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NASA's Search to Understand Extreme Gravity: Black Holes and Active Galactic
Nuclei
Credit: Alfred Kamajian |
This artis's impression of a supermassive black hole
highlights the accretion disk of gas and stars swirling around the black hole
and the jets of material ejected along the poles. |
Constellation-X, a group of 4 to 6 satellites acting in unison, is a NASA
mission currently in development (http://constellation.gsfc.nasa.gov/). The
satellites will measure how forces of extreme gravity operate near a black
hole by mapping the distortions of space-time predicated by Einstein's
Theory of General Relativity. The Observatory will study the two classes of
black holes: both galactic black holes, the remains of massive stars
10-100 times the size of our Sun, and supermassive black holes, the
powerhouses in the centers of galaxies that range up to billions of solar
masses in size. Our own Galaxy harbors thousands of stellar black holes,
and new observations show that most galaxies, including possibly our own,
have a supermassive black hole at their core. Within both classes of black
hole, space and time as we know it collapse. Thus, black holes are cosmic
laboratories, allowing us to explore the ultimate limits of our physical
laws and of gravity.
One unavoidable challenge when studying black
holes, however, is that they're almost invisible. A black hole is defined by a
surface called the event horizon, the point at which gravity is so strong
that nothing, not even light, can escape. The stellar matter itself is
crushed into a singularity at the center, hidden behind the event horizon.
The event horizon of a galactic black hole is only a few miles across. In
supermassive black holes, it is only about the size of our Solar System.
How do we go about observing black holes if they are so compact and emit no
visible light? There are a couple of tricks. Stellar black holes are
often part of a binary star system, two stars revolving around each other.
What we see from Earth is a visible star orbiting around what appears to be
nothing. In reality, it is orbiting around the black hole. We can infer
the mass of the black hole by the way the visible star is orbiting around
it. The larger the black hole, the greater the gravitational pull, and the
greater the effect on the visible star.
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Galactic (stellar sized) black holes are often found in a binary system.
This artist's conception shows how the accretion disk forms as material is
pulled from the companion star and swirls into the black hole. |
Credit: Margaret Masetti (GSFC) |
Another way we can "see" a black hole is by observing X-rays generated
around it. Because a black hole has such a powerful gravitational force, a
galactic black hole in a binary system can literally tear apart its
companion star. Gas from the companion swirls into the black hole like
water down a drain. The swirling gas is what we call an accretion disk.
As the gas gets closer to the black hole, it heats up from the friction of
ever faster moving gas molecules. Just outside the black hole's event
horizon, the gas heats to temperatures in the range of millions of degrees.
Gas heated to these temperatures releases tremendous amounts of energy in
the form of X-rays.
Supermassive black holes also have an accretion disk. This is formed not
by a single star, as in a binary system, but by the great amounts of gas
present in the regions between stars. In about 10% of supermassive black
holes, jets of energized matter thousands of light years in length shoot
out in opposite directions. This can be detected in radio, visible, X-ray
and gamma-ray wavelengths. These jets accelerate matter to nearly the
speed of light through a mechanism not well understood.
Credit: National Radio Astronomy Observatory/AUI [VLA] |
This radio image of the nearby active galaxy Cygnus A shows
its prominent jets that extend for almost 18,000 light years in either
direction. |
From small to large-scale
black holes, many questions abound:
- How is material fed directly into the black hole?
- How do jets form?
- Why do some AGN have jets, while many more do not?
- What keeps the jets powered for millions of years?
- Why is the AGN phenomenon -- intense variations in X-ray output
from an energy source -- more common in the past than it is today?
- How do supermassive black holes participate in the formation and
evolution of galaxies?
- What are the mass and spin of black holes?
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Constellation-X's large X-ray collecting area and superior resolution, or
clarity, will provide the most detailed, quantitative observations of the
region surrounding a black hole. Data from current telescopes can take us
near a black hole, but Constellation-X will take us to within a few miles
of its edge. It will be able to measure the mass and spin of a black hole,
two of its defining characteristics. Such measurements will enable
scientists to begin to answer the many questions that remain about the
formation and evolution of black holes, and about how the laws of physics
behave in extreme environments.
| This simulated image shows what the accretion disk around
a black hole might look like. The distortions of time and space by the intense
gravity of the black hole and motion of the material at close to the speed of
light cause emission to be shifted to longer and shorter wavelengths.
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Credit: Ben Bromley
(Harvard-Smithsonian Center for Astrophysics) |
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