Hunting for Black Holes

By Ken Croswell • Illustrated by Tom Powers

A space traveler notices a black spot looming ahead, a place where no stars can be seen. The space-ship veers toward this dark object, which seems to pull on the ship. The blackness grows bigger, blotting out more and more stars. With a burst of rocket fire, the ship barely escapes from the powerful pull of the black hole.

If we had spaceships that could carry us far away from our Solar System, a black hole would be every space voyager’s nightmare. A black hole is so strong that it swallows everything that enters it. Once you fall into one, you can’t get out again. Not even light—the fastest thing in the universe—can escape a black hole. That’s why a black hole is black.

No one has ever gone into a black hole. Even if some space travelers did, they couldn’t tell anyone about it, since they couldn’t leave. And if they tried to send a message, those radio waves could not escape either. But astronomers think they know what black holes are like because the key to every black hole’s power is gravity, the same force that pulls objects toward Earth.

Every object has gravity. The more mass something has, the more powerful its “pull.” A black hole has the mass of at least three Suns, but most black holes are as heavy as ten Suns. All of the material in a black hole is compressed into a very small space by the force of its own gravity.

Except for the matter at the very center, most of a black hole is empty space. From the center, gravity reaches outward in all directions. As a result, the center has a boundary around it—the outer limit of the black hole, which is an imaginary bubble called the event horizon.

Any spot on the event horizon is a “point of no return.” Anything outside the event horizon will feel the pull of the black hole’s gravity, but with enough speed, it can escape. Once something goes inside the event horizon, it cannot escape.

Crunched Stars
Black holes are the remains of massive stars that have collapsed. Throughout the life of a star, huge amounts of energy are re-leased inside. This energy pushes outward, preventing the star from falling in on itself. Once the star’s fuel is used up, the star collapses. The heaviest stars have enough mass to become black holes.
There might be 100 million black holes scattered throughout our galaxy, the Milky Way. This is about how many large stars have turned into black holes during the galaxy’s long life.

Black holes are hard to find. Unlike normal stars, they give off no light. But by being clever, astronomers have found objects that appear to be black holes.
Suppose another star is going around a black hole. The star is too far away to be pulled in all at once. Instead, the black hole tears material away from the closer side of the star. This material falls faster and faster toward the black hole. As the particles fall, they rub against one another and glow hotter and hotter. A stream of this extremely hot material gives off energy, including X-rays, as it plunges into the black hole.

In 1971, astronomers found X-rays coming from a blue star in the constellation Cygnus, which appears as a large cross in the summer sky. Astronomers also discovered that this blue star was going around something that no one could see. The star was moving fast, which meant that it was caught in the grip of something as massive as a large star.

But the mystery object was not a normal star. Stars of this size are bright, and astronomers could see nothing. They concluded that the object must be a black hole, and they named it Cygnus X-1.

Since then, astronomers have found four or five other black holes in our galaxy. In each case, a normal star circles a dark object that has the mass of a black hole, and X-rays come from it.

Hub of the Galaxy
These objects are ordinary black holes, with a few times more mass than the Sun. But at the center of our galaxy there may be a black hole with more mass than a million Suns.

All stars in the Milky Way, including the Sun, circle around the center of the galaxy, just as the Earth goes around the Sun.

At the center of the galaxy, millions of stars are crammed together, and they are moving fast around something that may be a huge black hole.

Here is how such a black hole might have formed. Long ago, a big star died and became an ordinary black hole, like Cygnus X-1. In the crowded center of the galaxy, this black hole swallowed nearby stars and grew bigger and grabbed even more stars. For-tunately, it will grab only the closest stars. The Earth and Sun will never fall into this black hole, nor will most of the rest of the galaxy.

Astronomers do not know for sure that a black hole lurks in the hub of the galaxy. But I wouldn’t take any chances. If you are ever speeding through the Milky Way, be extra careful when you’re near the center!