Water is plentiful here on the Earth, but the cold nebulae from which stars form appear bone dry. Astronomers have wondered: where does all the water come from? A satellite called SWAS (Submillimeter Wave Astronomy Satellite), launched just this past December, has provided an answer. SWAS has documented how water is created during the early stages of star formation and has shown that the water is not just a byproduct; in fact, it plays a surprisingly active role in spurring the birth of stars. The results were presented at the January meeting of the American Astronomical Society in Atlanta.
Stars form when a relatively dense portion of a nebula begins to collapse. As the cloud grows smaller it heats up and so resists further collapse. At this point, a miraculous thing occurs: water suddenly appears in the nebula. Water molecules are very efficient at radiating energy away from the cloud, continuously cooling it so that it can collapse into a star. SWAS determined that some of the water is hidden in ice grains and individual molecules that were invisible to all previous telescopes. Mostly, however, the water forms in a "big bang" chemical reaction. Heat and shock waves cause hydrogen and oxygen atoms in the nebula to combine rapidly into water. The water in turn siphons off energy; in response, the cloud shrinks further and creates still more water. SWAS has revealed how efficient this process is. In a star-forming region like the Orion Nebula, "it's water, water everywhere," says Gary Melnick of the Harvard-Smithsonian Center for Astrophysics. Some of that water will end up on planets around the infant stars--more good news for finding life elsewhere in the universe.
