A study of more than 1,000 newly discovered star clusters has revealed that these stellar villages have a more complex and diverse process of formation than previously recognized. The findings could change the way we think about our Sun's relationship to the stars with which it was born.
Considering how much time amateur astronomers spend entranced by the beauty of “embedded clusters” (groups of stars surrounded by interstellar gas and dust) it is surprising how little we know about them. While the globular clusters that surround the Milky Way have been the subject of extensive study, clusters that lie within the galaxy itself remain more of a mystery, partly because dust lanes block so many from our view. Yet embedded clusters are thought to be where almost all stars are born; understanding them is essential to knowing where we come from.
Professor Denilso Camargo of Brazil's Colegio Militar de Porto Alegre, has been conducting a search for undiscovered clusters using visual inspection of the images collected with the Wide-field Infrared Survey Explorer. His first 437 revealed that the galaxy probably has four arms, not two as has been proposed. Camargo also identified two clusters whose locations and behavior differ dramatically from any seen before.
Camargo has now more than doubled the number of clusters that bear his name, with a paper to be published in Monthly Notices of the Royal Astronomical Society (preprint available on arXiv.org) detailing the clusters named Camargo 447 to Camargo 1098.
These clusters, mostly made up of just 10-20 stars support the previous claims for a four armed galaxy. More significantly, much has been learned in the process about how embedded clusters evolve.
Regions such as the Orion Nebula are giving birth to stars before our eyes. The Trapezium Cluster at the nebula's heart consists of very young stars with halos of the gas from which they form. Embedded clusters that survive intact until all their gas has turned to stars are known as open clusters. Since stars in clusters like this usually have the same age and formed from chemically similar material, they have been thought to have a fairly simple history, being born in the same nursery before eventually drifting apart.
However, Carmargo proposes instead that some clusters are “aggregates” that come from two or more clusters merging. After studying a representative sample of his clusters in detail he and his coauthors found several examples where clusters appear to be made from stars with slightly different origins, indicating a merger. Others, Camargo told IFLScience, “present signs of ongoing merge events.”
The paper suggests this process may explain why some clusters have previously been found to have a surprisingly wide spread of ages among their stars.
“We find evidence that star formation processes span a wide range of sizes,” Camargo told IFLScience, “From populous dense clusters to small compact embedded ones, sparse stellar groups or in relative isolation.”
The authors propose that many clusters undergo what they call “collect and collapse events” where supernovae explosions, or the powerful stellar winds from very large stars, drive stars apart until the cohesiveness of the cluster is lost.
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