What is a main sequence star
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Understanding Main Sequence Stars
Definition and Characteristics of Main Sequence Stars
Main sequence stars are the most common type of stars in the universe, representing a phase in stellar evolution where stars are in a state of hydrostatic equilibrium, fusing hydrogen into helium in their cores. This phase is characterized by a stable balance between gravitational forces pulling inward and the pressure from nuclear fusion pushing outward6.
Physical Properties of Main Sequence Stars
Main sequence stars exhibit a range of physical properties that vary with their mass. For instance, G-type main sequence stars, like our Sun, show strong positive correlations between mass, radius, and luminosity. More massive stars tend to be larger and more luminous4. The mass-radius and mass-luminosity relationships are fundamental in understanding the structure and evolution of these stars.
Spectroscopic Evolution
The spectroscopic properties of main sequence stars also evolve over time. For massive stars, the spectral type and luminosity class change as they progress along the main sequence. For example, the earliest O-type stars appear only above a certain mass threshold, and as mass increases, a larger portion of the main sequence is spent in higher luminosity classes8. This spectroscopic evolution provides insights into the physical processes occurring within these stars.
Main Sequence in Galactic Context
Star-Forming Main Sequence (SFMS)
In the context of galaxies, the term "main sequence" is also used to describe the relationship between the star formation rate (SFR) and stellar mass of galaxies. This relationship, known as the star-forming main sequence (SFMS), indicates that galaxies with higher stellar masses tend to have higher star formation rates1 2. The SFMS is a crucial tool for understanding galaxy evolution, as it highlights the connection between star formation activity and the growth of stellar mass.
Molecular Gas Main Sequence (MGMS)
Recent studies have introduced the concept of the molecular gas main sequence (MGMS), which relates the surface density of molecular gas to the stellar mass surface density in star-forming regions. This relationship suggests that the local gas mass traces the gravitational potential set by the local stellar mass, or that both quantities follow the underlying total mass distributions2. The MGMS, along with the SFMS, helps in understanding the physical conditions that regulate star formation in galaxies.
Sub-Galactic and Nuclear Main Sequences
Further refinement of the main sequence concept has led to the identification of sub-galactic main sequences (SGMS) and nuclear main sequences. The SGMS relates the star formation rate surface density to the stellar mass density within distinct regions of galaxies, including their nuclei. This relationship holds down to kiloparsec scales and varies with galaxy morphology7. The nuclear main sequence, in particular, is useful for studying bulge growth and feedback processes connecting active galactic nuclei (AGN) and star formation.
Pre-Main Sequence and Transition Phases
Pre-Main Sequence Stars
Before stars enter the main sequence, they go through a pre-main sequence (PMS) phase, characterized by contraction and heating until nuclear fusion begins. Observations of PMS stars, such as T Tauri and Herbig Ae/Be stars, provide valuable data on the early stages of stellar evolution9.
Transition Phase Stars
There is also a transitional phase where stars evolve from the PMS to the main sequence. These transition phase (TP) stars are identified by their intermediate properties between classical Be stars and Herbig Ae/Be stars. They exhibit specific rotational velocities and color excess values, indicating their ongoing evolution towards the main sequence10.
Conclusion
Main sequence stars are a fundamental class of stars, crucial for understanding stellar and galactic evolution. Their physical properties, spectroscopic evolution, and the relationships they form in the context of galaxies provide deep insights into the processes governing star formation and growth. Observations of pre-main sequence and transition phase stars further enrich our understanding of the early and intermediate stages of stellar development.
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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.
Reconciling mass functions with the star-forming main sequence via mergers
Combining star formation and merging can reconcile galaxy evolution, allowing for a model in which hierarchical merging is disconnected from environmentally independent star formation.
On the Physical Properties of G-Type Main Sequence Stars
More massive G-type main sequence stars tend to be larger and more luminous, with strong positive correlations between mass, radius, and luminosity.
VINCI / VLTI observations of Main Sequence stars
VINCI/VLTI interferometric observations of Main Sequence stars provide precise angular diameters, constraining detailed models and revealing physical processes at play.
The 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.
Spectroscopic 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.
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.
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