Images reveal possible origin of young stars

From Washington, D.C., at a meeting of the American Astronomical Society

Newborn stars have no business being anywhere near the monster black hole that lies at the center of the Milky Way. The tidal forces that the black hole exerts on the cold, low-density clouds of gas and dust that give rise to stars are so enormous that they would rip the cloud to shreds long before stars could emerge.

Nonetheless, the galactic center contains hundreds of massive, newborn stars, a puzzle that Andrea Ghez of the University of California, Los Angeles calls the paradox of youth (SN: 6/21/03, p. 394: Available to subscribers at Mystery in the Middle). Ghez and her colleagues say that they have now solved the puzzle.

Astronomers have had two leading theories to explain the young stars’ existence. According to one model, the stars didn’t form in their current locations but instead were ferried in as part of a massive star cluster with enough gravity to stick together even as it entered the black hole’s arena. In this scenario, the star cluster broke apart soon after its arrival, sprinkling the region with young, massive stars in either circular or elliptical orbits around the galactic center.

In another model, the young stars formed where they now reside but arose within a flattened disk of gas and dust that managed to stay intact despite its proximity to the black hole. In this case, all the stars should have the circular orbits they would have had when born. The stars would lie too far apart for their mutual gravity to have altered their orbits from circular to elliptical.

Ghez and her colleagues recorded a series of unusually sharp images of stars at the galactic center taken by a laser system on one of the world’s largest visible-light telescope, the Keck II telescopes atop Hawaii’s Mauna Kea. Beamed into space, the laser light acts as a virtual star, enabling a rapidly adjustable mirror on the telescope to continuously change shape to compensate for the blurring caused by Earth’s turbulent atmosphere. The team tracked the motion of more than 30 slow-moving, young stars that lie a few light-months from the galaxy’s core. That’s just far enough away from the center that the black hole couldn’t have distorted the shapes of the star’s original orbits.

The images reveal that most of the 30 massive stars have elliptical orbits, indicating that, as migration theory suggests, these stars did indeed travel to the galactic center after forming elsewhere, says UCLA astronomer Jessica Lu, a member of Ghez’ team.