Star Cluster Study Inspires New Ideas On How Sun-Like Stars Form
May 8, 2014

Star Cluster Study Inspires New Ideas On How Sun-Like Stars Form

Brett Smith for - Your Universe Online

The prevailing theory on star cluster formation states that gravity causes a giant cloud of gas and dust to condense to concentrations that trigger the ignition of stars at the center of the cloud.

However, according to a new study based on data from NASA's Chandra X-ray Observatory and infrared telescopes, some of the oldest stars in known clusters sit on the exterior of their groups. The study researchers focused on two clusters where sun-like stars currently are forming – NGC 2024, located in the center of the Flame Nebula, and the Orion Nebula Cluster.

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

The researchers designed a novel two-step technique to reach their conclusion. They started by using Chandra information on the intensity of X-rays coming from stars, which would help to find out their masses. Next, the team established the infrared brightness with ground-based telescopes and information from NASA's Spitzer Space Telescope. By incorporating this data with theoretical models, the team calculated the ages of the stars throughout the two clusters.

The study team found their results were unlike what previous models had predicted. At the center of NGC 2024, stars were approximately 200,000 years old, while those on the borders of the cluster were around 1.5 million years old. In the Orion Nebula, star ages varied from 1.2 million years in the center of the cluster to just about 2 million years near the perimeters, according to a report on the research published in The Astrophysical Journal.

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

The study team offered a three-point process that could explain their results. First, they said star formation will continue to occur in the interior regions, as star-forming gas inside the cloud is denser in comparison to the more diffuse outer locations. Over time, gas density drops below a threshold where it cannot produce any more stars, resulting in the cessation of star creation in the outer perimeters. Meanwhile, stars continue to ignite inside the cloud, resulting in a centralization of younger stars.

The team also suggested that old stars have had additional time to move away from the heart of the group, or be knocked outward by interactions with other stars. Or, young stars could be grown in enormous filaments of gas that collapse toward the center of the cluster, the astronomers said.

The researchers noted that the Orion Nebula Cluster revealed hints of this reversed age spread in the past. However, earlier research was based on limited data. The new research offers the first evidence of this age distribution in the Flame Nebula.

"The next steps will be to see if we find this same age range in other young clusters," said study author Michael Kuhn, a graduate student at Penn State.