A Steward Observatory astronomer recently completed the largest survey of dense molecular gas in the Milky Way galaxy — the regions where stars are formed.

Yancy Shirley, an assistant professor of astronomy at the University of Arizona, said the study will allow astronomers to better understand how stars form and galaxies evolve.

A paper about the project is available on the website of Astrophysical Journal and scheduled for print publication next month.

The team used a radio telescope on Mount Graham, in the Pinaleño Mountains of Southeastern Arizona, to observe spectroscopic signals of different molecules at their individual wavelengths. Every molecule has a unique fingerprint — a set of frequencies at which it emits light, Shirley said.

For example, if Shirley and his team wanted to look at regions in the sky with carbon monoxide in them, they would turn their radio telescope to carbon monoxide’s specific frequency.

By observing different molecules, the team was able to determine which areas are the densest and which have the most dust emissions.

“Those little dense regions are the sites of current and future star formation,” Shirley said.

The team also determined the distances to more than 1,500 regions for the first time.

Shirley said it’s necessary to know the distances in order to calculate basic physical properties of the regions, such as size, mass and luminosity.

“We’ve determined some basic properties of these clouds now,” Shirley said. “We know that typically, the average mass of one of these clouds is a few hundred times the mass of our sun, so they’re not going to form one single star. They’re actually the sites of future clusters of stars.”

Shirley said this information is important to understand how our sun and solar system formed.

Star formation takes a few million years, but the team can now piece together the sequence of events by observing objects in different stages of the evolutionary process, he said.

The team started its observations in 2009 and completed them last December. They observed a total of 6,194 regions — about a quarter of the Milky Way galaxy — that are visible from the Northern Hemisphere.

The team used the Heinrich Hertz Submillimeter Telescope on Mount Graham and worked closely with the Arizona Radio Observatory to complete the observations.

Shirley called it the most complete and accurate survey to date.

He said, however, that there are still some biases — “we can’t see every tiny little star-forming region, but we can see all the regions that are essentially going to form large clusters.”

The most interesting part of the project for Shirley was that he and his team are now finding hundreds of regions in the earliest stage of star formation. He said the team calls these “starless clumps.”

“Not very many of these have been identified or are known,” he said, “and we are now finding them in large numbers.” Before the survey, only a handful of starless clumps had been found. Now the team knows of about 500 starless clumps.

Shirley said he and his team will focus their research on this area for the next couple of years.

Brian Svoboda, a UA graduate student of astronomy, said his goal is to understand how high-mass stars — 100 times bigger than our sun — begin to form in these starless clumps.

That formation influences how galaxies evolve and change, he said.

“If we want to understand the holistic picture of star formation and galaxy evolution as a whole, we really need to understand how high-mass stars form,” he said.

Shirley said the results of the team’s observations will be useful to astronomers who study galaxy formation.

“They now have a much larger set of objects in evolutionary stages that they can select to do follow-up studies,” he said.

“Now we have this much-less-biased view of how this process is working,” he said, “so we should be able to put together a much clearer picture of how something like a solar system will form in a very clustered environment.”

Drew McCullough is a NASA Space Grant intern.