Nebulae and star clusters
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Nebulae and Star Clusters: A Comprehensive Overview
Embedded Star Clusters in Nebulae
Characteristics and Formation
Embedded star clusters represent the youngest group of star clusters in the Galaxy, often found within reflection nebulae or HII regions. These clusters are typically low mass systems, with total stellar masses ranging from 20 to 220 solar masses. Most of these clusters are younger than 2 million years and lack stars of spectral types earlier than B, indicating their recent formation1. The gravitational instability of these clusters suggests they are likely to dissolve within a few million years1.
Infrared Star Clusters and Stellar Groups
A significant number of star clusters are identified through infrared observations, particularly those embedded in dark clouds like Taurus-Auriga and Chamaleon I. A catalogue of 189 infrared clusters and 87 embedded stellar groups highlights the importance of multiplicity in star cluster formation, with 25% of these clusters forming in pairs or triplets2.
The Orion Nebula Cluster (ONC)
Formation History and Dynamics
The Orion Nebula Cluster (ONC) is a prime example for studying star cluster formation. It exhibits three distinct age populations and anisotropic velocity dispersions, which are indicative of its complex formation history. Simulations suggest that the ONC formed through hierarchical processes involving the mergers of subclumps, which brought cold gas into the cluster center, enhancing star formation3 9. This process also contributes to the anisotropy in velocity dispersions and the formation of runaway stars3.
Star Formation and Age Distribution
The ONC is characterized by a continuous star formation process over the last few million years, with the most active period of formation occurring within the last 2 million years. The cluster's stars range from 0.1 to 6.0 solar masses, and the age gradient observed suggests that star formation is propagating into the cloud7 8. The presence of a rotating central cluster and the net infall of young stars into the cluster's gravitational potential well further illustrate the dynamic nature of the ONC7.
Runaway and Walkaway Stars
The ONC is also notable for its runaway young stars, which are likely the result of dynamical ejections from unstable young triple systems or close three-body encounters. Observations have identified 26 candidate stars with high proper motions, many of which can be traced back to the densest regions of the ONC, such as the Trapezium5.
Cloud-Cloud Collisions
Recent studies propose that the formation of high-mass stars in the ONC may have been triggered by cloud-cloud collisions. Analysis of molecular gas data suggests that collisions between clouds at different velocities could have initiated the formation of O and B stars in the region6.
Historical Context and Observational Advances
Early Studies and Catalogues
The study of star clusters has a rich history, with early catalogues documenting numerous nebulae, nebulous stars, planetary nebulae, and star clusters. These catalogues have significantly enriched our understanding of the structure and composition of the heavens10. The pioneering work of astronomers like Harlow Shapley in the early 20th century brought globular clusters into the forefront of astronomical research, highlighting their importance in understanding the scale and structure of the Galaxy4.
Conclusion
The study of nebulae and star clusters, particularly embedded clusters and regions like the Orion Nebula Cluster, provides critical insights into the processes of star formation and the dynamic evolution of stellar systems. Advances in infrared observations and simulations continue to enhance our understanding of these complex and fascinating astronomical phenomena.
Sources and full results
Most relevant research papers on this topic
Near infrared photometric and optical spectroscopic study of 22 low mass star clusters embedded in nebulae
Low mass star clusters embedded in nebulae are gravitationally unstable and expected to dissolve within a few Myr, with most cluster ages younger than 2 Myr.
A Catalogue of infrared star clusters and stellar groups
This catalogue includes 189 infrared clusters and 87 embedded stellar groups, highlighting the importance of multiplicity in star cluster formation.
Highly CitedSIRIUS Project. IV. The formation history of the Orion Nebula Cluster driven by clump mergers
The Orion Nebula Cluster's formation history is driven by clump mergers, which enhance star formation and contribute to anisotropy in velocity dispersions, but disappear within 0.5 Myr.
Runaway Young Stars near the Orion Nebula
26 young stars with high proper motions in the Orion Nebula are likely to have experienced dynamical ejection from unstable triple systems or close three-body encounters in the last 1 Myr.
A New Look at the Molecular Gas in M42 and M43: Possible Evidence for Cloud–Cloud Collision that Triggered Formation of the OB Stars in the Orion Nebula Cluster
The Orion Nebula Cluster's high-mass star formation may have been triggered by cloud-cloud collisions within the last 0.1 Myr, as observed in molecular data.
Highly CitedStar Formation in the Orion Nebula Cluster
Star formation in the Orion Nebula Cluster began at a low level 107 years ago and has gradually accelerated to the present epoch, with the most active formation period confined to a few 106 years.
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