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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
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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.
WHITE DWARFS
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.
PULSARS
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.
BLACK HOLES
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.
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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.
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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.
WORMHOLES
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