Galactic engines

Natural History, May, 1997 by Neil de Grasse Tyson

The extraordinary luminosity of quasars and other active galaxies is powered by activity in a very small region of their nuclei.

Galaxies are phenomenal objects in every way. They are the basic organization of visible matter in the universe, which contains as many as a hundred billion of them. They each commonly pack hundreds of billions of stars. Many are found solo in space; others are found in gravitationally linked pairs, familial groups, and clusters. They can be spiral, elliptical, or irregular in shape. Most are photogenic. Their morphological diversity has prompted all manner of classification schemes, supplying a conversational vocabulary for astrophysicists. One variety, the "active" galaxy, emits an unusual amount of energy in one or more bands of light. This excess energy usually comes from the galaxy's center. The center is where you will find a galactic engine. The center is where you will find a supermassive black hole.

The names for active galaxies read like a manifest for a carnival grab bag: BL Lacertae galaxies, Seyfert galaxies (type I and II), blazers, N-galaxies, LINERs, radio galaxies, and of course, the royalty of active galaxies--quasars. What they have in common is that their extraordinary luminosities appear to be driven by mysterious activity from a very small region buried deep within their nuclei.

Quasars, discovered in the early 1960s, are up to a thousand times as luminous as our own Milky Way galaxy, yet their energy hails from regions that would fit comfortably within the planetary orbits of our solar system. Curiously, none are nearby The closest one is about 1.5 billion light-years away--its light takes 1.5 billion years to reach us. Most are over 10 billion light-years away, yet we can see even farther out into the universe to a time before quasars turned on. Possessed of small size and extreme distance, they are almost indistinguishable on photographs from the pointlike images left by stars in the Milky Way. Quasars were first found with radio telescopes. And because stars are not known for emitting copious amounts of radio waves, it was clear that a new class of object had been discovered. In the we-call-them-as-we-see-them tradition of astrophysicists, these objects were dubbed Quasi-Stellar Radio Sources or, more affectionately, "quasars."

What manner of beast are they?

One's ability to describe and understand a new phenomenon is always limited by the contents of the prevailing scientific and technological toolbox. An eighteenth-century person who was briefly thrust into the twentieth century would return to describe a car as a horse-drawn carriage without a horse, and a light bulb as a candle without a flame. With no knowledge of internal combustion engines or electricity, the possibility of true understanding would be remote indeed. With that as a disclaimer, allow me to declare that we think we understand the basic principles of what drives a quasar. In what has come to be known as the standard model, black holes have been implicated as the engines of quasars and of all active galaxies.

Within a black hole's invisible boundary--its "event horizon"--the concentration of matter is so great that space curves back on itself and not even light can escape. When you fall into a black hole, you fall in for good, even if you are made of light.

How, you might ask, can something that emits no light be a power source for something that emits more light than anything else in the universe? When many of the exotic properties of black holes were explored in the late 1960s and 1970s, people swiftly realized that they provided a remarkable addition to the theorist's toolbox. According to some well-known laws of gravitational physics, gaseous matter funneling toward a black hole must heat up and radiate profusely before it descends through the event horizon. The energy comes from the efficient conversion of gravity's potential energy into heat.

While it is not a household notion, most people have, at some point, had a run-in with converted gravitational potential energy. If you have ever dropped something to the floor and broken it, or if you have ever set a pie to cool on a windowsill and watched it fall and splatter on the ground below, then you understand the power of gravitational potential energy. As objects fall, their potential energy is continuously being converted to energy of motion. And if something stops the fall, all the energy the object has gained swiftly converts into the kind of energy that breaks or splatters things. This is the reason that you are more likely to die if you jump off a tall building instead of a short one.

But if something prevents the object from gaining speed, and the object continues to fall, then the converted potential energy reveals itself in some other way--usually in the form of heat. Good examples include space vehicles and meteors that heat up as they plunge through Earth's lower atmosphere--they want to speed up, but air resistance prevents it. In a famous experiment, the nineteenth-century English physicist James Joule created a device--rotating paddles powered by falling weights--that stirred a jar of water. The potential energy of the weights was transferred into the water and successfully raised its temperature. Joule describes his effort: