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Black Holes: Starving and Misunderstood

The lonely black hole.

It is, perhaps, the most misunderstood entity in all of astronomy.

The popular image of the black hole, fostered by books and movies, is of a giant, swirling maw vacuuming up space, sucking in everything far and wide as it dangerously marauds through the galaxy. The reality, however, is not quite that sinister.

In isolation, black holes are just overly dense bits of matter with, alas, no planet-eating superpowers. And the science fiction idea of using them to tunnel through space is, well, fiction. They can’t change the orbits of pulsars any more effectively than stars of the same mass. All they can do is sit there, emitting no light, and hope that maybe something passes by, just close enough, just by accident, and happens to fall into their gravitational gullets.

Black holes are so dense that their escape velocity is greater than light’s speed of 300,000 kilometers per second. Not even Superman can go fast enough to fly off of that. And anything can become a black hole if it’s made dense enough. An atom, a star, it’s all the same to physics. Make the volume of something—anything—small enough and it will become a black hole.

So consider, for instance, our own planet Earth. As it stands now, our world is a people hole. I, in all my athletic might, might try to jump to the stars. But at best I’ll make it three feet up (if I were a basketball player, which I’m not). I’d lift off at an amazing seven point six meters per second, but instantly, gravity would pull me back down, reducing my velocity to zero before a second had even passed. Bottom line: even if I jump as hard as I can, I will never escape Earth’s gravitational pull.

Now, Superman on the other hand, well, he has super powers. With his superhero legs he can lift off at speeds greater than a speeding bullet, an amazing eleven point two kilometers per second, over 1,000 times faster than me. Only an object with that amazing starting velocity (or that can constantly accelerate like, say, a rocket) can make it off the Earth. So Earth is a human hole, but supermen and Saturn rockets can escape because of their great initial velocity and their great engines respectively.

But even our mighty Earth, placed in a large enough interstellar trash compactor, could be compressed into a black hole.

Now, in the world of heavenly bodies, size does matter. Make our planet four times smaller, shrinking the radius to something a bit less than our Moon’s, and suddenly you’ll have to go twice as fast to get off Earth’s face. Shrink it again, 1,000 times smaller, reducing the radius to the size of a small town, and the escape velocity will magnify to 1,120 kilometers a second. Now, this is still a far cry from 300,000 kilometers per second, but if you just keep shrinking the volume, shrinking and shrinking the planet all the way down to just eighteen millimeters North Pole to South, you will eventually hit the point where the escape velocity from the surface of this new pea-sized Earth is the speed of light.

As an interesting side note, if this did happen, the Moon would just keep orbiting. This is because the Earth’s pull—even a pea-sized Earth’s pull—on the moon only depends on mass. In making the Earth into a black hole, all we changed was the radius. The Moon never has, and never will, care about the Earth’s size. So, conversely, if we grow fat with age (and global warming bloats out our oceans a bit around the middle), the Moon still won’t care.

And in the event that our Earth was to shrink to the radius of an Earth-massed black hole, it might actually eat less than it eats today. Every moment our planet sweeps up interplanetary dust as it orbits the Sun. Every day small pebbles burn up in the Earth’s atmosphere. And every once in a while, boulders careen all the way to our planet’s surface. How much stuff we sweep up is determined in part by our size. Just like a janitor’s broom might clean an entire hallway is a single swipe, while a small hand broom will only sweep a small swath, so, too, a tiny black hole actually won’t sweep up as much stuff in space as a full-sized planet.

But most black holes aren’t so small.

In the wild, black holes form when giant stars die and collapse in on themselves. The smallest known black hole, unpoetically named XTE J1650-500, is just three point eight times the mass of our Sun and 10,000 light years away in the direction of the constellation Ata. This tiny real-universe black hole is just fifteen miles (twenty-five kilometers) in diameter, and is close enough to a companion star that it’s able to slowly grow by sucking matter off this sorely abused neighbor. This is an unusually lucky-to-be-feeding black hole. Most are not so lucky.

For instance, the largest black hole in the Milky Way sits swirled in the center of our galaxy. Coming in at three million times the mass of the Sun, this giant object has nothing to eat. It is literally starving. Stars orbit nearby, at distances not too different from the range of the Voyager space probe to the Sun. That’s about 120AU or seventeen light hours. Or, if you like your numbers really large, eighteen point four billion kilometers. And remarkably, those stars are safe just where they are. Yes, occasionally, like moths to a lamp, some random objects do plunge into the Milky Way’s center and get consumed, but this, unfortunately for the black hole, is rare.

Black holes do want to eat. They want to be that swirling maw of death we imagine gobbling up all the stars and planets and heavenly bodies in their way, filling their greedy, dense gullets like pigs at a trough. Their gravities pull and pull and pull all the time, trying to suck things in. But in our modern universe, black hole food is scarce and stars tend to orbit at safe and stable distances. Billions of years ago, when mighty Quasars ruled the heavens, food was more abundant and black holes fed more often as they sat lazy and full in the centers of many galaxies.

Today though, like some kind of cosmic parasite, most black holes are, sadly, starving and misunderstood.

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Pamela L. Gay

Pamela GayDr. Pamela L. Gay is an astronomer, writer, and podcaster focused on using new media technologies to engage people in science and technology. You can learn more about astronomy each week through AstronomyCast.com. Want to do science? Help Pamela and other scientists through IceHunters.org.