holes were theorized more than 200 years ago, and later were
predicted by Einstein's theory of general relativity. The
discovery of active galaxies forced astronomers to think that
monstrous black holes really do exist and are the "engines"
at the heart of these fireworks. The gushers of light and
other radiation from such objects could not be explained by
definition, a black hole is very hard to find. It is a celestial
object that squeezes a lot of material into a very small space.
The resulting gravitational pull is so intense that anything
passing nearby, even light, is trapped forever.
a ghost in a mystery story, a black hole's presence must be
inferred by the effects on its surroundings. Its powerful
gravity will influence the motion of neighboring stars. The
closer the stars are to the black hole, the faster they should
be moving, just as orbiting planets move faster the closer
they are to the Sun. If no black hole is present, the speed
of the stars should slow toward the hub of a galaxy, because
most of the gravity influencing their motion would come from
the other stars in the galaxy.
the speed of the entrapped material is measured, astronomers
can calculate the mass of the black hole using the simple
laws of gravity, just as the orbital speed of the Moon can
be used to calculate Earth's mass. If it turns out that there
is far more mass present than there are stars, the matter
must be tucked away in something that is invisible and compact.
observations have been made with ground-based telescopes since
the mid-1980s; but having to look through the Earth's turbulent
atmosphere severely limits the accuracy of such telescopes
for detecting and measuring a large, central mass. While the
ground-based data give ambiguous lower limits to the central
mass, NASA Hubble Space Telescope observations are decisive
for accurately measuring the mass and ruling out all other
first black hole confirmation was nailed down when the space
telescope uncovered a spiral disk of gas swirling around the
hub of the giant elliptical galaxy M87. The shape alone suggested
that the material was caught in a gravitational whirlpool.
Using Hubble's spectrographs, astronomers were able to measure
the velocity of the gas by a method known as Doppler shift.
As the disk spins like a carousel, one side of it approaches
us and is blueshifted, while the other side rotates away and
concluded that the gas is whirling at more than a million
miles an hour. This information can be used to calculate how
much mass is packed into the core of M87. It turns out that
the mass of two billion Suns is compressed into a region of
space no bigger than our solar system. Hubble has made similar
observations in two other elliptical galaxies, NGC 4261 and
NGC 3115. These monstrous black holes weigh in, respectively,
at 200 million solar masses and two billion solar masses.
of galaxy nuclei in both active and quiescent galaxies suggest
black holes are common to virtually all galaxies. The mystery
is how the black holes formed in the first place. The abundance
of quasars in the early universe, objects at the hearts of
galaxies that pour out a torrent of radiation, suggest that
monstrous black holes must have formed very early, though
it is still not known how this happened.
images of quasars show that they reside in a variety of galaxies,
both spiral and elliptical. Many but not all of the quasar
host galaxies, are engaged in a collision or interaction with
other bypassing galaxies. The infall of gas resulting from
such collisions fuels the monster black holes.
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