Science Goals
Black Holes
What happens close to a black hole?
Finding an eyewitness to answer this question would certainly be a tough task. No one here on Earth has ever visited a black hole, which is a good thing, because a black hole is not something you would want to get close to. With each satellite mission, however, NASA peers a little bit farther into the belly of these beasts. Scientists are now piecing together the rough-and-tumble journey of stars and other bits of matter that wander too close.
Below is a description of a black hole and what physicists think happens in the black hole neighborhood. If you get confused, then you're right on par with the physicists, for black holes throw them for a loop too.
- What is a black hole?
- What kinds of black holes do we know of?
- How do we "see" black holes?
- What happens close to a black hole?
What is a black hole?
The term "black hole" describes a location in space with extremely high gravity. There is no hole per se, just a lot of gravity. Think of it as a gravitational well. The gravity is so strong that nothing can escape its pull, not even light, so it would appear black. The Earth's gravity is nothing compared to a black hole's. A rocket merely travels at 25,000 miles an hour to escape our planet's pull. That rocket could travel 671 million miles an hour, the speed of light, and still not escape from a black hole. Since nothing can travel faster than light, nothing can escape a black hole once it is in its clutches.
Black holes have a boundary of no return. Once an object crosses the boundary, called the event horizon, the object is theoretically lost forever. (No light can escape, hence no information about the object's whereabouts can escape.) Outside of the event horizon, or even close to the edge, an object still has a fighting chance to escape. The gravity is strong there, but light can get away. Within the event horizon, the game is over. All matter is infinitely stretched and, at the very center of the black hole, it is crushed to what physicists call a singularity. A singularity is a mathematical term to characterize matter infinitely compressed to a single point.
What kinds of black holes are there?
Scientists have identified three kinds of black holes – small, medium, and large. The small black holes go by several different names. Some scientists call them low-mass black holes; others say stellar or galactic black holes. These low-mass black holes form from collapsed stars that were once 100 times as massive as our Sun. When such a massive star runs out of energy, it throws off its outer shell and is left with a core that is still 15 times as massive as the Sun. The incredible mass collapses further to a region only a few miles across. This is this black hole, and the event horizon – that boundary of no return – is a few miles across.
Artist's concept of a galactic black hole and its companion star.
Low-mass black holes are more common than you might think. Scientists believe that our Milky Way Galaxy contains tens of thousands of these black holes, hence the name "galactic" black hole. They tend to live in binary star systems, with a "living" companion star losing healthy portions of its gas to the black holes extreme gravity. Scientists can see this gas as it funnels toward the black hole. That's one way they know black holes exist.
The large black holes are called supermassive black holes. They have the mind-boggling mass of one million to one billion Suns confined in a region no larger than our Solar System. This region marks out the event horizon. Once an object crosses the event horizon... well, you know. Singularity.
Supermassive black holes reside in the center of galaxies. Some scientists speculate that most galaxies, including our own, harbor a supermassive black hole. The theory is that these large-mass black holes either formed from collapsed gas when the host galaxies formed, or they formed later, growing with time as the result of galaxy and black hole mergers. The supermassive black hole is the source of large amounts of energy. All of the gas that pours into a black hole puts up quite a fight. The gas, under the force of extreme gravity, heats to millions of degrees and releases bright light across most of the electromagnetic spectrum, from radio waves through optical light and into X-ray light. This is matter's final cry before it is lost forever. Some black holes produce enormous jets of matter shooting away for millions of miles at nearly light-speed via a poorly understood mechanism. We cannot see the black hole, but we can see all the fireworks shooting out from the center of galaxy with a black hole. The brightest of these galaxies are called quasars, all at the farthest reaches of the Universe and all powered by black holes.
The mid-size black holes don't have a catchy name yet. This is because they were just discovered, and the scientific jury hasn't reached a decision yet as to what these may be. The massive, compact objects are 100 to 10,000 times as massive as our Sun. They are most likely produced by black hole mergers. They are also much closer than quasars, residing in several nearby spiral galaxies.
How do we "see" black holes?
One unavoidable challenge when studying black holes is that they're almost invisible. How do we go about observing black holes if they are so compact and emit no visible light? Well, there are a couple of tricks. Depending on its mass and rate of spin, a black hole exerts a noticeable influence on its surrounding environment. 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.
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, it can capture material that wanders nearby, like a spider that lives hidden until an unlucky insect stumbles into its nest. 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 called 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.
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.
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.
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. As this gas plunges beyond the event horizon, a fraction of its mass is converted into energy. Accretion is an energy powerhouse, producing energy 10 times more efficiently than nuclear burning in stars. Scientists have long believed that most of the energy seen in the Universe was produced by stars. Now the tide is turning, and some scientists speculate that up to 50% of all the Universe's power may come from black hole accretion.
NASA astronomers have recently found evidence for matter actually falling into a black hole. As random material falls anywhere between us and the black hole, it blocks some of the X-rays in the accretion disk from reaching us. These X-rays were produced by super-heated iron atoms. By analyzing the spectra of the iron X-rays that reached us and that were absorbed along the way, the astronomers could determine that the iron was heading straight for the black hole at over 600 million miles an hour.
In about 10% of supermassive black holes, jets of energized matter thousands of light-years in length shoot out in opposite directions. These jets are detected in radio, visible, X-ray, and gamma-ray wavelengths. The process seems contradictory: a black hole, the ultimate gravity machine, accelerating matter away at nearly the speed of light. Scientists are now probing the jet phenomenon to understand how this process works.
It's not all fireworks, though. Black holes can also be quiet. Such objects do not accrete matter, probably because they have long ago used up their fuel supply. They can still be detected by their gravitational influence on their environment. Our own galaxy probably contains a dormant supermassive black hole at its center. Weak X-ray emission emanates from near the galactic center. This appears to be a fossil "footprint" of a once-active black hole.
So, what happens close to a black hole?
This image from the Chandra X-ray observatory shows an enormous X-ray jet powered by a distant quasar.
Close to a black hole, matter is tumbling toward the event horizon, light is making a getaway, sometimes jets of matter are somehow shooting away at nearly light-speed, and space-time is being warped. It's a busy place; easy to get lost. Our only map is the radiation, often X-ray light, emitted from the vicinity. This is what scientists use to piece together what happens close to a black hole.
A prized tools for scientists studying black holes is the X-ray signature of iron atoms in the black hole accretion disk. The signature is the emission spectrum, characteristic features within the X-ray light that reveal tidbits about the iron's temperature, velocity, and direction. Iron atoms at different locations within the accretion disk of hot gas exhibit different properties. Thus, with lots of iron-borne light samples, scientists form a picture of this otherwise invisible and unwelcoming world.
The Constellation-X mission 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. Heavy stuff, huh? Space-time is everything, a four-dimensional reality that combines our three-dimensional view of the world with another dimension called time. Gravity, Einstein predicted, warps the fabric of space-time. Near a black hole, the gravity is strong enough to bend space itself and slow time. The closer to a black hole, the more extreme the warp in space and the slower the movement of time. At the center of a black hole, that point of singularity, space and time as we know it collapse.
