How are the stars born
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The Birth of Stars: A Comprehensive Overview
Molecular Clouds and Star Formation
Stars are born within giant molecular clouds composed primarily of hydrogen and helium, along with dust and other elements . These clouds are cold and dense, providing the perfect environment for star formation. The process begins when regions within these clouds experience gravitational collapse, often triggered by external forces such as galactic shocks or turbulence .
Hierarchical Fragmentation and Cluster Formation
The formation of stars is not a solitary event but rather a collective process. Most stars, especially massive ones, are born in dense stellar clusters. These clusters form through the hierarchical fragmentation of turbulent molecular clouds. This fragmentation leads to the creation of numerous small subclusters, which eventually interact and merge to form larger stellar clusters. The high density of stars in these subclusters results in frequent dynamical interactions, affecting the properties of the newly formed stars, such as truncating circumstellar discs and altering binary systems.
Role of Magnetic Fields and Ambipolar Diffusion
Magnetic fields play a crucial role in supporting the clumps of molecular gas against gravitational collapse. Over time, these fields slip relative to the neutral gas through a process known as ambipolar diffusion, leading to the formation of dense cloud cores. These cores then undergo gravitational collapse, typically giving rise to low-mass stars. The onset of powerful stellar winds, driven by the energy released during the protostellar phase, eventually halts the accretion flow, determining the final mass of the new star.
Chemical and Physical Processes in Molecular Clouds
The chemistry and physics of the molecules within these clouds are vital to the star formation process. Molecules such as carbon monoxide (CO) and ammonia (NH3) are used to probe the conditions within the clouds, providing insights into the initial stages of stellar birth. These molecular processes influence the overall evolution of the parent clouds and the formation of stellar systems.
Evolution and Lifespan of Stars
Once formed, stars evolve through various stages, which are mapped on the Hertzsprung-Russell diagram. The life cycle of a star, from its birth in molecular clouds to its eventual death, is primarily determined by its mass. Low-mass stars evolve into red giants and then white dwarfs, while high-mass stars end their lives in spectacular supernovae, leaving behind neutron stars or black holes.
Conatal Stars and Galactic Dynamics
Stars that are born together, known as conatal stars, often remain in close proximity and share similar velocities and metallicities. These stars can be identified throughout the Galaxy, providing insights into the clustered nature of star formation and the dynamical state of the galactic disk. Studies using data from Gaia and LAMOST have identified numerous comoving pairs, supporting the idea that these stars were likely born in the same cluster.
Conclusion
The birth of stars is a complex and multifaceted process involving the interplay of gravitational forces, turbulence, magnetic fields, and molecular chemistry. Stars are typically born in clusters through the hierarchical fragmentation of molecular clouds, and their evolution is heavily influenced by their initial mass and environmental interactions. Understanding these processes is crucial for unraveling the mysteries of our Universe and the formation of galactic structures.
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