Recipe for a heavyweight: making a massive star.

Astronomers have a good idea of how small stars such as our sun form. First, a spinning gas cloud collapses to become a dense core surrounded by a flattened disk of gas and dust. Matter from the disk then falls onto the central body, which becomes massive and dense enough to ignite nuclear reactions. As most of this material careens inward, some of it also spews outward in a pair of jets.

For rarer, heavier stars, the process isn't as clear. Some scientists have argued that as stars get bigger, they emit intense radiation whose pressure breaks up the surrounding disk, limiting the star's final size. These astronomers suggest instead that massive stars might arise from the merger of several smaller stars.

In the Sept. 1 Nature, two teams weigh in on the debate. Each team found evidence of a disk surrounding a still-forming massive star, along with outflowing jets. The findings support the notion that at least some massive stars form as their smaller siblings do, by packing on infalling material from a circumstellar disk. This accretion process creates and sustains disks and narrow jets, whereas mergers destroy disks and spew only diffuse jets, if any, comments Barbara Whitney of the Space Telescope Science Institute in Boulder, Colo.

One of the research groups, led by Nimesh Patel of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., discovered a flattened disk around the young protostar HW2, which is about 15 times as massive as the sun and located about 2,400 light-years away from Earth. The team spotted HW2's disk with the Sub-millimeter Array telescope atop Mauna Kea in Hawaii, a network of eight far-infrared detectors exquisitely sensitive to dust and highly ionized molecular gas in dense, circumstellar disks.

Follow-up observations with the Very Large Array radio telescope near Socorro, N.M., revealed outflowing jets, additional evidence that the protostar is growing by accreting matter.

Using a different technique, Zhibo Jiang of the Purple Mountain Observatory in Nanjing, China, and his collaborators detected signs of a similar disk around an object known as the Becklin-Neugebauer protostellar object. At least seven times as heavy as the sun, the object resides some 1,500 light-years from Earth.

Jiang's team used the Subaru Telescope atop Manna Kea to measure the scattering and polarization of near-infrared light from the protostar. These polarized images reveal regions of high-density dust typical of a circumstellar disk.

In her commentary accompanying the new reports, Whitney agrees that for the two observed protostars, all the ingredients are in place for growth by accretion. But she adds that both scenarios--accretion and mergers--may play a role in forming massive stars. Which one dominates depends on the cosmic neighborhood, she proposes.

Whitney suggests that accretion reigns in sparsely populated regions but that both accretion and mergers may play a role within dense clusters of newborn stars, where collisions between neighbors are likely.