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They say that truth is stranger than fiction,
and it turns out that nature is stranger than science fiction.
More than a dozen black holes have already been discovered in our
Milky Way galaxy - out of more than a million black holes estimated
to exist there. And a giant black hole, heavier than millions
of stars, has been discovered at the center of our Milky Way galaxy.
The same picture appears to hold for other galaxies in the universe:
black holes scattered through the galaxies, with a giant black
hole at their center. Black holes turn out to be the most
powerful objects in the universe. Exploring them is one
of the truly exciting frontiers of astronomy.
If black holes are black, how can we see them? How do we know
they are black holes? Though we cannot see black holes directly,
they are so powerful that we can see their unmistakable, dramatic
effects on the matter around them. Here are three lines of evidence
that black-hole hunters look for:
A blaze of X-rays
Matter that comes
too close to a black hole – matter such as gas and dust, or even
a whole star – is drawn towards the hole. As the matter spirals
in towards the edge of the hole, it heats up, reaching millions
degrees before plunging into the black hole. When gas is
this hot, it glows in X-ray light, and can be seen by an X-ray
space telescope such as Chandra.
This glowing disc of gas is a fingerprint for the presence of
a black hole.
Super-powerful jets of matter
Giant jets of matter – the most powerful beams in the universe
– are observed to shoot out from a galaxy's core at almost the speed
of light. The only known source powerful enough to produce such jets,
is a giant spinning black hole. The jets seem to originate from just
outside the black hole's edge ("horizon"), where temperatures
and magnetic fields are at their most intense. How the black hole
creates these jets is not known.
Rapidly moving stars
The gravitational
field of a black hole tugs on the stars in its vicinity. A super-massive
black hole will make whole swarms of stars whip around as they
fall under its influence. By following the
motions of the orbiting stars, astronomers can deduce the
location, and size, of the central black hole they cannot see.
From these and other lines of evidence, astronomers are convinced
that black holes are real, and that they play an important role in
the universe. What secrets do black holes promise to reveal? Explore
more in the next section > |
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