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BLACKHOL.DOC - Article on Black Holes

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. 


  1. ————————————————————

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. 

– more – 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. 

       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 

– more – 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. 

       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. 


– more – 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. 


       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 

– more – 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, 

– more – 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 

– more – (ten times the mass of our Sun) after exhaustion of its nuclear

  fuels, the Schwarzschild radius is about 30 kilometers. 
  1. ——————————————————————–

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 

– more – 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 

       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? 


       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 

– more – 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 


       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 

– more – universe. Black holes are theorized to play relativistic tricks with

  space and time. 

       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. 


       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 

– more – would fan out across our galaxy like a death ray, destroying life on

  Earth and wherever else it may have occurred. 

       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 

       Our universe is theorized to have begun with a bang that sent 
  pieces of it outward in all directions.  As yet, astronomers have not 

– more – 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 

       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. 
/data/webs/external/dokuwiki/data/pages/archive/fun/blackhol.nas.txt · Last modified: 1999/09/29 13:55 (external edit)