What is a black hole made of?

Although we have yet to detect a black hole directly, most scientists are convinced of their existence. The particulars -- that is, what does a black hole look like, exactly -- are still debated.

Albert Einstein's theories tell us that black holes are made of pure gravitational energy. They have mass and spin, but contain no matter. Anything that falls into a black hole is converted to energy.

Black holes represent a point of infinite density, called a singularity. They have no surface; they are not dark, black orbs (although they are often depicted this way). Black holes are, well, like holes. The perimeter is called the event horizon. We can see light outside this perimeter; the inside is forever black.

Photo courtesy: Chris Reynolds

In contrast, a neutron star has a surface. Neutron stars are dense spheres about ten miles across made mostly of neutrons. Both neutron stars and black holes are created in massive star explosions. A quick history: When a massive star runs out of fuel to burn, it no longer has outward radiation pressure to support its mass. The core of such a star collapses, while shock waves cause the outer parts to explode in space (what we see from a safe distance as a beautiful supernova).

The core, containing the mass of several of our suns, keeps collapsing,getting denser and denser, squeezing its protons and electrons into neutrons The packing together of neutrons, like marbles in a jar, can prevent the star from collapsing further. At this stage, it is a neutron star with a surface. Stars about two to eight times more massive than our Sun collapse into neutron stars. But if the core has a little more mass,not even the neutrons can support this star. The core keeps collapsing beyond the density of a neutron star to a point of infinite density, the black hole. Stars over eight times more massive than our Sun, theory states, will collapse into a black hole.

If our spacecraft were parked near a black hole, what would we see? We might see gas pouring into the black hole. In fact, scientists infer the existence of black holes by observing the light emitted from hot gas presumably falling toward the void. We would see the event horizon, or perimeter, marking the point were light seemingly disappears, like water going over a waterfall from the angle of rafter upstream.

That light isn't really disappearing. Gas will freely cross the border of the event horizon without needing to present its passport. Just before the event horizon, the tug of gravity is strong, but light emitted from the gas can still escape the region. Once the gas crosses the event horizon, it is likely still emitting light, but we cannot see this light. This is because gravity is so strong that it prevents light from shooting out towards us. Gravity sucks the light back. Thus, the event horizon marks the regions of what we can and cannot see. Matter and light will continue to fall toward the center of the black hole, the singularity. All matter and energy will be compressed to a single point in the center of the black hole.

If this were a neutron star, we could fly "under" and around the star, much like we could do with the Moon or Sun. The black hole, as hard as this is to visualize, is a four-dimensional object. The region has length, width,depth and the fourth dimension, time. Farther out, we can discern some structure We might see that this black hole is in a binary star system;its neighbor might be a fiery red giant. If so, we might see that this invisible black hole is pulling gas away from its companion. That gas swirls around the black hole in an accretion disk, like water doing down a drain. So from farther out, we can fly "above" and "below" the accretion disk. All is "normal."

Gas in the accretion disk can maintain an orderly, innermost stable orbit around the black hole. This will appear like a ring around the black hole. There is a gap, perhaps several miles wide, between the accretion disk's innermost stable orbit and the event horizon. In this gap, gas falls freely at varying rates and directions. It's a chaotic free-for-all at this point. Observations get messy.

Closer to the black hole event horizon, our view is distorted. These are the particulars that scientists debate. Recently there has been evidence that black holes spin, like neutron stars. If black holes have no surface,this means that the fabric of space, a four-dimensional concept called space-time, is spinning with the black hole. Imagine walking on a moving walkway at an airport. Gas, already moving quickly, orbits the black hole even faster on this moving walkway. Gravity distorts light in bizarre ways,too. Light from behind the black hole is warped by gravity, and from our spacecraft we would see gas from both the front and back of the black holes simultaneously

Close in, from every angle we approach it, the black hole would look essentially the same: a dark dome created by the effect of light from behind bending up and around. Time comes to a standstill, too. If we were unfortunate enough to cross the event horizon, we would see no change in time. But from a safe vantage point, as we gaze toward the black hole, we might see an object (one of the unlucky spacecraft) taking an infinite amount of time to cross the event horizon. This is because light emitted from the spacecraft is being pulled back by gravity. The spacecraft may be long gone, but its light is having a difficult time reaching our eyeballs to tell us this. Light, in many ways, is running the wrong way on the moving walkway, up the down escalator. The closer to the event horizon, the faster this down escalator is moving. The event horizon is the point were no light can escape. At the nanometers outside the event horizon, light essentially takes an infinite time to reach us.

All of this visualization, of course, assumes there is gas (or an unlucky spacecraft with taillights) feeding the black hole. If there is no companion star -- or no gas donated by the companion star -- then the black hole region is pitch black. The blackness is defined by the event horizon, a border that grows as more matter enters into the black hole.

So the answer to "what does a black hole look like," is that it looks like nothing. Gas falling into the black hole reveals its shape, in a sense, but even that view is distorted by gravity's effect on light. From any direction you approach it, the black hole is like a dark funnel. The answer to "what is the black hole made of," is that it is made of nothing but energy. Enter the funnel, and you will be squeezed into gravitational energy.

This week's question is provided by Chris Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.