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What a black hole would look like
Nov. 4, 2005
Special to World Science
Astronomers are sure that they’ve seen black holes in space—almost.
What they’ve really seen is objects that, judging by their gravitational pull on things nearby, are so dense that they seem like they must be black holes. But
the pictures are unclear.
What astronomers would really like is to capture on camera something that actually looks
the way they think a black hole should look.
As part of that effort, researchers on Nov. 3 published a computerized image of what they’re looking for, and reported
that they have almost found it. A slight improvement in their telescopes’ resolving should let them
catch the real thing, they say.
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Simulation of what a black hole would look
like in radio waves, seen from Earth. (Courtesy of Eric Agol, Univ. Washington.)
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Black holes are objects so massive and dense that their gravity sucks in anything
nearby permanently, including light. This is why they’re traditionally called black.
In reality, from a distance they wouldn’t look black, because their powerful gravitational energy heats up the infalling matter so that it radiates
intense light.
Standard physics predicts black holes should exist throughout the universe. Many of them would be remains of massive stars that have crushed themselves to a tiny size through the force of their own gravity, after having burnt out the fuel that kept them puffed up.
In seeking to capture a definitive image of a black hole, astronomers are setting their sights on the best-known candidate black hole nearby—a big one thought to lurk at the center of our Milky Way galaxy.
The newly published image is “a simulation of what we might see” with the right telescopes, said University of Washington astronomer Eric Agol, who created the simulation, though he wasn’t involved in the new research.
The image doesn’t actually represent the visible light that the object would give off. It represents
radio waves that it would emit. Radio waves are simply a type of light with lower energy than visible light. The radio-wave image has been translated into
visible colors for the benefit of human viewers.
Astronomers say the image shows—and they should eventually be able to see—the black hole’s signature characteristic,
the “event horizon.” This is a place where, once something has crossed that place, it
loses any chance of escaping the hole’s grip.
That place would truly look black if we were unlucky enough to be able to see it from the
inside, researchers think. From our perspective on Earth, it wouldn’t be black, but it would be a little darker than surrounding region.
The image consists of a bright ring-like shape that gives off a glow outside of itself,
with a darker area inside.
Both the ring and the area inside it represent light emitted by material falling into the black hole,
explained Chris Reynolds, an astronomer at the University of Maryland, College Park. The darker appearance of the inside results from the fact that the black hole sucks up any light that comes from behind it, with respect to us. This light never reaches us and thus the region is a sort of shadow. That isn’t the case for the area of the ring and elsewhere.
The dark region’s border represents the event horizon, said Reynolds, who wrote a commentary on the new findings published along with Agol’s image in the Nov. 3 issue of the research journal
Nature.
The fact that there is light within the ring at all, he said, is because there is infalling matter on our side of the hole, which releases some light in our direction, but not enough to make that area as bright as the ring.
The ring itself represents light waves from
material that from our perspective, is just outside the edges of the black
hole. These light waves just miss falling into the hole and aren’t blocked
from our view.’
The extra brightness on one side of the ring in the image represents the possibility
that the black hole would be spinning, in which case it would throw more light waves at us from the side that’s moving toward us than from the other side.
Astronomers didn’t produce a visible-light counterpart to the images, because we wouldn’t be able to see it in real life anyway. Dust clouds in our galaxy would get in the way and keep us from capturing such an image from Earth. It would be possible to see it in radio waves, though, because
they worm their way through the dust.
Astronomers have confirmed that that apparent black hole at the center of our galaxy, known as Sagittarius A*, is four million times more massive than our Sun, Reynolds wrote. This calculation is based on the speeds of nearby stars whose motions are influenced by its gravity.
In the same issue of the journal, Zhi-Qiang Shen of Shanghai Astronomical Observatory, China, and colleagues published data showing the putative black hole is no wider than the distance between the Earth and the Sun.
This requires such a stupendous concentration of mass that any explanation besides a black hole looks highly unlikely, Reynolds wrote.
Previous studies had also narrowed down the object’s size to a relatively
small area, he added, but the new findings strengthen the case by narrowing it
down still further.
Yet an even “more discerning test,” Reynolds noted, would be to capture an
image like the one in the simulation. The same radio telescope that Shen and colleagues used for their measurement should be able to capture such an image within 10 years thanks to increases in resolving power, he predicted.
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