【1993 Archives】

Black holes993 Archivesoften be misunderstood. Or mistaken for something evil.

What are they, actually? They're intensely fascinating objects in space, places where matter has been crunched down into an intensely compact area. If Earth was (hypothetically) crushed into a black hole, it would be under an inch across. Yet the object would still be extremely massive, as it would contain the entirety of our planet's mass.

The result? A place with a gravitational pull so strong, not even light can escape. (Things with more mass have stronger gravitational pulls.)

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This can make black holes seem like omnipotent, terrifying objects, with an insatiable diet for stars and planets. But this isn't so. They aren't menaces in the cosmos. As astrophysicist Misty Bentz told Mashable, following the first image ever taken of a black hole: "We tend to anthropomorphize these things. But really, black holes aren’t evil, mean, or scary. They just are."

Below we address misconceptions about black holes, including Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. Astronomers recently captured an unprecedented image of this cosmic behemoth.

A NASA illustration showing the superhot accretion disk around a black hole. Also shown is a jet of eneregy released from matter outside the black hole. Credit: NASA SEE ALSO: Behold the Milky Way's supermassive black hole in first-ever photo

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Black holes don't have special gravitational powers

Nothing we know of can escape from inside a black hole. Something would have to move faster than the speed of light — traveling from Earth to the moon in roughly a second— to breakout. That might make it seem like black holes wield inordinate gravitational power. But that's not exactly the case.

"There is nothing exceptionally special about the gravity of a black hole," Douglas Gobeille, an astrophysicist and black hole researcher at the University of Rhode Island, told Mashable.

In fact, if the sun were replaced with a black hole of the same mass, most planets would continue their motion around the Sun just as they are now, with only the closest planets noticing some tidal forces from the black hole. And if Earth were replaced with a black hole of the same mass, the moon's orbit wouldn't change much either. That's because the mass they're orbiting around remains the same.

But the situation changes when something ventures neara black hole ("near" is relative and depends on the size of the black hole). What's unique about black holes is how close something can get to the entiretyof such an intensely compact, massive object. If you somehow visited the surface of the sun, you still wouldn't be immediately next to an object with nearly the density of a black hole. For supermassive black holes, which are millions to billions of times more massive than the sun, "relatively close" could mean 100 million miles away.

"You would feel exceptional gravity if you moved close to a black hole," said Gobeille.

The first image of Sagittarius A*, the black hole at the center of our galaxy. Credit: Event Horizon Telescope Collaboration

Black holes aren't relentlessly sucking everything up

Just because black holes can exert a powerful gravitational pull on objects passing by, it doesn't nearly mean black holes are out there "sucking" things up in the cosmos.

"Some people imagine that they're Hoovers [vacuum cleaners] in the sky," Jean Creighton, an astronomer and the director of the Manfred Olson Planetarium at the University of Wisconsin–Milwaukee, told Mashable. "Of course, that's not true." If it were, the supermassive black hole at the center of the Milky Way would continuously inhale new stars which, fortunately for us, it doesn't. "They are not vacuum cleaners, otherwise we'd be in one," agreed Gobeille.

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But matter or light passing close by can get pulled around a black hole. Yet only a small amount of this stuff actually falls into a black hole and is "consumed," never to return.

"Black holes are terrible at eating things. They are notoriously picky eaters," said Gobeille.

Black holes are terrible at eating things.

As matter draws close to a black hole, however, things do get intense. Objects like stars are literally stretched apart, or "spaghettified," by gravitational tugging. This material collects in a ring, called an accretion disk, where the material spins around rapidly and is superheated to millions of degrees. (A hot accretion disk allowed astronomers to image the first-ever black hole; the disk revealed the black hole.) Eventually, some of this accumulated stuff does spiral into the black hole, but much of it gets spit back out into space: The fast, rotating disk's natural motion ejects material.

It's certainly a messy dining situation. "It's pretty hard for black holes to feed in any efficient manner," explained Gobeille. Only about one percent of the cosmic material pulled around the Milky Way's supermassive black hole Sagittarius A* actually falls in, notes NASA.

But when something does fall into a black hole, that means it passed a point of no return called the "event horizon." "That's the last point," Marco Ajello, an astrophysicist at Clemson University who researches supermassive black holes and galaxies, told Mashable. Hypothetically, he explained, a person could still use a flashlight just outside the event horizon. But once they cross over, that light can't escape back into the universe.

"Most black holes are sitting there quietly."

The majority of black holes, however, aren't actively eating anything. That's because they aren't seeking anything out, or sucking anything in. Compared to the galaxies they occupy, even supermassive black holes occupy tiny spaces. Things have to wander by.

"Most black holes are sitting there quietly," Ajello explained.

Black holes aren't exactly holes — or are they?

Black holes clearly contain an exceptional amount of matter. They have a shape (spherical). And other matter interacts with black holes. So astrophysicists often classify them as objects, though unusual ones. "It's a fantastically weird object," said Ajello.

Labeling a black hole as an "object" or "thing" is appropriate, Dominic Pesce, an astrophysicist at the Center for Astrophysics-Harvard and Smithsonian who researchers supermassive blackholes, told Mashable. And others might reasonably choose to describe them as a "region," he noted

But if someone insists black holes are indeed "holes," they still have a reasonable argument, too.

"I even think there's a case to be made for black holes being referred to as 'holes' in the observable Universe, in the sense that they enclose a region of spacetime about which external observers cannot glean any information," Pesce said.

An artist's concepton of a black hole. Energy is released outside the black hole as hot matter spirals around in a disk. Credit: XMM-Newton / ESA / NASA

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Black holes aren't relentless cosmic vacuum cleaners with unnatural gravitational powers. But the common conception that they're deeply eerie is definitely real. Many aspects of black holes remain mysterious, particularly their insides.

"We don't have a way of probing the interior of a black hole," explained the astronomer Creighton. Researchers can only theorize what might transpire there, a realm where space and time are thought to break down.

What we know about black holes comes from how things interact with them — outside their event horizon, of course. When a black hole shreds apart or consumes a star, for example, the hole's swirling disk of superheated material can glow or eject bursts of energy into space. At times, these invisible objects can essentially scream into the cosmos. Our specialized telescopes and radio antennas, like those used by the astronomers who recently imaged the black hole in the center of our galaxy, detect this energy, which reveals their activity or existence.

In the years ahead, these giant instruments will continue to unveil more secrets about the curious black holes in our universe, and capture more unprecedented imagery. Without them, we'd be in the dark.

"What humans can literally see and hear in the universe is almost nothing," said Gobeille.