–which gets its name because of its location in the Vela (Sails) constellation–pulses at wavelengths shorter than radio. Like the Crab pulsar, the Vela pulsar also pulses at optical and gamma ray wavelengths. However, unlike the Crab pulsar, it is not an X-ray pulsar. Aside from the mystery generated by these differences, scientists also debate the reasons for the pulses at gamma, X-ray and optical frequencies. As noted earlier, they agree on the origin of the radio pulses.
When a star has three or more solar masses left after it exhausts its nuclear fuels, it can become a black hole.
Like the white dwarf and neutron star, this star's density and gravity increase with contraction. Consequently, the star's gravitational escape velocity (speed needed to escape from the star) increases. When the star has shrunk to the Schwarzschild radius, named for the man who first calculated it, its gravitational escape velocity would be nearly 300,000 kilometers per second, which is equal to the speed of light. Consequently, light could never leave the star.
Reduction of a giant star to the Schwarzschild radius represents an incredible compression of mass and decrease in size. As an example, mathematicians calculate that for a star of 10 solar masses (ten times the mass of our Sun) after exhaustion of its nuclear fuels, the Schwarzschild radius is about 30 kilometers.
According to the Law of General Relativity, space and time are
warped, or curved, by gravity. Time is theorized TO POINT INTO THE BLACK HOLE FROM ALL DIRECTIONS. To leave a black hole, an object, even light would have to go backward in time. Thus, anything falling into a black hole would disappear from our Universe.
The Schwarzschild radius becomes the black hole's "event
horizon", the hole's boundary of no return. Anything crossing the event horizon can never leave the black hole. Within the event horizon, the star continues to contract until it reaches a space-time singularity, which modern science cannot easily define. It may be considered a state of infinite density in which matter loses all of its familiar properties.
Theoretically, it may take less than a second for a star to collapse into black hole. However, because of relativistic effects, we could never see such an event. This is because, as demonstrated by comparison of clocks on spacecraft with clocks on Earth, gravity can slow, perhaps even stop, time. The gravity of the collapsing star would slow time so much that we would see the star collapsing for as long as we watched.
Once a black hole has been formed, it crushes into a singularity anything crossing its event horizon. As the black hole devours matter, its event horizon expands. This expansion is limited only by the availability of matter. Incredibly vast black holes that harbor the crushed remains of billions of solar masses are theoretically possible.
Evidence that such superdense stars as white dwarfs and neutron stars do exist has supported the idea that black holes, representing what may be the ultimate in density, must also exist. Potential black holes, stars with three or more times the mass of our Sun, pepper the sky. But how can astronomers detect a black hole?
HOW BLACK HOLES MAY BE INDIRECTLY DETECTED
Scientists found indirect ways of doing so. The methods depends upon black holes being members of binary star systems. A binary star system consists of two stars comparatively near to and revolving about each other. Unlike our Sun, most stars exist in pairs.
If one of the stars in a binary system had become a black hole, the hole would betray its existence, although invisible, by its gravitational effects upon the other star. These effects would be in accordance with Newton's Law: attractions of two bodies to each other are directly proportional to the square of the distance between them. The reason is that outside of its event horizon, a black hole's gravity is the same as other objects'.
Scientists also have determined that a substantial part of the energy of matter spiraling into a black hole is converted by collision, compression, and heating into X- and gamma rays displaying certain spectral characteristics. The radiation is from the material as it is pulled across the hole's event horizon, its radiation cannot escape.
Some scientists speculate that matter going into a black hole may survive. Under special circumstances, it might be conducted via passages called "wormholes" to emerge in another time or another universe. Black holes are theorized to play relativistic tricks with space and time.
NASA ORBITING OBSERVATORY OBSERVATIONS
Black hole candidates–phenomena exhibiting black hole effects–have been discovered and studied through such NASA satellites as the Small Astronomy Satellites (SAS) and the much larger Orbiting Astronomical Observatories (OAO) and High Energy Astronomical Observatories (HEAO). The most likely candidate is Cygnus X-1, an invisible object in the constellation Cygnus, the swan. Cygnus X-1 means that it is the first X-ray source discovered in Cygnus. X-rays from the invisible object have characteristics like those predicted from material as it falls toward a black hole. The material is apparently being pulled from the hole's binary companion, a large star of about 30 solar masses. Based upon the black hole's gravitational effects on the visible star, the hole's mass is estimated to be about six times of our Sun. In time the gargantuan visible star could also collapse into a neutron star or black hole or be pulled piece by piece into the existing black hole, significantly enlarging the hole's event horizon.
BLACK HOLES AND GALAXIES
It is theorized that rotating black holes, containing the remains of millions or billions of dead stars, may lie at the centers of galaxies such as our Milky Way and that vast rotating black holes may be the powerhouses of quasars and active galaxies. Quasars are believed to be galaxies in an early violent evolutionary stage while active galaxies are marked by their extraordinary outputs of energy, mostly from their cores.
According to one part of the General Theory of Relativity called the Penrose Process, most of the matter falling toward black holes is consumed while the remainder is flung outward with more energy than the original total falling in. The energy is imparted by the hole's incredibly fast spin. Quiet normal galaxies like our Milky Way are said to be that way only because the black holes at their centers have no material upon which to feed.
This situation could be changed by a chance break-up of a star cluster near the hole, sending stars careening into the hole. Such an event could cause the nucleus of our galaxy to explode with activity, generating large volumes of lethal gamma radiation that would fan out across our galaxy like a death ray, destroying life on Earth and wherever else it may have occurred.
BLACK HOLES AND GALACTIC CLUSTERS
Some astronomers believe that the gravity pulls of gigantic black holes may hold together vast galactic clusters such as the Virgo cluster consisting of about 2500 galaxies. Such clusters were formed after the Big Bang some 10 to 20 billion years ago. Why they did not spread randomly as the Universe expanded is not understood, as only a fraction of the mass needed to keep them together is observable. NASA's Hubble Space Telescope and AXAF Telescope, scheduled for a future Shuttle launch, will provide many more times the data than present ground and space observatories furnish and should contribute to resolving this and other mysteries of our Universe.
BLACK HOLES AND OUR UNIVERSE
Our universe is theorized to have begun with a bang that sent pieces of it outward in all directions. As yet, astronomers have not detected enough mass to reverse this expansion. The possibility remains, however, that the missing mass may be locked up in undetectable black holes that are more prevalent than anyone realizes.
If enough black holes exist to reverse the universe's expansion, what then? Will all of the stars, and galaxies, and other matter in the universe collapse inward like a star that has exhausted its nuclear fuels? Will one large black hole be created, within which the universe will shrink to the ultimate singularity?
Extrapolating backward more than 10 billion years, some cosmologists trace our present universe to a singularity. Is a singularity both the beginning and end of our universe? Is our universe but a phase between singularities?
These questions may be more academic than we realize. Scientists say that, if the universe itself is closed and nothing can escape from it, we may already be in a black hole. e. Scientists say that, if the universe itself is closed and nothing can esc