Before stars begin forming in them, interstellar clouds are cold and are made up mostly of hydrogen atoms. As part of a cloud begins to contract, potentially into several clumps that could form stars, these atoms begin to combine and form molecules, which are heavier.

"These may be the first observations of a cloud that is in the transition between the neutral atomic hydrogen and molecular phases," said NRAO scientist Felix J. Lockman, who conducted the study with colleague Anthony H. Minter. The research was presented here earlier this month at a meeting of the American Astronomical Society.

The observation was made with the National Science Foundation's 140-foot radio telescope at the NRAO in Green Bank, West Virginia, and maps out different temperatures in the cloud. Hotter temperatures indicate greater density.

The researchers called the cloud "uncharacteristically massive," at some 500 light-years across and with an estimated mass of atomic hydrogen some 100,000 times that of our Sun. Other clouds this massive have been found to already be forming stars, a hint that this one is likely on the brink of doing so.

"We think we have caught something in a special state," Lockman said. "It could be one of the missing links in the cycle of star formation."

Another recent study observed an interstellar cloud at a slightly later stage, as individual globs of material had begun to form. Combined, the two new studies should help astronomers zero in on exactly what happens when stars are born.

The newly observed cloud is about 16,300 light-years from Earth and sits along the inner plane of our galaxy. It appears to be near one of the galaxy's spiral arms, and Lockman and Minter speculate that as the cloud slams into the arm, a shock wave results that could be the catalyst for the converting atomic hydrogen to the molecular state.

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