Black holes

They were talking about black holes recently at the 189th Meeting of the American Astronomical Society in Toronto. New evidence of their existence was presented, along with evidence of black holes at the centers of three typical galaxies.

This may prompt you to ask the question, “So what’s a black hole, and why should I care?”

A black hole is an object with surface gravity so strong that nothing, not even light, can travel fast enough to escape it.

All objects in the universe, even you and I, exert gravity. The more mass an object has, the more gravity it exerts. We don’t notice our personal gravity because, not having much mass (except right after Christmas), we don’t exert very much of it. The much greater gravity of the Earth swamps it. The Earth’s gravity, in turn, is nothing compared to the sun’s, which holds the whole solar system in orbit.

Even the sun, though, can’t hang on to light–it takes strong gravity indeed to hold on to something moving at the absolute speed limit of the universe. The way you get that much gravity is by cramming a lot of mass into a very small space.

An object’s surface gravity is its total gravity divided by its surface area. It’s like a layer of glue on a ball. The amount of glue that makes a thin layer on a basketball would make a thicker layer on a softball. Put it on a marble, and the glue is so thick it’ll hold anything. Similarly, decrease any object’s surface area enough, and while its total gravity won’t change, its surface gravity will become so strong it, too, will hold anything — even light.

The point at which that happens is called the Schwarzschild Radius, and every object has one. You could turn the sun into a black hole by shrinking it from 1.4 million kilometres in diameter to six, or turn the Earth into one by shrinking it to the diameter of a dime.

Black holes are created when stars collapse under their own weight. A star’s size is determined by the balance between the gravitational pull of its mass, trying to shrink it, and the heat it produces, which tries to expand it. As a star cools it shrinks, but as it shrinks it also burns hotter, so the smaller it becomes, the harder it is to make it smaller still. Only very big stars–much bigger than our sun–have enough mass to collapse all the way to a black hole.

Even the Hubble Space Telescope can’t show us black holes directly. To find them, astronomers look for indirect clues. A paper presented by astronomer Ramesh Narayan and colleagues in Toronto provided new evidence for black holes by showing that binary star systems that emit X-rays probably consist of ordinary stars orbiting black holes. As matter from the ordinary star falls into the black hole, it heats up and produces the X-rays. But the X-rays emitted are only a fraction of what should be generated by this process, which indicates that most of the X-rays are being sucked in past the black hole’s “event horizon,” the point beyond which no energy escapes.

The astronomers who announced finding black holes at the centers of three galaxies used the Hubble Space Telescope to measure the velocities of stars orbiting the galaxies’ centers. They found stars moving so fast that they concluded they must be orbiting huge black holes, millions of times as massive as our sun. (Similar observations have indicated there’s a giant black hole at the center of our own galaxy.)

This leads scientists to think that black holes probably lie at the center of all large galaxies, and may even have played an integral part in the still-mysterious process of galaxy formation

What’s inside a black hole? No one can ever know, because no information about events inside one can every reach us. One theory, however, suggests that matter might flow through a completely collapsed black hole–one that has no radius at all, and therefore infinite density–into another universe, emerging through a “white hole” to amaze and astound observers there.

It all sounds pretty queer, but as British scientist J. B. S. Haldane once said, “My own suspicion is the Universe is not only queerer than we suppose, but queerer than we CAN suppose.”

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