Stellar Atmospheres and Spectra
Almost everything we know about a star is read from the thin outer layer where its light escapes; the spectrum imprinted there encodes the star's temperature, gravity, composition, and motion.
Definition
A stellar atmosphere is the outer region of a star from which radiation escapes to space, and a stellar spectrum is the distribution of that radiation with wavelength, carrying the continuum and absorption or emission lines used to characterize the star.
Scope
The area covers the physics of stellar atmospheres and the radiative transfer that shapes the emerging light, the classification of stars by their spectra, the quantitative analysis of spectral lines to derive temperatures, gravities, and chemical abundances, and the photometric measurement of brightness and color that underpins the cosmic distance scale.
Sub-topics
Core questions
- How does light escape from a star's outer layers?
- Why do stars have different spectral types?
- How are temperature, gravity, and composition read from a spectrum?
- How does stellar light yield distances?
Key concepts
- radiative transfer
- photosphere
- spectral line formation
- spectral type
- effective temperature
- chemical abundance
- photometry
Key theories
- Radiative transfer in stellar atmospheres
- The emerging spectrum is governed by the equation of radiative transfer through the atmosphere, where absorption and emission by atoms and ions, set by temperature and pressure, sculpt the continuum and the spectral lines that diagnose the star.
- Spectral classification and stellar composition
- The strengths of spectral lines order stars into a temperature sequence of spectral types; Payne showed that these differences arise from ionization and excitation rather than composition, establishing that stars are made overwhelmingly of hydrogen and helium.
Mechanisms
Radiation generated in the interior diffuses outward until it reaches the atmosphere, where the gas becomes transparent and photons stream into space. As they leave, atoms and ions absorb light at characteristic wavelengths set by the local temperature and pressure, imprinting absorption lines whose strengths and shapes encode the star's properties.
Clinical relevance
Stellar spectra and photometry are the primary observational gateway to stellar physics: they yield temperatures, gravities, abundances, velocities, and distances, underpin the classification and cataloging of stars, calibrate the cosmic distance ladder, and enable surveys that map the composition and structure of the Galaxy.
History
Fraunhofer mapped the solar absorption lines, Cannon devised the spectral classification system, Saha's ionization equation explained the temperature sequence, and Payne demonstrated in 1925 that stars are mostly hydrogen, founding the quantitative analysis of stellar atmospheres advanced later by Mihalas and others.
Key figures
- Cecilia Payne-Gaposchkin
- Annie Jump Cannon
- Meghnad Saha
- Dimitri Mihalas
Related topics
Seminal works
- mihalas1978
- payne1925
Frequently asked questions
- Why do absorption lines appear in stellar spectra?
- Light from the hot, dense interior passes through the cooler, more transparent atmosphere, where atoms and ions absorb specific wavelengths corresponding to their energy levels; this removes light at those wavelengths and leaves the dark absorption lines we observe.
- How can a spectrum reveal what a star is made of?
- Each chemical element absorbs at a unique set of wavelengths, so the pattern and strength of the absorption lines in a star's spectrum, interpreted with the physics of ionization and excitation, reveal which elements are present and in what amounts.