A main sequence star
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Main Sequence Stars: Characteristics and Evolution
Star-Forming Main Sequence (SFMS) in Galaxies
The star-forming main sequence (SFMS) is a well-established relationship between the star formation rate (SFR) and stellar mass of galaxies. This relationship is observed both globally and on smaller scales within galaxies. Studies using 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 including composite, Seyfert, and LINER galaxies. This sequence suggests an evolutionary pathway where star formation is quenched by active galactic nuclei (AGN)1.
Molecular Gas Main Sequence (MGMS)
The ALMaQUEST survey has revealed that the SFMS is closely related to the molecular gas main sequence (MGMS). The MGMS indicates a strong correlation between the local gas mass and the gravitational potential set by the local stellar mass. This relationship, along with the Schmidt-Kennicutt (SK) relation, suggests that the resolved SFMS is a consequence of these more fundamental relations2.
Habitable Zones Around Main Sequence Stars
Main sequence stars less massive than the Sun have continuously habitable zones that are closer and narrower compared to those around the Sun. Calculations show that for stars with masses around 0.83 times that of the Sun (K1 stars), the inner and outer boundaries of the habitable zone converge, indicating no continuously habitable zone for most K and M stars3.
Evolution of Massive Main Sequence Stars
The evolution of massive stars on the main sequence is complex and depends on several parameters. Observational data and evolutionary models show that the earliest O-type stars appear only above 50 solar masses. As the mass increases, a larger portion of the main sequence is spent in luminosity class III, and supergiants appear before the end of core-hydrogen burning. This distribution aligns well with empirical data of OB stars, although some discrepancies exist for supergiants due to wind densities affecting luminosity class diagnostics4.
Transition from Pre-Main Sequence to Main Sequence
Pre-main-sequence (PMS) stars evolve into main-sequence (MS) stars over time. Studies have identified a rare group of stars in the transition phase (TP) from PMS to MS. These TP candidates, identified through photometric analysis, have ages between 0.1-5 million years and masses between 2-10.5 solar masses. They exhibit rotational velocity and color excess values intermediate between classical Be stars and Herbig Ae/Be stars5.
Spatially Resolved Star Formation
The SAMI Galaxy Survey has provided detailed star formation rate maps for galaxies, revealing that galaxies below the SFMS tend to have flatter star formation profiles. Early-type galaxies show a split into two populations based on their star formation activity, supporting a two-step quenching mechanism where halo mass first cuts off gas supply, followed by stabilization of remaining gas against further star formation9.
Starbursts and the Main Sequence
High-resolution ALMA images have identified two regimes of starbursts: classical starbursts above the SFMS with enhanced gas fractions and short depletion times, and a sub-population within the SFMS experiencing compact star formation. These starbursts, often associated with AGNs, suggest a connection between star formation and AGN activity8.
Conclusion
Main sequence stars and their associated phenomena, such as the SFMS and MGMS, play a crucial role in understanding galaxy evolution and star formation. The transition from PMS to MS, the spatial distribution of star formation, and the role of AGNs in quenching star formation are key areas of ongoing research. These insights help us comprehend the complex processes governing the life cycles of stars and galaxies.
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Most relevant research papers on this topic
Quenching star formation: insights from the local main sequence
Star formation quenching by active galactic nuclei (AGN) plays a key role in the evolutionary pathway of galaxies, from blue cloud to red-and-dead galaxies.
The ALMaQUEST Survey: The Molecular Gas Main Sequence and the Origin of the Star-forming Main Sequence
The molecular gas main sequence (MGMS) may be the least physically fundamental factor in the star-forming main sequence, with the resolved SFMS being the result of the combination of the Schmidt-Kennicutt and MGMS relations.
Habitable zones about main sequence stars
A main sequence star less massive than the Sun has a narrower, closer-in habitable zone, with initial stellar mass strongly influencing the width of this zone.
Highly CitedSpectroscopic evolution of massive stars on the main sequence
The evolution of massive stars is influenced by parameters like mass, spectral type, and wind densities, with a lower mass limit for dwarf stars and a higher mass limit for supergiants.
Identification of emission-line stars in transition phase from pre-main sequence to main sequence
This study identified 98 'transition phase' stars, which are evolving from pre-main sequence to main sequence, with ages between 0.1-5 Myr and masses between 2-100.5 M.
Hipparcos observations of pre-main-sequence stars
Hipparcos observations reveal the evolutionary status of Herbig Ae/Be stars and provide weighted mean parallaxes for nearby star-forming regions.
Highly CitedThe sub-galactic and nuclear main sequences for local star-forming galaxies
The sub-galactic main sequence (SGMS) reveals that star formation rates and stellar masses peak in the nucleus of star-forming galaxies, with circumnuclear clusters being the most active and massive regions.
Starbursts in and out of the star-formation main sequence
ALMA images reveal two distinct starburst regimes in galaxies, with compact star formation in the main sequence and active galactic nuclei ubiquitous in compact bursts.
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