Black holes are the
cold remnants of former stars, so dense that no matter—not even light—is able
to escape their powerful gravitational pull.
While most stars end
up as white dwarfs* or neutron stars*, black holes
are the last evolutionary stage in the lifetimes of enormous stars that had
been at least 10 or 15 times as massive as our own sun.
When giant stars reach
the final stages of their lives they often detonate in cataclysms known as supernovae*.
Such an explosion scatters most of a star into the void of space but leaves
behind a large "cold" remnant on which fusion no longer takes place.
In younger stars,
nuclear fusion creates energy and a constant outward pressure that exists in
balance with the inward pull of gravity caused by the star's own mass. But in
the dead remnants of a massive supernova, no force opposes gravity—so the star
begins to collapse in upon itself.
With no force to check
gravity, a budding black hole shrinks to zero volume—at which point it is
infinitely dense. Even the light from such a star is unable to escape its
immense gravitational pull. The star's own light becomes trapped in orbit, and
the dark star becomes known as a black hole.
Black holes pull
matter and even energy into themselves—but no more so than other stars or
cosmic objects of similar mass. That means that a black hole with the mass of
our own sun would not "suck" objects into it any more than our own
sun does with its own gravitational pull.
Planets, light, and
other matter must pass close to a black hole in order to be pulled into its
grasp. When they reach a point of no return they are said to have entered the event
horizon*—the point from which any escape is impossible because it requires
moving faster than the speed of light.
Small But Powerful
Black holes are small
in size. A million-solar-mass hole, like that believed to be at the center of
some galaxies, would have a radius of just about two million miles (three
million kilometers)—only about four times the size of the sun. A black hole
with a mass equal to that of the sun would have a two-mile (three-kilometer)
radius.
Because they are so
small, distant, and dark, black holes cannot be directly observed. Yet
scientists have confirmed their long-held suspicions that they exist. This is
typically done by measuring mass in a region of the sky and looking for areas
of large, dark mass.
Many black holes exist
in binary star systems*. These holes may continually pull mass from
their neighboring star, growing the black hole and shrinking the other star,
until the black hole is large and the companion star has completely vanished.
Extremely large black
holes may exist at the center of some galaxies—including our own Milky Way.
These massive features may have the mass of 10 to 100 billion suns. They are similar
to smaller black holes but grow to enormous size because there is so much
matter in the center of the galaxy for them to add. Black holes can accrue
limitless amounts of matter; they simply become even denser as their mass
increases.
Black holes capture
the public's imagination and feature prominently in extremely theoretical
concepts like wormholes*. These "tunnels" could allow
rapid travel through space and time—but there is no evidence that they exist.
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