Nome: Patrícia Gomes T2811E1
Nome: Thadeu Theodoro de Sillos C625DB1
Black Holes
Don't let the name fool
you: a black hole is anything but empty space. Rather, it is a great amount of
matter packed into a very small area - think of a star ten times more massive
than the Sun squeezed into a sphere approximately the diameter of New York
City. The result is a gravitational field so strong that nothing, not even
light, can escape. In recent years, NASA instruments have painted a new picture
of these strange objects that are, to many, the most fascinating objects in
space.
Although the term was
not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so
massive and dense that light could not escape it has been around for centuries.
Most famously, black holes were predicted by Einstein's theory of general
relativity, which showed that when a massive star dies, it leaves behind a
small, dense remnant core. If the core's mass is more than about three times
the mass of the Sun, the equations showed, the force of gravity overwhelms all
other forces and produces a black hole.
Scientists
can't directly observe black holes with telescopes that detect x-rays, light,
or other forms of electromagnetic radiation. We can, however, infer the
presence of black holes and study them by detecting their effect on other
matter nearby. If a black hole passes through a cloud of interstellar matter,
for example, it will draw matter inward in a process known as accretion. A
similar process can occur if a normal star passes close to a black hole. In
this case, the black hole can tear the star apart as it pulls it toward itself.
As the attracted matter accelerates and heats up, it emits x-rays that radiate
into space. Recent discoveries offer some tantalizing evidence that black holes
have a dramatic influence on the neighborhoods around them - emitting powerful
gamma ray bursts, devouring nearby stars, and spurring the growth of new stars
in some areas while stalling it in others.
One Star's End is a Black Hole's Beginning
Most black holes form from the remnants of a large
star that dies in a supernova explosion. (Smaller stars become dense neutron
stars, which are not massive enough to trap light.) If the total mass of the
star is large enough (about three times the mass of the Sun), it can be proven
theoretically that no force can keep the star from collapsing under the
influence of gravity. However, as the star collapses, a strange thing occurs.
As the surface of the star nears an imaginary surface called the "event
horizon," time on the star slows relative to the time kept by observers
far away. When the surface reaches the event horizon, time stands still, and
the star can collapse no more - it is a frozen collapsing object.
Even bigger
black holes can result from stellar collisions. Soon after its launch in
December 2004, NASA's Swift telescope observed the powerful, fleeting flashes
of light known as gamma ray bursts. Chandra and NASA's Hubble Space Telescope
later collected data from the event's "afterglow," and together the
observations led astronomers to conclude that the powerful explosions can
result when a black hole and a neutron star collide, producing another black
hole.
Babies and Giants
Although the
basic formation process is understood, one perennial mystery in the science of
black holes is that they appear to exist on two radically different size
scales. On the one end, there are the countless black holes that are the
remnants of massive stars. Peppered throughout the Universe, these
"stellar mass" black holes are generally 10 to 24 times as massive as
the Sun. Astronomers spot them when another star draws near enough for some of
the matter surrounding it to be snared by the black hole's gravity, churning
out x-rays in the process. Most stellar black holes, however, lead isolated
lives and are impossible to detect. Judging from the number of stars large
enough to produce such black holes, however, scientists estimate that there are
as many as ten million to a billion such black holes in the Milky Way alone.
On the other
end of the size spectrum are the giants known as "supermassive" black
holes, which are millions, if not billions, of times as massive as the Sun.
Astronomers believe that supermassive black holes lie at the center of
virtually all large galaxies, even our own Milky Way. Astronomers can detect
them by watching for their effects on nearby stars and gas.
Historically, astronomers have long believed that no
mid-sized black holes exist. However, recent evidence from Chandra,
XMM-Newton and Hubble strengthens the case that mid-size black holes do
exist. One possible mechanism for the formation of supermassive black
holes involves a chain reaction of collisions of stars in compact star clusters
that results in the buildup of extremely massive stars, which then collapse to
form intermediate-mass black holes. The star clusters then sink to the center
of the galaxy, where the intermediate-mass black holes merge to form a supermassive
black hole.
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