Hanny van Arkel had been using the Galaxy Zoo Web site less than a week when she noticed something odd about the photograph of IC 2497, a minor galaxy in the Leo Minor constellation. “It was this strange thing,” she recalled: an enormous gas cloud, floating like a ghost in front of the spiral galaxy.

A Dutch schoolteacher with no formal training in astronomy, Ms. van Arkel had joined tens of thousands of other Web volunteers to help classify photographs taken by deep-space telescopes. Stumped by the unusual image on her computer screen, she e-mailed the project staff for guidance. Staff members were stumped, too. And thus was christened the celestial body now known to astronomers worldwide as Hanny’s Voorwerp (Dutch for “object”).

Stories like Ms. van Arkel’s are becoming more common, as the Internet opens up new opportunities for so-called citizen scientists. And as millions of people get involved in these participatory projects, scientists are grappling with how best to harness the amateurs’ enthusiasm.

Some critics argue that citizen science projects are often little more than ploys to stimulate public interest rather than advance scientific knowledge. Others fret over the quality of data generated by nonspecialists. But scientists must weigh such risks against the benefits of a powerful new research tool: a vast computer network that can parcel out complex projects into small tasks that can be completed by individuals with relatively limited training.