4.2 Stellar atmosphere models and opacity

Stellar opacity is crucial for understanding how stars work. It measures how easily light travels through a star's interior, affecting energy transfer and the star's structure. This concept is key to grasping stellar evolution and atmospheres.

Atmosphere models are essential tools for studying stars. They help scientists predict what stars look like from Earth, determine their properties, and understand their chemical makeup. These models connect theory with what we actually see in the night sky.

Stellar Opacity and Atmosphere Models

Concept of stellar opacity

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  Evolution from the Main Sequence to Red Giants | Astronomy View original

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  Using Spectra to Measure Stellar Radius, Composition, and Motion | Astronomy View original

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  17.4 Using Spectra to Measure Stellar Radius, Composition, and Motion | Astronomy View original

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  Evolution from the Main Sequence to Red Giants | Astronomy View original

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  Using Spectra to Measure Stellar Radius, Composition, and Motion | Astronomy View original

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  17.4 Using Spectra to Measure Stellar Radius, Composition, and Motion | Astronomy View original

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• Stellar opacity measures resistance to radiative energy transfer within star's interior
• Opacity coefficient ($\kappa$) quantifies absorption and scattering per unit mass ($\text{cm}^2/\text{g}$)
• Optical depth ($\tau$) gauges stellar material transparency defined as $d\tau = -\kappa \rho ds$
• Controls atmospheric temperature structure influencing emergent spectrum
• Determines energy transport mechanisms in different stellar layers (radiative vs convective)
• Impacts stellar evolution timescales and internal structure

Sources of atmospheric opacity

• Bound-bound transitions create absorption lines (Hydrogen Balmer lines, metal lines)
• Bound-free transitions (photoionization) contribute to continuous opacity (H$^-$ in Sun)
• Free-free transitions (Bremsstrahlung) significant in hot, ionized gases
• Electron scattering dominates in hot atmospheres (Thomson scattering, Compton scattering)
• Molecular opacity crucial for cool stars (TiO, H2O, CO bands)
• Negative hydrogen ion (H$^-$) major opacity source in solar-type stars
• Rayleigh scattering important in UV and visible regions of cool stars

Role of atmosphere models

• Bridge theoretical stellar structure and observations predicting emergent spectra
• Solve radiative transfer equation describing radiation propagation
• Determine stellar parameters ($T_\text{eff}$, $\log g$, [Fe/H])
• Enable detailed abundance analysis of chemical elements
• Facilitate stellar population synthesis for galaxies and clusters
• Provide input for stellar evolution models and isochrones
• Help interpret asteroseismological data and stellar pulsations

Limitations of atmosphere models

• Local Thermodynamic Equilibrium (LTE) assumption breaks down in low-density atmospheres
• Plane-parallel approximation unsuitable for extended atmospheres (giants, supergiants)
• Static atmosphere assumption neglects stellar winds, pulsations, and convection
• 1D models oversimplify complex 3D structures and convection patterns
• Non-LTE effects crucial for accurate spectral line modeling require extensive atomic data
• Classical models often omit chromospheres and coronae affecting high-energy phenomena
• Magnetic fields frequently excluded despite impact on active stars
• Limited treatment of dust formation and opacity in cool stellar atmospheres
• Microturbulence parameterization may oversimplify small-scale velocity fields