Recent data captured by NASA's Chandra Observatory and Spitzer Space Telescope has cast doubt on the basic model that accounts for the creation of star clusters. The data, collected from studies of NGC 2024, located in the Flame Nebula and the Orion Nebula Cluster, will require scientists to think up an entirely new approach as to how these celestial bodies come into creation.
The traditional model for the formation of star clusters has the first stars forming at the center of the gas cloud, eventually making them the oldest of the cluster. Gravity from these first stars would then draw in the material that will ultimately coalesce to form layers of outer stars, with this process continuing until the gas is depleted, giving birth to the final star.
The team were able to determine the age of the stars within the cluster via a two-step approach. First the researchers used the Chandra Observatory to obtain x-ray measurements in order to ascertain the brightness of the stars. Then, using a combination of ground-based telescopes and NASA's Spitzer Space Telescope, the researchers measured the brightness of the same stars in the infrared spectrum of light. By combining the readings, the team was able to estimate and compare the ages of the stars throughout the clusters.
It was found that stars on the edge of the two clusters were in fact older than those in the center, the polar opposite of what was to be expected under the current formation model. "Our findings are counterintuitive," states 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."
The stars on the outer perimeter of NGC 2024 were estimated to be 1.5 million years old, while those in the center of the cluster were only estimated to have formed 200,000 years ago. Similarly stars on the outer edge of the Orion Nebula were estimated to be 2 million years old, while those in the center were only formed 1.2 million years ago.
Three explanations have been proffered to help explain the new findings. The first states that the gas used to create the stars is denser in the center of the cloud. Therefore the more scarce areas of gas on the outside are used up much faster in the formation of stars, leaving the edges of the cluster depleted while the denser concentration of gas in the center continues to birth new stars.
The second theory holds that, given the extended life of the older stars, they are more prone to wandering away from the center of the cluster, leaving the impression that they were created there in the first place. The final explanation stipulates that the youngest stars were formed in huge filaments of gas that then fall towards the center of the cluster.
The team members intend to take measurements of more clusters, in order to establish a normative age model for these stellar bodies, thus allowing them to further hone their theories.
The two papers detailing the team's findings on the disparate ages of star clusters are available online in The Astrophysical Journal.