Mystery in the middle: a stellar riddle turns up at the Milky Way's core

Science News, June 21, 2003 by Ron Cowen

For nearly a decade, Andrea M. Ghez has tracked the motion of stars at the Milky Way's core. The great speed with which these centrally located stars whirl around provides the best evidence to date for the existence of an extremely dense and massive object--a supermassive black hole--right in the bull's-eye of the galaxy. Like most astronomers, Ghez, who is based at the University of California, Los Angeles, had assumed these closely orbiting stars were relatively old and lightweight.

Last summer, she had her first inkling that something was wrong with this picture. In June, the prime month for viewing the galactic center with the Keck Telescope's ultra-sharp optics on Hawaii's Mauna Kea, her team took the highest-quality spectrum ever of any of these close-in stars. So puzzling were the results that the young astronomer passed up an opportunity last July to share the new data at a research conference. By August, however, Ghez was confident: The spectrum of the star dubbed SO-2 confirmed that it and the other stars circling near the galaxy's core are unusually young and massive--some 15 to 20 times as heavy as the sun. One of the stars lies as close to the galactic center as twice Pluto's distance from the sun.

Such stars have no business being anywhere near that close to the galactic center. Massive stars are short-lived. They burn their nuclear fuel so fast that they can last no more than 10 million years, Ghez and her colleagues note in the April 1 Astrophysical Journal Letters.

"These are stars ... that are short-lived, in astronomical terms, in a region that's incredibly inhospitable to star formation," notes Ghez. "In fact, given our current understanding of how stars form and the properties of the galactic center, it's not allowed to happen"

Under ordinary conditions, no star could be born so close to a supermassive black hole. Stars arise from the gravitational collapse of cold clouds of mostly hydrogen gas. A typical cloud outside the galactic center has a diameter of 30 light-years and a low density. The tidal forces exerted by the supermassive black hole--the differences in the gravitational tug the black hole exerts on the different parts of such a cloud--would be so enormous that it would rip the cloud to shreds long before stars could emerge.

On the other hand, models in which massive stars are born at a safe distance from a black hole--about 100 light-years--and then migrate toward its galaxy's core have their own problems. Since massive stars last only a few million years, they should die out long before they complete the journey inward.

Making sense of the massive stars near the Milky Way's core "is a real challenge," says Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Genzel has tracked the motion of stars orbiting the galactic center since the late 1980s. Theorists, he notes, first began to worry a decade ago, when researchers found massive stars about a light-year out from the core. Now, with the find of apparently massive stars at one-hundredth that distance from the core's black hole, theorists are faced with a much tougher puzzle.

SUPERDENSE GAS CLOUDS There's an outside chance that stars might form in the immediate vicinity of a supermassive black hole, but the birthing process would be unlike any other known in the galaxy, says Mark Morris of UCLA. For example, if a gas cloud can compress to a million times the usual starforming density, it should withstand the black hole's tidal forces 5 and remain intact long enough to make stars.

If such a process actually unfolded at the Milky Way's center, however, the gas density would have to have been dramatically larger 5 million to 7 million years ago, when the stars were forming, than it is today.

Nonetheless, Morris notes, there's a possible source for such gas clouds: a ring of gas, about 6 light-years in diameter, that encircles the galactic core. This ring might even be responsible for multiple episodes of star birth at the galaxy's center, he speculates.

As Morris envisions it, the gravity of the supermassive black hole lures the ring inward, compressing parts of the ring's gas to enormously high densities. When the ring approaches the center, its density might be high enough to form stars like the massive ones that astronomers have recently imaged.

Massive stars often have fierce winds, notes Morris. In the Milky Way, such winds might have pushed the ring back outwards, where it now resides. However, a few million years from now, after the massive stars at the core have died out and the winds have vanished, the black hole may pull the ring of gas back toward it, ripe for yet another round of star formation.

MIGRATING STARS For scientists who are reluctant to justify a change in the common view of how dense gas clouds near supermassive black holes can behave, there's another scenario. In this model, notes Morris, the stars now in the neighborhood of the supermassive black hole weren't born there.

The main challenge to this model is that if these massive stars formed beyond the gravitational grasp of the black hole, they must have traveled to the center in a hurry or they would have burned out. If each star were initially a member of a heavy, tightly bound cluster, however, this fast migration would be possible. A cluster that forms some 10 to 30 light-years from the core would lose energy as it passes through fields of stars and spiral rapidly inward, lured by the collective mass of all the stars and gas that lie closer to the galactic center. If the cluster were still intact as it came within 4 light-years of the core, within the gravitational grasp of the supermassive black hole, it would soon disintegrate, spilling its stars into orbits around the black hole.