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 ultrasharp optics on Hawaii’s Mauna Kea, her team took the highest-quality spectrum ever of any of these close-in stars.
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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.
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“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 star-forming density, it should withstand the black hole’s tidal forces 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.
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.
There’s now evidence that such clusters exist. Over the past decade, researchers have found two stellar groupings that lie within 100 light-years of the galaxy’s center. If such a cluster of stars were to migrate within a few light-years of the supermassive black hole, the black hole’s tidal forces could split it up. In that case, the cluster’s most massive members, now traveling solo or perhaps in pairs, would journey even further inward.
In an upcoming Astrophysical Journal, Andrew Gould of Ohio State University in Columbus and Alice C. Quillen of the University of Rochester in New York describe details of one such migration scenario. Their model focuses on the final step–a cluster of stars that rips apart as it comes within 0.1 light-year of the supermassive black hole.
The researchers calculate that for a massive star from that cluster to come as close to the galactic center as the stars that Ghez’ and Genzel’s teams have studied, it must have a partner several times as heavy, perhaps 60 times the sun’s mass. The black hole at the galaxy’s center will ultimately disrupt this pas de deux, and in some cases, will capture the lighter star into an orbit about 25 times as large as that of Pluto, Gould and Quillen say.
The most intriguing part of this model, notes Quillen, is that the massive, short-lived stars found at the galactic center may have had companions that were even more massive and therefore shorter lived. That would mean that the stars that now closely orbit our galaxy’s supermassive black hole are widows of much more massive stars.
These survivors could harbor important clues about their former megapartners. Little is known about the evolution of such extreme heavyweights, but cosmologists speculate that such stars first lit up the universe almost 13 billion years ago.
After considering all the known populations of stars that lie near the Milky Way’s core, Brad Hansen of UCLA and Milo Milosavljevic of the California Institute of Technology in Pasadena find that none can exert a strong enough gravitational tug to rapidly draw in a group of massive, short-lived stars born a few light-years from the core. Instead, the astronomers propose in an article posted online June 3 (http://xxx.lanl.gov/abs/astro-ph/0306074) the most likely gravitational source is an unidentified intermediate-mass black hole, some 1,000 to 10,000 times the mass of the sun. This middleweight, which would closely orbit the supermassive black hole, could easily form at the center of a dense star cluster that formed farther out and then sank towards the center, dragging young stars along with it, the researchers note.
Supporting evidence for an intermediate-mass black hole may come from high-resolution radio wave maps that track the motion of the supermassive black hole at the galaxy’s core. Its motion across the sky would be slightly altered by the presence of an intermediate-mass black-hole companion, Hansen and Milosavljevic suggest.
Making that model more credible is the first evidence for an intermediate-mass black hole residing near our galaxy’s center. Morris and his collaborators have made as yet unpublished observations with the Chandra X-ray Observatory that hint that such a black hole resides about a light-year from the core, Morris told Science News.
In early May, Ghez latched onto another hypothesis that she thought might solve the conundrum of the central stars.
In this model, the stars that her team has observed aren’t really young and massive. They just look that way. Because stars that have lower mass are much longer-lived, they could have formed far from the galaxy’s core and then taken hundreds of millions of years to migrate there.
To flesh out this idea, Ghez says, “we need some kind of astronomical Botox, some way of making stars look young when they’re, in fact, old.” One potential mechanism for such a disguise intrigues Ghez: Bash together two elderly stars known as red giants. Although they’re no more massive than the sun, these stars have bloated atmospheres. A collision would blast away the stars’ atmospheres and expose their hot, bright cores. From a telescopic perspective, such a stripped-down star, which looks hot and bright, could well be mistaken for a young, massive star, notes Ghez.
The model would also appear to explain one puzzling feature of the stars nearest the supermassive black hole. No two orbits lie in the same plane. Multiple collisions involving red giants and other stars might yield the random orbits her team has observed.
But by early June, Ghez told Science News, she was no longer confident of that model’s promise. Calculations by other astronomers have shown that red giants whose envelopes are stripped by collisions cool down in 10,000 years–a mere blink of an eye in astronomical terms. That means the youthful-looking stars that closely orbit the supermassive black hole are too hot to have formed this way.
Furthermore, a closer analysis by Ghez and her colleagues has revealed that the orbits of the close-in stars are not random in all their properties. Although no orbit is in the same plane as another, all the orbits are elongated and many are elongated in roughly the same direction.
Researchers have proposed other explanations for how old stars at the galactic center may look young. In one model, several elderly, low-mass stars merge to form a single massive body that appears bright and young. A more exotic model posits the existence of small black holes not far from the supermassive one at the galactic center. Each of these black holes might attract gas and stars around it, creating a spherical atmosphere. To a telescope, the combination of a small black hole and an atmosphere could resemble a young star.
“Scientifically, this has been one of the most exciting few weeks of my life,” says Ghez. “I’ve been bouncing ideas off colleagues, and we’re all trying to come up with a [satisfying] theory.”
Whatever the solution to the mystery of the massive stars in the galaxy’s middle, the answer “is bound to be a new and incredibly interesting twist on how star formation and movement can take place in the extreme environment surrounding a supermassive black hole,” says Morris.
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