Two hundred years before black holes were proven to exist, scientists were imagining collapsed objects so dense that gravity would prevent light from escaping them. The first of these scientists was John Michell in 1784. About a dozen years later, Pierre-Simon Laplace included a similar notion in his book Exposition du Système du Monde. These speculations were based upon Isaac Newton’s theory of gravity, which had been published a century before, but not much was made of them. After all, how can anyone study a distant object in space if it’s invisible?

After the publication of Albert Einstein’s general relativity in 1915 there was a new way to think about collapsed objects. However, Einstein himself was not the one to fully explore these ideas. Instead, Karl Schwarzschild, a German astrophysicist, was first to obtain complete solutions of Einstein’s equations applied to a fully collapsed spherical object. Schwarzschild had enlisted in the German army at the outbreak of World War I, and so his investigations were undertaken while he served in the trenches of the Eastern Front.

But Schwarzschild could not leave the Eastern Front to present his work to his colleagues in person. Instead, he sent it to Einstein, who performed this task for him. Though Schwarzschild’s work was enthusiastically received, he never learned of it. Schwarzschild died in 1916 of a disease contracted while serving at the front. Nonetheless, because of the fundamental nature of his work, the size of the event horizon of a black hole, the boundary where light is no longer able to escape, is called the Schwarzschild radius in his honor.

To give one example of the bizarre nature of black holes: a black hole with the mass of the Sun would have a Schwarzschild radius of about 3 kilometers. That means it has all of the Sun’s mass crammed into a space smaller than a small town! If you, or anything else, were to venture any closer to a solar-mass black hole than 3 km, you would cross the event horizon and would never be able to escape.

For nearly six decades, collapsed objects remained a sideline of physics, and they played no part at all in astrophysics. How could an object that emitted no radiation be studied in other than theoretical terms? In recognition of this predicament, in 1967 the physicist John A. Wheeler dubbed these collapsed objects “black holes.” The objects were thought to leave no mark on the outside world other than their intense local gravity. Many scientists (including, for his entire life, Albert Einstein) did not even think such objects existed!

All that began to change in the late 1960s with the launch of the first rockets that carried X-ray detectors. The first black hole candidate object, Cygnus X-1, was discovered during one of these early rocket flights. Additional evidence resulted after the 1970 launch of the Uhuru satellite, the first to survey the entire sky in X-rays. Contrary to the expectations of many scientists, it found sources all over the sky. Many of these sources were quite bright, emitting much more X-ray energy than the Sun emits in all wavelengths combined. The discovery of these sources had to wait for the arrival of space-based X-ray detectors because X-rays cannot penetrate the Earth’s atmosphere. Scientists used the gravitational effects of the unseen object on its companion star to weigh the invisible X-ray source, quickly deducing that the heaviest of these objects were indeed black holes.

Today scientists know a lot more about black holes than they did at the launch of Uhuru. There have been dozens of satellites launched to study the sky, and their sensitivities have ranged from infrared all the way through gamma rays. In addition, observations from the ground in optical, infrared, and radio wavelengths have been indispensable in studying black holes.

Since black holes do not emit any detectable radiation directly, they must be detected indirectly by the effects they have on nearby observable material. For example, the X-rays observed from black holes are actually emitted by a disk of hot gas called an accretion disk. As the material spirals into the black hole, it heats up and begins to emit at many wavelengths, from radio up through X-rays. The glow from these accretion disks is detected by X-ray telescopes such as Chandra, Swift, and XMM-Newton. In addition, one of the earliest observations made by the Hubble Space Telescope was of an accretion disk around a black hole in the center of the galaxy M87. This was the most massive black hole observed at that time, with a mass over 3 billion times that of our Sun.

More recently, observations using Hubble, Chandra, Spitzer and other satellites have been combined with ground-based observations at optical and radio wavelengths to study a black hole at the center of our own galaxy. What has been revealed is a chaotic region encircling an inner one mostly devoid of gas. This inner zone has a dozen or so stars orbiting an unseen object. By measuring the motions of these stars over several years it has been possible to determine their orbits. Then, using the same physics that applies to the motions of the planets around the Sun, scientists have learned that there is a 3-million-solar-mass object at the center of the Milky Way. Such a massive, dark and compact object can only be a black hole.

But, not all black holes are the size of these central-galaxy monsters. Our own galaxy contains at least a dozen well-studied black holes outside the center. They are members of binary-star systems in which a normal star and black hole are locked in orbit around each other. Sometimes material will be pulled from the normal star onto the black hole, creating an accretion disk which then emits X-rays and other radiation. These black holes tend to be only about 10 times the mass of the Sun. They are often referred to as stellar black holes because they are formed by the collapse of stars. We even see these sorts of objects in other galaxies. In fact, large galaxies like the Milky Way probably contain at least a couple hundred of these X-ray binary systems.

The detection of the central Milky Way black hole is the most definitive detection of a “super-massive” black hole to date, and today all but the most reluctant scientists have been forced to the conclusion that black holes really are part of our universe. In fact, observations with Hubble and ground-based telescopes have shown that all large galaxies contain “supermassive” black holes. These monstrous black holes have a mass of millions or even billions of times the Sun’s mass, and their properties indicate that they are intimately related to the formation of the galaxies in which they reside.

One emerging idea is that the supermassive black holes in galaxy centers help to seed the chemicals of life throughout their host galaxies. This might seem contradictory at first; how can an object from which nothing can escape possibly spread material throughout a galaxy? One of the best analogies to understand this might be the “filling a dog dish with a fire hose” model. If you envision trying to undertake this rather absurd act, you will probably realize that, while you might get some water in the dish, most of it by far will end splashing all over the place. In a similar way, a black hole is like a very small container into which a tremendous amount of material is falling. Most of this material just goes splashing back out again in high speed jets and massive outflows. This is another paradoxical and weird aspect of black holes.

So scientists have learned a lot about black holes since they were first postulated. But we are still learning more. The Swift satellite currently studies the formation of black holes during gamma-ray bursts. These events can occur when extremely massive stars go supernova, forming a black hole in their core in the process. Another kind of gamma ray burst is thought to result when two neutron stars in a binary system spiral into each other and merge, again forming a black hole. And we are learning more about the super-massive black holes in galaxy centers with the Fermi Gamma-ray Space Telescope. Fermi is the most sensitive gamma-ray telescope to survey the sky at extreme gamma-ray energies. It is currently discovering many new giant black holes in the centers of distant galaxies.

Future space telescope missions are planned that will take the study of black holes to new levels of detail. Some will even be able to make images of the material that is part of the inner accretion disk, just before it plunges through the event horizon. One such mission is the Nuclear Spectroscopic Telescope Array, or NuSTAR, which will extend our imaging capabilities to much X-ray higher energies than has been possible. Additionally, still in the early planning stages is the International X-ray Observatory, or IXO. IXO will go beyond the capabilities of Chandra, with increases in sensitivity and resolution that will allow us to make the best-ever observations of the accretion disk region of black holes. So look to the future as we uncover additional secrets of these mysterious and bizarre objects that first began to captivate scientists more than two centuries ago.

 

Artist rendering of Fermi in orbit.

Fermi’s First Light Sky Map.

Launch of Swift on Delta II rocket.

Artist rendering of Swift viewing an Active Galaxy.

Artist rendering of XMM-Newton.

Artist rendering, Chandra X-ray image, and ESO optical image of black hole binary system.

Artist rendering of Cyg X-1 black hole binary system.