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Issue 1, July 2002
In Search of Darkness: The quest to observe one of nature's most curious astronomical constructions
Rudy Montez
Astronomy and Physics, University of Texas at Austin
montez@jyi.org
Imagine
walking through a carnival. You hear laughing children, the bustle
of the crowd, and a man shouting, "Witness the immeasurable strength
of the ultimate attraction between matter and light! Behold the inescapable
grip that pervades the Universe! Only a dollar a head!" Intrigued,
you pay the small fee and enter a trailer. You find a table in the
center of the room with a black opaque glass case. You stare baffled,
waiting for something to happen, until you glance at the label and
read in big black letters, "THE BLACK HOLE." You realize you've been
had... or have you?
How could you tell? If physicists and astrophysicists have developed
the theory of black holes correctly, then it seems that a black hole
will never be "observed." By definition, light cannot escape the surface,
or event horizon, of a black hole. When you consider that to see an
object, light has to be either emitted or reflected by it, determining
whether a black opaque glass case harbors a black hole seems an impossible
task.
Theoretically, any object could become a black hole. However, it is
not easy. For a black hole to exist, the velocity required to break
free from the black hole's gravitational bind must be greater than
the speed of light. From the escape velocity we can calculate the
maximum radius a given mass must occupy to be a black hole. For instance,
to turn the earth into a black hole would require compressing it to
the size of a sphere with a radius less than 10 millimeters. Similarly,
if all the mass of the Sun were contained in a sphere with a radius
less than 3 kilometers it would become a black hole. The current size
of the sun is more than 690,000 kilometers.
How can mass be compressed to such a high density? How would an object
that does not emit any light be observed? As it turns out, astronomers
now know how black holes form and how to observe them. Black holes
were formulated as a theoretical solution to Einstein's theory of
general relativity. Astronomers came up with the idea that a black
hole could be formed during the death of large stars. The theory plays
out like this: During the lifetime of a star, its core is constantly
converting hydrogen to helium. This conversion releases tons of energy,
creating an outward radiation pressure. The gravity of the star's
mass wants to compress the star inward. Most of a star's lifetime
is spent in thermodynamic equilibrium, where the outward radiation
pressure equals the inward gravitational compression. Eventually,
as a star exhausts its supply of hydrogen, it loses the tug-of-war
to gravity. Gravity's victory results in a gravitational collapse.
A massive star undergoing gravitational collapse will crash down with
enough momentum to press matter beyond physical boundaries, forming
a black hole.
So, going back to the black opaque case at the carnival, if a table
is able to support it, chances are it does not contain a black hole.
If we assume the black hole's mass is small enough to be supported
by the table, its gravitational collapse would not have enough momentum
to form a black hole and it would just destroy the object. But if
it did manage to form, it would be a microscopic black hole that would
pass through the molecules of the glass and the ground on its way
to the earth's core.
So how does one observe a black hole? By calculating and predicting
what the consequences of a black hole are on the surrounding matter.
The surrounding matter can be observed because its light can escape.
A black hole's gravity will accrete, or gather matter, onto its surface,
which is called an event horizon. The effect of this is similar to
water draining from a sink. The water swirls around outside of the
drain, and as it approaches the drain, it eventually spirals into
it. With a black hole, gases orbit around the black hole's center
and remain in a stable orbit around the black hole, called an accretion
disk. Friction heats and ionizes the atoms, which release high-energy
photons, or x-ray light. These photons escape because they are not
at the event horizon. The photons reach the earth and are collected
by x-ray satellite observatories.
Occasionally a blob of gas will break from the stable orbit and enter
a chaotic orbit that takes the blob of gas on a wild spiraling path
toward the event horizon. If the angle of the accretion disk and event
horizon is just right, astronomers can observe the photons from the
blob spiral around the front and back of the event horizon. These
"pulse trains" can be used to characterize the rotation and size of
the black hole. The intensity dims as the blob is occulted, or blocked,
by the event horizon, similar to a solar eclipse where the moon blocks
the sun's light. Once the blob reappears on the other side of the
event horizon, its intensity increases again. The maximum peak in
these fluctuations decreases as the blob's orbit shrinks and the blob
approaches the event horizon.
In order for astronomers to observe the accretion disk around a black
hole, there must be a plentiful supply of matter for the black hole
to accumulate. The best supply of mass comes by way of another star
orbiting the center of mass of the black hole/star binary system.
An old, dying, expanding star, also called a Red Giant, can provide
an adequate source of mass that can be readily accreted by the black
hole. The gravitational dynamic dance between the members of a binary
system is mathematically well understood. For a black hole/star binary
system, the star is usually bright and visible, but the black hole
remains unseen. By understanding the dynamics of such a binary system,
astronomers can determine upper and lower limits for the mass of each
object. If the mass of the unseen object is greater than three times
the mass of our sun then it is large enough to be converting hydrogen
to helium and should be easily seen. If it is not seen, it is dynamically
determined to be a black hole.
To date, there are only a handful of stellar mass black hole candidates.
They are only candidates because the current observations are either
not sensitive enough to determine if the masses are definitely black
hole masses, or the observations haven't been corroborated. Two concrete
examples are Cygnus X-1 and GRO J1655-40. These two objects were first
identified by their x-ray emissions. Astronomers could not find optical
counterparts, or visible stars corresponding to the location of the
x-ray sources. Instead, they discovered that the x-ray sources were
the unseen companions of a binary system. By determining the dynamic
properties of each system, astronomers deduced that the unseen companions,
the black holes, each have masses so large they could not be anything
but black holes.
Astronomers are searching for - and finding - one of nature's most
curious constructions despite the obvious difficulty posed by their
absence of light. The indirect clues (x-ray emissions detected by
x-ray satellite observatories, orbital dynamics, and pulse trains)
are adding up and the only information missing from the picture is
that of the actual event horizon. The path to this Holy Grail is along
the chaotic orbits that create pulse trains. With advancing x-ray
observation technology and multi-wavelength observations, the realization
of this seemingly impossible task is here, and astronomers, as always,
are embracing the darkness.
Journal of Young
Investigators. 2002. Volume Six.
Copyright © 2002 by Rudy Montez and JYI. All rights reserved.
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JYI is supported by: The National Science Foundation,
The Burroughs Wellcome Fund, Glaxo Wellcome Inc., Science Magazine, Science's
Next Wave, Swarthmore College, Duke University, Georgetown University,
and many others. Copyright
©1998-2003 The Journal of Young Investigators, Inc. |