All awhirl about a nearby spiral galaxy.

Its starlit arms forming a giant pinwheel in space, M51 stands out as one of the most striking spiral galaxies ever observed. But despite the fame of this nearby swirl of stars, aptly nicknamed the Whirlpool, its center has remained a mystery, hidden behind a shroud of dust. Viewing M51 in the near-infrared, however, a trio of astronomers has attained the first unadulterated peek at the galaxy's core.

In the July 22 NATURE, the team reports that M51's spiral arms wrap around the galaxy nearly three times -- twice as far as observed in visible light in any spiral galaxy -- and reach closer to the galactic center than theory had suggested. The researchers also find that M51 has a bar-shaped structure at its core, a feature that visible-light images only hinted at.

The infrared portraits are more than just pretty pictures. They "pose a strong challenge to our theoretical understanding of . . . spiral structure in galaxies," writes Jeffrey Kenney of Yale University in a commentary accompanying the report.

Dennis Zaritsky of the Carnegie Observatories in Pasadena, Calif., and his colleagues observed M51 in the near-infrared because they knew that such radiation, unlike visible light, can pass easily through the dust that cloaks the galaxy's core. In addition, infrared detectors pick out more low-mass stars -- the dominant stellar population -- providing a more complete view of the size and shape of the spiral arms. To make their observations, Zaritsky, Hans-Walter Rix of the Institute for Advanced Study in Princeton, N.J., and Marcia Rieke of the University of Arizona in Tucson used a state-of-the-art infrared array and the Steward Observatory's 90-inch telescope atop Kitt Peak near Tucson.

The leading theory to explain the formation of a "grand-design" spiral -- a symmetrical pair of spiral arms like that in M51 -- invokes the idea of a density wave. In this model, a disturbance in a rotating galaxy -- akin to the ripples created by a rock tossed into a pond -- causes gas and stars to pile up, producing regions of higher-than-average density. The galaxy's rotation then shapes these high-density regions into spirals. In one version of the theory, regions of high density move through the galaxy like ocean waves; in another version, the regions stay put.

But in either case, although "it's easy to explain how to get the basic spiral shape, no one has ever shown how you can get spiral arms [that make] three full revolutions," notes Kenney. Calculations suggest that M51's spiral arms can't penetrate as far into the galaxy's center as the new images show they do, says Zaritsky. Yet both he and Kenney caution that the discrepancies between modeling and observations don't mean that astronomers must discard their models about spiral galaxies; they may only need to refine them. Current theories, says Kenney, provide "an understanding of only the very simplest things about spiral galaxies."

Zaritsky notes that the gravitational tug of a companion galaxy, NGC 5195, may have helped form M51's spiral arms. But it remains unclear whether this outsider could have triggered the spiral pattern so near the nucleus, he adds. Zaritsky says that infrared observations of spiral galaxies that lack the complication of a companion may shed more light on how spirals take shape.