Watery Stellar Nurseries

Water may be the key ingredient for the successful birth of a star.

Tuesday, July 01, 1997

Were it not for the fusion-powered heat and radiation rushing from their cores, stars would collapse under their own weight. This balancing act between gravity and radiation allows stars like our sun to chug along for billions of years before they use up their fuel and cave in on themselves. But how did this finely tuned equilibrium get started? Astrophysicists believe that stars form from the gravitational collapse of giant clouds of gas and dust. The problem is that long before these imploding clouds become dense enough to form stars, they should heat up, making the gas and dust expand, preventing any further collapse. As physicist Daniel Zajfman of the Weizmann Institute in Rehovot, Israel, says, When the clouds start to collapse, you need to find a way to get rid of some energy.

Zajfman has recently found evidence that suggests a solution to the problem. Astronomers have speculated that water may be the key ingredient to a successful stellar birth. Water molecules in interstellar clouds might absorb enough heat to allow the cloud to continue to collapse and form a star. Unfortunately, no one was sure whether interstellar clouds held enough water to do the trick--the water vapor in our own atmosphere makes it difficult for ground telescopes to measure whatever water the clouds may contain.

But astronomers can see molecules in the clouds that can combine to make water, says Zajfman. In particular, a charged molecule called hydronium, which has three hydrogen atoms and one oxygen ion, can transform into water (plus an independent hydrogen atom) if it captures a free- floating electron. Unfortunately for theorists, it can also form other combinations of hydrogen and oxygen. But if astronomers knew the rate at which hydronium converts to water, then they could estimate the amount of water in the clouds by measuring hydronium, which can be detected by radio telescopes.

Zajfman, Weizmann colleague Oded Heber, and Lars Andersen of the University of Aarhus in Denmark decided to make a simplified model of an interstellar cloud to see how much water would be made. They injected some hydronium into a particle accelerator and brought it up to the temperatures and pressures found in a typical cloud. They then merged the hydronium with a beam of electrons and measured how often hydronium converted into water. It turns out to occur about a third of the time, says Zajfman, more than enough to rid collapsing interstellar clouds of their excess heat.

Zajfman says the question may be resolved when the swas (short for submillimeter wave astronomy satellite) begins looking for water in interstellar clouds later this year. If its results match ours, then we do understand a very important part of the star formation process, Zajfman says. If the results are different, we have to go back to zero and start over.

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