Turning the galaxy's heart to a watermelon.

Turning the galaxy's heart to a watermelon 

If forced to move along a rotating disk shrouded in smog, you'd find it tough to see what's behind or in front of you and nearly impossible to decipher the pattern of objects looming above. That explains the problem facing Earth-bound astronomers as they attempt to map the overall shape of our dusty galaxy, the Milky Way. Scientists have known since the 1700s that many of the Milky Way's star clusters concentrate along a flat plane, somewhat like a cosmic phonograph recrd; astronomers early in this century deduced that a spherical concentration of stars and gas sits above and below the disk's center like two halves of a grapefruit. Two astronomers now say the shape of this central structure may actually resemble a watermelon rather than a sphere--a suggestion with surprising implications.

The gravitational force exerted by such a nonspherical distribution, the researchers say, causes the solar system to orbit the galactic centr along an ellipse rather than a circle as commonly assumed. The scientists also say their computer-derived model explains anomalies in the observed velocities of galactic gases that have puzzled astronomers.

Leo Blitz of the University of Maryland at College Park and David N. Spergel of Princeton (N.J.) University base their model on data from several research groups that observed the Milky Way at radio wavelengths, which can penetrate the galaxy's dusty byways. During the 1970s, in observing radio emissions from atomic hydrogen near the galaxy's edge, those groups noted that some of the gas seemed to be heading inward -- toward the solar system and the galactic center--at a rate of 15 kilometers per second. That phenomenon, says Blitz, can be explained if the sun is now heading away from the galactic center and toward the gas, in the elliptical orbit predicted by the watermelon model. Supporting evidence for the model comes from other observations of molecular clouds orbiting the galaxy close to its center, he says. Despite the near-perfect circular shape of the gas clouds' orbits, researchers observed that portions of these clouds seemed to be speeding away from the solar system, also at 15 km per second. Both the atomic hydrogen and gas cloud observations fit with the same elliptical orbit for the sun, Blitz says.

"We didn't know about this [inner gas velocity] problem until later, when we were looking for confirmation," Blitz says. He and Spergel reported their findings this week at the annual meeting of the American Astronomical Society in Arlington, Va. "Our model makes a clear prediction about this phenomenon," Blitz contends, but some researchers remain skeptical about the watermelon scenario.

MIT astronomer Alar Toomre says the watermelon-shaped mass distribution is plausible but "is on equal footing" with other possible explanations for the radio-emission findings, including the notion that the Milky Way contains pockets of gas devoured from other galaxies. The gravitational force exerted from the extra gas might similarly cause the solar system to follow an elliptical orbit, Toomre says. Blitz argues that such a scenario, while possible, could not explain all the observations. Further calculations, he adds, may resolve another possible objection cited by Toomre by showing how the watermelon distribution of stars could have retained its shape over billions of years.

"The model of [Blitz and Spergel] is very plausible," comments astronomer Car Heiles of the University of California, Berkeley. However, he adds, "I'd like to see even more velocity measurements to verify the picture."