Utilizing data from an X-ray space observatory and ground-based telescopes, astronomer say they're getting new ideas on how star clusters form -- and it's not how they once thought.

The telescope observations and information gathered by NASA's Chandra X-ray Observatory suggest previous theories of star cluster formation don't hold up.

Those earlier notions suggested cluster form in the condensing of giant gas clouds, with the cloud's center continuously pulling material in until it becomes dense enough for stars to begin forming.

Under this scenario, stars in the cluster's middle, having formed first, should be the oldest.

But that's not what Chandra saw, researchers say; in two clusters observed in which Sun-like stars are still forming, those in the outer regions of the cluster were found to be the oldest.

"Our findings are counterintuitive," says study leader Konstantin Getman of Penn State University. "It means we need to think harder and come up with more ideas of how stars like our Sun are formed."

In one of the clusters, known as NGC 2024, stars in its center were around 200,000 years old, compared to those on its outer edges which were around 1.5 million years old.

In the Orion Nebula star cluster, star ages were found ranging from 1.2 million years in the center to nearly 2 million years out near its edges.

"A key conclusion from our study is we can reject the basic model where clusters form from the inside out," says study co-author Eric Feigelson, who is also at Penn State. "So we need to consider more complex models that are now emerging from star formation studies."

One of those models suggests that as the density of the gas cloud decreases, star formation in the outer reaches of the cluster ceases while it continues in the center, eventually making younger stars in the majority there.

Another suggestion is that older stars have been around longer with more chance of drifting out of the cluster's center or of being kicked toward the outer regions from the gravitational effects of other stars.

More studies of yet more clusters could help refine those ideas, the researchers say.

"The next steps will be to see if we find this same age range in other young clusters," says Penn State graduate student Michael Kuhn, also a participant in the study published in The Astrophysical Journal.