Black hole lies at galaxy’s center

For years, astronomers speculated that a giant, mysterious force lay at the center of the Milky Way, but it wasn’t until four years ago that UCLA astronomer Andrea Ghez definitively showed what it was.

Using new techniques for peering into the dusty heart of the galaxy, Ghez’s observations proved that scores of stars were rapidly orbiting what could only be a black hole. But it wasn’t the kind of garden-variety black hole created when a star explodes and dies; it was hundreds of thousands of times as powerful — a “supermassive” black hole, as they are now known.

Her discoveries, along with the work of scientists studying other galaxies, have in a short time led researchers to the surprising conclusion that most, if not all, of the universe’s hundreds of billions of galaxies have a supermassive black hole at their core. Even more striking, the astronomers have found that the black holes’ mass and nature are closely related to the size and makeup of the surrounding galaxies.

It also appears that these cosmic monsters — which can “eat” stars whole — are key to understanding how galaxies were formed and are still being formed today.

“Many of these discoveries were unexpected,” said Ghez, a self-described “telescope junkie” and rising astronomy star who does much of her galaxy-gazing at the W.M. Keck Observatory in Hawaii, the world’s largest optical telescope. “There’s tremendous interest in this field now because of the potential that it can tell us so much about the dynamics of very basic galaxy creation.”

Black holes appear, for instance, to be both creators and destroyers — swallowing stars or gases that come too close while also spewing out jets of super-high-energy particles and radiation generated by this violent feeding process. The jets, which can be millions of light-years in length, are believed to seed galaxies with the mass and energy that will, in time, become new stars and perhaps even planets.

With many promising areas to research, the supermassives are drawing astronomers and astrophysicists back into black hole research. In 1915, based on purely theoretical calculations, Albert Einstein laid the groundwork for the existence of these bizarre phenomena, which have such strong gravitational pull that not even light can escape them. But research on them languished for decades because there was no way to observe them directly.

The Hubble Space Telescope provided the first real evidence of the existence of supermassive black holes — revealing in 1994 that something was orbiting rapidly around the nuclei of some distant galaxies, suggesting the presence of a huge mass contained in a very small area.

Since then, the Hubble, NASA’s orbiting Chandra X-Ray Observatory, the Keck and other very large, high-resolution ground telescopes have begun to unravel more about these central black holes — which can be as large as the distance from the sun to well past Mars, and as small as New Jersey.

Because nobody knows what happens after a star or gas is swallowed by a black hole, astrophysicists have focused instead on learning and theorizing more about its outer structure. They believe that black holes have an “event horizon” — the point where anything will be inexorably captured by the gravitational pull — and that they have “accretion disks,” a vast, swirling region where matter is funneled into the hole. The process creates intense friction and heat, and as a result energy and matter can get supercharged and shot out in jets.

As supermassive black holes go, the one at the center of the Milky Way (about 27,000 light-years, or 158 trillion miles, away from our exurbanite sun) is dormant and small. It is believed to have the mass of almost 4 million suns and does not appear to be sending out jets of radiation. Some of the larger supermassives are hundreds of millions to many billions times as massive as our sun. (A typical stellar black hole has five to 10 times the mass of the sun, although researchers this week reported discovering an exploded star that was a record two to three times as massive as that.)

To the enormous surprise of those who study the universe, the size of a supermassive black hole appears to have a direct and unusual correlation to the galaxy around it. Researchers calculated a decade ago that the mass of a supermassive black hole appeared to have a constant relation to the mass of the central part of its galaxy, known as its bulge. This relationship supports the notion that the evolution and structure of a galaxy is closely tied to the scale of its black hole.

“Something very profound is going on here, and the formation of black holes and galaxies is related in some way,” said Juna Kollmeier, an astrophysicist and fellow with the Observatories of the Carnegie Institution.

“This is an exciting new wrinkle on the old concept of black holes, and that’s why so many researchers are drawn to it,” she said.

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