How can we see a black hole when, by definition, black holes harbor a gravitational force so powerful that not even light can escape?

Although we cannot see a black hole directly, we can detect all the commotion that a black hole causes.

Black holes eagerly swallow gas or whole stars that happen to venture too close to them. This swallowing of matter releases a tremendous about of energy. In fact, astronomers estimate that up to half of all the light in the universe is generated by black holes swallowing (or, scientifically speaking, accreting) matter.

Most scientists are convinced of the existence of black holes of all sizes throughout the universe. The theory is that most galaxies, including our Milky Way galaxy, contain a supermassive black hole at the core, along with stellar-size black holes peppered throughout the galaxy.

Stellar-size black holes contain the mass of about ten suns or more. These form from the collapse of very massive stars, much larger than our Sun. When such a massive star runs out of fuel to burn, it becomes unstable. The star no longer has the energy to support its massive weight. The outer part of the star explodes into space, a colorful event called a supernova. Meanwhile, the core of the star collapses. The mass is so great that nothing stops the collapse. All matter is squeeze to a point of infinite density, called a singularity. This is the black hole, a singularity, a single point in space. There is no surface to such an object.

Supermassive black holes contain the mass of millions to billions of suns confined to a region no larger than our solar system. Scientists aren't sure about how these form, but supermassive black holes likely were created in the early universe from the collapse of unimaginably large gas clouds.

Black holes are invisible -- or as a scientist is likely to say, quiescent -- until matter passes by. Think of a black hole as a bowling ball on a soft mattress. If a marble gets to close to the bowling ball, it will roll right into the well that the heavy ball creates. In space, gas and whole stars roll into the gravitational well created by the black hole. This concept comes from Albert Einstein, who proposed that the force of gravity is really the effect of matter distorting the fabric of space, a concept called spacetime.

As matter falls towards a black hole -- be it stellar-sized or supermassive -- the matter picks up velocity, bangs into other bits of matter, and all together grows very hot. Where there is heat, there is energy and light. This is what we see when we look towards a black hole: the light emitted from matter being ripped apart as it falls into the void.

Like planets around a star, matter often orbits a black hole. This is called an accretion disk -- a disk of gas swirling around the black hole analogous to water going down a drain. The disk varies greatly in temperature, growing hotter and more energetic closer to the black hole. Gas in the outer part of an accretion disk often glows in optical and ultraviolet light. The Hubble Space Telescope detects this type of light. Closer to the black hole, the gas is so hot and so energetic that it actually glows in X-ray radiation. This gas is thousands of times more energetic than the gas emitting optical light, with a temperature often hotter than the surface of the Sun. The Chandra X-ray Observatory detects this type of light very close to a black hole.

Also, although black holes are notorious for swallowing matter, they somehow manage to push matter away in collimated jets moving at near the speed of light. These jets, racing off perpendicularly from the flow of matter in the accretion disk, stretch for light years and are detected by radio and gamma-ray observatories. Thus, with a multitude of telescopes -- radio, infrared, optical, ultraviolet, X-ray and gamma-ray -- scientists can piece together a picture of the black hole behind all the commotion.

When you think about a galaxy, you might imagine a bulge of light in the middle. That bulge is likely caused by a supermassive black hole. Scientists have shown that the larger the bulge, the larger the black hole. This is because the black hole itself is generating light, like a dynamo, and also because millions of stars are slowly falling into the gravitational well of the black hole. They are all hovering near the core because that's the greatest source of gravity in the region. Scientists can estimate the mass of a black hole by calculating the velocity of stars orbiting it.

Black holes are the ultimate expression of gravity. The reason light cannot escape a black hole is because the pull of gravity is too strong. On Earth, it takes massive rockets to propel the Space Shuttle into orbit -- that is, to break free of Earth's gravity. The shuttle must reach great speeds, or else it will fall back. In a black hole, the escape velocity is greater than the speed of light. Since nothing is faster than light, nothing can escape a black hole.

Black holes might be observed directly in two ways. The first is through gravitational waves. These waves, like light, travel at light speed. This goes back to Einstein too. Einstein proposed that all moving objects emit gravitational waves, like waves on an ocean. The waves make matter bob up and down like a buoy on the ocean. The waves are so minuscule though -- altering the distance between the Earth and Moon by less than an atom -- that Einstein himself figured we could never detect them.

Never say never. The technology is within reach. NASA and the European Space Agency are considering a mission called LISA that would detect gravitational waves from black holes, ripples in spacetime. LISA's three satellites would "float" in space like buoys, tethered by a laser used to sense changes in the distances between satellites. NASA Goddard is now constructing instruments that can measure minute distance changes. The most massive objects produce the largest, or loudest, waves; and LISA would be tuned to galaxy mergers (with merging supermassive black holes), among other phenomena. Gravitational waves have not yet been detected directly. This represents a whole new window to the Universe.

The other way to directly image a black hole is to image its event horizon. This is the theoretical border of a black hole, the point where light and matter cannot return once crossed. Current X-ray observatories get us close to the event horizon. A proposed NASA mission called MAXIM would use the collection power of several X-ray mirrors in space to observe the X-ray light at the event horizon -- falling into the void or escaping via the jets. Capturing the innermost ring of light around a black hole is tantamount to seeing the void itself.

Scientists first found evidence of black holes only 30 years ago. In another decade or so, we may indeed take a virtual journey to the rim of a black hole.