Magazine article American Scientist

On the Trail of Monster Black Holes

Magazine article American Scientist

On the Trail of Monster Black Holes

Article excerpt

Collapsed objects weighing millions or billions of times as much as the Sun lie at the heart of nearly all galaxies. New images are finally stripping the mystery from these spheres of warped space.

Across the universe there are trillions of black holes-objects so compact that their gravitational pull prevents anything from escaping, even light. Some of them formerly blazed brilliantly as stars before collapsing catastrophically. Others, far more massive and ancient, sit at the center of galaxies, including our own. When they draw in material from their surroundings, such monster black holes can shoot out jets of energetic particles that stretch for hundreds of thousands of light-years, as in the galaxy Hercules A.

Theoretical predictions about the existence of black holes date back nearly a century, to Albert Einstein's general theory of relativity. In 1915 one of Einstein's colleagues, Karl Schwartzchild, interpreted the relativity equations and showed that a sufficiently massive object could curve space and time in on itself, cutting itself off from the rest of the universe. Even Einstein had a hard time imagining such a possibility. It took another half-century before scientists started accepting that black holes exist, and only recently have they grasped the full cosmological importance of these extreme objects.

Because black holes, by definition, cannot emit any light, researchers such as myself are forced to study them indirectly. Much of our knowledge comes from observing gas as it is pulled into or ejected from the black hole. But we really want to know what is happening right at the event horizon, the outer boundary that defines a black hole's point of no return. That is the goal of the Event Horizon Telescope, which links together radio antennas around the world to create, in effect, a new telescope the size of the Earth. Even in its preliminary form it can resolve details as small as 60 microarcseconds, equivalent to seeing a baseball on the surface of the Moon.

The next year will be particularly exciting for studying black holes: A cloud of gas is nearing our galaxy's black hole, called Sagittarius A*, and may interact strongly with it. Using the Event Horizon Telescope and other instruments, astronomers will be able to see directly how a black hole gets fed. Stay tuned for the results, which will certainly be groundbreaking-or maybe universe-breaking.

Sagittarius A*: The Beast in Our Back Yard

Sagittarius A*, the massive black hole at the heart of the Milky Way, is an oddly quiescent beast. Stars orbit around it, but not much matter reaches the hole itself. It is so subdued that it is almost always invisible in optical images or x-ray maps of the center of our galaxy (bottom). Astronomers only became aware of Sagittarius A* in 1974 from its radio emissions.

It is in many ways easier to study smaller black holes, weighing just a few times as much as the Sun. In contrast to monsters like Sagittarius A*, whose formation was probably intertwined with the birth of our galaxy, the more modest black holes began their lives as bright stars but then exploded and collapsed. When matter gets close to one of these stellar-mass black holes, it settles into a flattened formation called an accretion disk. The extreme gravity near the black hole then causes the gas to heat up by tens-of millions of degrees Celsius and emit x-rays. If the black hole has a nearby companion star, the star can get shredded and slowly consumed, creating a brilliant x-ray beacon. Around fifty such systems have been found so far. A schematic illustration (left) shows the key features of one of these x-ray binaries, called XTE J1550-564. It turns out to share basic traits with other, far more massive active black holes.

Many active black holes also shoot off supersonic jets of particles above and below the accretion disk. The jets produce copious amounts of radio emissions because they contain high-speed particles and strong magnetic fields. …

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