Main sequence star definition
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Main Sequence Star Definition
Introduction to Main Sequence Stars
The concept of the main sequence (MS) is fundamental in the study of stellar and galactic evolution. It refers to a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. This sequence is crucial for understanding the lifecycle of stars and the processes governing star formation and evolution.
Main Sequence of Star-Forming Galaxies
The main sequence of star-forming galaxies is characterized by a tight relationship between the star formation rate (SFR) and the stellar mass of a galaxy. This relationship is pivotal in driving galaxy evolution and is often used to classify galaxies based on their star-forming activity. However, discrepancies in the normalization, slope, and shape of the MS arise due to different selection criteria for isolating star-forming galaxies. To address this, an objective definition of the MS has been proposed, which does not rely on pre-selecting star-forming galaxies. Instead, it uses a 3D SFR-mass-number plot to define the MS as the ridge line of the star-forming peak, facilitating more consistent comparisons across different studies .
Hydrogen Main Sequence (H-MS)
The traditional main sequence, often referred to as the hydrogen main sequence (H-MS), consists of stars that are in a state of complete equilibrium, primarily burning hydrogen in their cores. This sequence is a special case of a more generalized concept, where stars of different compositions and evolutionary stages can also form main sequences. These generalized main sequences, although less significant for observed stars, provide valuable insights into the properties and evolution of certain types of evolved stars .
Star Formation Quenching and the Main Sequence
The star-forming main sequence also helps in understanding the quenching of star formation in galaxies. By studying local galaxies and their emission line ratios, researchers have identified a sequence that extends from actively star-forming 'blue cloud' galaxies to 'red-and-dead' galaxies. This sequence supports the idea that active galactic nuclei (AGN) play a crucial role in quenching star formation, marking an evolutionary pathway for galaxies .
Spatially Resolved Star Formation Main Sequence
Recent studies have extended the concept of the main sequence to spatially resolved properties of galaxies. Using integral field spectroscopic (IFS) observations, researchers have analyzed the star formation rate surface density and stellar mass surface density in individual regions within galaxies. This local relation exhibits a high degree of correlation, irrespective of the dominant ionization source or the galaxy's integrated stellar mass. The spatially resolved main sequence provides a more detailed understanding of star formation processes within galaxies, highlighting the integrated star formation main sequence and identifying sequences formed by galaxies dominated by old stellar populations .
Conclusion
The main sequence is a critical concept in astrophysics, providing insights into the lifecycle of stars and the evolution of galaxies. From the traditional hydrogen main sequence to the star-forming main sequence of galaxies, and the spatially resolved main sequence, these relationships help astronomers understand the processes governing star formation and the evolutionary pathways of galaxies. By adopting objective definitions and considering spatially resolved properties, researchers can achieve more consistent and comprehensive insights into the nature of main sequence stars and galaxies.
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