The sounds of spirals.

Like giant pinwheels, spiral galaxies feature curving arms that rotate around a dense core. The arms of starlit gas and dust are believed to arise when a chance gravitational impulse--the tug of a nearby galaxy, for example--triggers a wave of compression that travels around the galaxy. This disturbance is known as a density wave because it draws material together by gravitational attraction. The galaxy's rotation twists the wave into a spiral shape.

Gas caught in the spiral arms is slowed and squeezed. As a result, the arms become a hotbed of glowing gas bejeweled by bright, newly formed stars.

That's why the network of spiral arms recently discovered at the core of the spiral galaxy NGC 2207 caught astronomers completely by surprise. Imaged by the Hubble Space Telescope when it zoomed in on the galaxy's center, these arms do not contain clutches of newborn stars. Moreover, the arms are shorter, less curved, and in greater number than the classic spiral arms imprinted over the outer regions of the galaxy.

The gas near the galaxy's nucleus is too diffuse for a density wave to have sculpted the spiral network, says Bruce G. Elmegreen of IBM's T.J. Watson Research Center in Yorktown Heights, N.Y. Instead, he and his collaborators assert, another type of wave--a sound wave--is the most likely source of the core's pinwheel pattern. In a sound wave, pressure rather than gravity pushes matter together.

Random noise, from a source that could be as mundane as the jostling of stars and gas, would tend to be amplified by the highly curved shape of the galaxy's core, the researchers argue in the Aug. 20 ASTROPHYSICAL JOURNAL LETTERS. Because the speed of material orbiting near the core is at most only a few times higher than the speed of sound, acoustic waves could push gas and dust around with relative ease, Elmegreen and his colleagues note.

He calculates that the wave has an extremely low frequency, about [10.sup.-14] cycles per second, roughly 56 octaves below middle C. Even if human ears were sensitive to such a tone, its volume would be inaudible--an estimated 65 decibels fainter than the quietest sound a person can hear, Elmegreen adds.

He notes that sound waves may play a role in shaping dust lanes near the center of galaxies far more active than NGC 2207. Amid the fireworks generated by central black holes and quasars in such galaxies, however, the gentle pressure exerted by acoustic waves may be much more difficult to identify.