Stars in the main sequence
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Understanding Stars in the Main Sequence
Star Formation Main Sequence: Relationship Between Star Formation Rate and Stellar Mass
The star formation main sequence (SFMS) is a well-established correlation between the star formation rate (SFR) and the stellar mass of galaxies. This relationship is observed in both local and high-redshift galaxies, indicating a fundamental aspect of galaxy evolution. Studies using data from the Sloan Digital Sky Survey have shown that galaxies form a sequence from actively star-forming 'blue cloud' galaxies to 'red-and-dead' galaxies, with intermediate stages involving composite, Seyfert, and LINER galaxies. This sequence suggests an evolutionary pathway where star formation is quenched by active galactic nuclei (AGN) .
Starbursts and the Main Sequence
High-resolution observations from the Atacama Large Millimeter Array (ALMA) have revealed that starbursts, or periods of intense star formation, can occur both within and above the main sequence. These starbursts are characterized by dense concentrations of dusty star formation near the galaxy centers. Two distinct regimes of starbursts have been identified: classical starbursts above the main sequence with high gas fractions and short depletion times, and a sub-population within the main sequence experiencing compact star formation. The latter may represent the final stages of star formation before the galaxy becomes passive .
Molecular Gas and the Resolved Main Sequence
The ALMaQUEST survey has provided insights into the molecular gas main sequence (MGMS), which correlates the surface densities of star formation rate (ΣSFR), stellar mass (Σ*), and molecular gas (ΣH2). This 3D linear relationship suggests that local gas mass traces the gravitational potential set by the local stellar mass. The resolved SFMS, which examines these relationships on kiloparsec scales, shows that the scatter in the SFMS is larger compared to the MGMS and the Schmidt-Kennicutt (SK) relation, indicating that the SFMS may be a consequence of the combination of these more fundamental relations .
Radial Star Formation Profiles
Using data from the MaNGA survey, researchers have confirmed that the correlation between stellar mass and SFR exists on kpc scales, forming a 'resolved' main sequence. Galaxies above the global main sequence show enhanced star formation throughout, particularly in their central regions. Conversely, quiescent galaxies exhibit a deficit of star formation, especially in their cores. This supports the 'compaction' scenario, where central starbursts lead to bulge growth and eventual quenching of star formation from the inside out .
Evolution of Massive Stars on the Main Sequence
The evolution of massive stars on the main sequence is complex and influenced by various parameters. Models show that the earliest O-type stars appear only above a certain mass threshold, and the distribution of stars across different luminosity classes varies with mass. For instance, supergiants can appear before the end of core-hydrogen burning in very massive stars. These models align well with observed distributions of stars, although some discrepancies exist, particularly in the classification of supergiants .
Habitable Zones Around Main Sequence Stars
The habitable zone (HZ) around a main sequence star, where conditions might support liquid water, varies with the star's mass. For stars less massive than the Sun, the HZ is closer and narrower. Calculations indicate that for stars with masses around 0.83 times that of the Sun (K1 stars), the inner and outer boundaries of the HZ converge, suggesting no continuously habitable zone for most K and M stars .
Conclusion
The main sequence of stars and galaxies provides a framework for understanding stellar and galactic evolution. From the tight correlation between SFR and stellar mass to the detailed radial profiles of star formation, these studies highlight the intricate processes governing star formation and quenching. Observations and models of massive stars further elucidate the evolutionary pathways on the main sequence, while the concept of habitable zones underscores the importance of stellar mass in determining potential habitability.
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