Imagine a black hole swallowing Earth, ending life in an instant. It's not only the stuff of pulp sci-fi novels but, scientists say, a looming possibility.

"It would be a bad day for the solar system if we got visited by a black hole," says Neil deGrasse Tyson, director of New York's Hayden Planetarium at the American Museum of Natural History.

In our vast galaxy there are billions upon billions of stars, each of which is at a different point in its life cycle. Citing the law of averages, some scientists believe at least one star dies every day.

And in death, stars occasionally give birth to black holes; when a massive celestial body's core collapses, it creates an immense gravitational pull not unlike an invisible cosmic vacuum cleaner. As it moves, it sucks in all matter in its way -- not even light can escape.

"If you shoot a nuclear weapon right into a black hole, the effect would be smaller than a pinprick compared to the enormous gravitational pull of a black hole itself," explains Michio Kaku, professor of theoretical physics at City University of New York.

A black hole lies in the heart of every large galaxy. A monstrous one sits in the center of our own Milky Way. Astronomers say probably more than 10 million black holes inhabit the cosmos.

Hell Breaking Loose

Initially, scientists believed we had nothing to fear, because black holes were thought of as fairly stationary.

"Then, in the year 2000, all hell broke loose," Kaku says. "At that point, we had conclusive evidence that there are wandering black holes -- nomads, renegades -- right next to us in our own backyard of a galaxy."

Fortunately, scientists say the probability of a black hole heading straight toward Earth and swallowing us whole is highly unlikely. But what would happen to us if one entered our solar system and came close to Earth?

Although scientists haven't directly observed a black hole (since a black hole swallows light), they have observed the effect of a black hole on surrounding material. Astronomers say the first sign of a black hole's approach would be subtle changes in the night sky. The gravity from a black hole would distort Earth's orbit and we'd begin to notice differences in the orbits of other planets and stars in the galaxy.

The closer a black hole gets to Earth, the worse the disruptions would become. But even from a billion miles away from our solar system, a black hole would still cause a disruption in Earth's orbit, leading to changes in our tides.

If a rogue black hole ever closed in on our solar system and crept up next to Earth, the resulting havoc would seem like the wildest science fiction. Either Earth would career out of its orbit, spinning out of the solar system, or in the opposite direction, toward the sun, and we'd suffer a deadly warming.

In either scenario, a black hole closing on Earth would cause our home planet to be literally ripped apart and swallowed whole.

" In a contest between a black hole and the Earth," Tyson says, "Earth would lose. It's that simple."