5.1 Stellar atmospheres and their chemical composition

Stellar atmospheres are the outer layers of stars, including the photosphere, chromosphere, and corona. These layers have unique properties that influence a star's appearance and behavior, from its color to phenomena like solar flares.

The chemical makeup of stellar atmospheres is crucial for understanding star formation and evolution. Hydrogen and helium dominate, but other elements are present in varying amounts. Spectroscopy helps scientists determine these compositions and uncover stellar secrets.

Stellar Atmospheres: Properties and Structure

Physical Characteristics

Top images from around the web for Physical Characteristics

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  Solar Activity above the Photosphere | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

1 of 3

Top images from around the web for Physical Characteristics

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  Solar Activity above the Photosphere | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

• 

  15.1 The Structure and Composition of the Sun | Astronomy View original

  Is this image relevant?

1 of 3

• Stellar atmospheres are the outermost layers of a star, consisting of the photosphere, chromosphere, and corona
• The temperature and density of stellar atmospheres decrease with increasing distance from the star's core
• Stellar atmospheres are in hydrostatic equilibrium, where the outward pressure gradient is balanced by the inward gravitational force
• The opacity of stellar atmospheres determines how easily photons can escape from the star and influences the temperature gradient

Atmospheric Layers

• The photosphere is the visible surface of the star, where most of the light is emitted
  • It has an effective temperature that determines the star's color (e.g., red, yellow, blue) and spectral type (e.g., O, B, A, F, G, K, M)
• The chromosphere is a thin, transparent layer above the photosphere, characterized by lower density and higher temperature than the photosphere
  • It is the site of various solar phenomena, such as solar flares and prominences
• The corona is the outermost layer of a star's atmosphere, extending millions of kilometers into space
  • It has a very high temperature (up to several million Kelvin) but low density

Chemical Composition of Stellar Atmospheres

Elemental Abundances

• Hydrogen (H) and helium (He) are the most abundant elements in stellar atmospheres, making up approximately 74% and 24% of the total mass, respectively
• Other common elements in stellar atmospheres include carbon (C), nitrogen (N), oxygen (O), neon (Ne), magnesium (Mg), silicon (Si), sulfur (S), and iron (Fe)
• The relative abundances of elements in stellar atmospheres are generally similar to the cosmic abundance pattern, which reflects the composition of the interstellar medium from which stars form

Variations in Composition

• The exact composition of a stellar atmosphere can vary depending on the star's initial composition, age, and evolutionary stage
• Some stars may show unusual abundances of certain elements due to processes such as atomic diffusion, accretion, or mass loss
  • For example, chemically peculiar stars (e.g., Ap and Bp stars) exhibit strong enhancements or depletions of specific elements in their atmospheres
  • Stars that have undergone mass transfer from a companion star may show altered atmospheric compositions

Determining Stellar Composition

Spectroscopic Analysis

• Spectroscopy is the primary method used to determine the chemical composition of stellar atmospheres by analyzing the absorption and emission lines in a star's spectrum
• Absorption lines are dark lines in a star's spectrum caused by the absorption of specific wavelengths of light by elements in the star's atmosphere
  • The wavelengths and strengths of these lines provide information about the presence and abundance of different elements
• Emission lines are bright lines in a star's spectrum produced when atoms in the atmosphere are excited and emit photons at specific wavelengths
  • These lines can also be used to identify the presence of certain elements

Abundance Determination Techniques

• The equivalent width of an absorption or emission line is a measure of the line's strength and is related to the abundance of the corresponding element in the atmosphere
• Curve of growth analysis is a technique used to determine elemental abundances by studying how the equivalent width of spectral lines varies with the number of absorbing atoms and the line's oscillator strength
• Model stellar atmospheres are used to interpret the observed spectra and derive the chemical composition by comparing the observed line strengths with theoretical predictions
  • These models take into account factors such as temperature, pressure, and opacity to simulate the physical conditions in the stellar atmosphere

Factors Influencing Stellar Composition

Initial Composition and Evolution

• The initial composition of a star is determined by the composition of the interstellar medium from which it formed, which can vary depending on the location and history of the star-forming region
• Stellar nucleosynthesis, the process by which elements are created in the cores of stars through nuclear fusion reactions, can alter the composition of a star's atmosphere over time as newly synthesized elements are mixed into the outer layers
  • For example, the carbon-nitrogen-oxygen (CNO) cycle in massive stars can lead to an enrichment of nitrogen in their atmospheres

Mass Loss and Accretion

• Mass loss through stellar winds or eruptions can affect the composition of a star's atmosphere by preferentially removing certain elements or by exposing material from deeper layers
  • For instance, Wolf-Rayet stars experience strong stellar winds that strip away their outer hydrogen-rich layers, revealing a helium-rich atmosphere
• Accretion of material from a companion star or the surrounding interstellar medium can introduce new elements into a star's atmosphere
  • This process is particularly relevant for stars in binary systems or those embedded in dense stellar environments

Atmospheric Processes

• Atomic diffusion, the process by which elements can sink or rise within a star's atmosphere due to the effects of gravity and radiation pressure, can lead to anomalous abundances of certain elements
  • In hot, chemically peculiar stars (e.g., Am and Fm stars), atomic diffusion can cause the accumulation of heavy elements in the atmosphere
• The presence of a strong magnetic field can influence the chemical composition of a star's atmosphere by affecting the transport of elements and the formation of spots or other surface features
  • Magnetic Ap stars, for example, often show enhanced abundances of rare earth elements in their atmospheres
• The evolutionary stage of a star can impact its atmospheric composition, as different processes (e.g., convection, mass loss, or dredge-up events) become more or less dominant throughout the star's lifetime
  • Red giant stars, which have expanded and cooled after exhausting the hydrogen fuel in their cores, may exhibit altered atmospheric compositions due to the dredge-up of material from inner layers