Mapping cloudy births in Orion.

Mapping cloudy births in Orion

In the most extensive mapping yet of radio-wave emissions from atomic hydrogen in a star-forming region, astronomers have made two significant discoveries: a rapidly expanding shell of warm hydrogen gas that appears to trigger star formation when it collides with a cloud of cooler molecular hydrogen, and a huge, diffuse "atmosphere" of atomic hydrogen surrounding the large molecular cloud. This complicated pattern of clouds and gas, only 1,500 light-years from Earth, lies within the constellation Orion.

To map this region, scientists used the 140-foot telescope at the National Radio Astronomy Observatory in Green Bank, W.Va., to observe the area for several days. A powerful computer converted the millions of readings into a detailed map showing the locations, velocities and densities of more than 20 atomic hydrogen clouds and filaments in the region.

The hydrogen shell, about 200 light-years across and expanding in all directions at 7 kilometers per second, lies near the three bright stars that make up Orion's belt. The forces driving the shell's motion have apparently carved a cavity out of an adjacent, dense molecular cloud and triggered a burst of star formation where the shell and cloud meet.

The discovery of an atomic hydrogen atmosphere surrounding the Orion molecular cloud provides important information about the early stages of star formation and the details of molecular cloud evolution, the researchers say. That atmosphere acts like a cocoon, shielding the more fragile hydrogen molecules in the molecular cloud from the disruptive effects of starlight and compressing the molecular gas in preparation for star formation.

The mapping was conducted by Debra M. Elmegreen of Vassar College in Poughkeepsie, N.Y., Bruce G. Elmegreen of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., and Federick R. Chromey of Vassar and IBM.