Stellar Physics
Stellar Spectroscopy
Decoding starlight — temperature, composition, motion all encoded in absorption lines
Stellar spectroscopy is the analysis of starlight to determine star's properties. Spectrum shows: continuous radiation from photosphere + absorption lines from cooler outer atmosphere. Lines reveal: (1) temperature (line strengths), (2) composition (which elements), (3) radial velocity (Doppler shift), (4) rotation (line broadening), (5) magnetic field (Zeeman splitting), (6) gravity (line wings). Massive surveys (SDSS, Gaia, LAMOST) record millions of spectra. Foundation of all modern stellar physics.
- DiscoveredJoseph von Fraunhofer, 1814 (lines in solar spectrum)
- Spectral typesO, B, A, F, G, K, M (hottest to coolest)
- Sun's classificationG2V (G-type, second subdivision, dwarf)
- Lines revealT, composition, velocity, rotation, gravity, B-field
- Modern surveysSDSS, LAMOST, Gaia, APOGEE
- Distance workSpectroscopic parallax (intrinsic luminosity from spectrum)
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Why spectroscopy matters
- Stellar properties. Most direct window into stars.
- Temperature. Wien's law, line strengths.
- Composition. Chemical evolution of universe.
- Velocity. Doppler shift; binary detection.
- Galactic kinematics. Map galactic motions.
- Cosmology. High-z spectroscopy probes early universe.
- Exoplanets. RV, atmospheric studies.
Common misconceptions
- Spectroscopy is just temperature. Reveals many properties.
- All stars look the same. Spectra are highly informative fingerprints.
- Spectra are visible bands. Multi-wavelength: UV, IR, X-ray crucial.
- Stellar lines from photosphere only. Different layers contribute.
- Spectra are hard to interpret. Standardized; modern computer analysis.
- Spectroscopy is qualitative. Highly quantitative — temperatures, abundances.
Frequently asked questions
What does a spectrum tell us?
Multiple. (1) Continuous spectrum: thermal radiation from photosphere. Peak wavelength → temperature (Wien's law). (2) Absorption lines: identifies elements + ionization states + temperature. (3) Doppler shift: line wavelengths shifted → radial velocity. (4) Line broadening: thermal motion, rotation, turbulence. (5) Zeeman splitting: magnetic field strength. (6) Gravity (logg): pressure broadening of line wings.
How are spectral types determined?
Strength of various absorption lines indicates temperature. O stars: He II lines. B: He I + H. A: H Balmer lines strong. F: Ca II. G: Sun-like — Ca II + many metal lines. K: TiO weak; CN bands. M: TiO strong; very cool. Temperature order: O hottest, M coolest. Numerical subclasses (e.g., G2 = Sun) for finer distinction.
How is composition measured?
Identify each element's spectral lines. Compare intensities to model predictions for various abundances. Stellar atmosphere modeling. Result: chemical composition (helium, metals, etc.). Very metal-poor stars (Pop II) found in galactic halo. Most stars: solar-like metal abundance.
What's a "metal" in astronomy?
Anything heavier than helium. Includes: Mg, Si, Ca, Fe, etc. Despite chemical inaccuracy, common usage. Sun's metal content: ~1.5%. Pop II stars: 0.001-0.1× solar. Population III stars (theoretical): zero metals — pure H + He from BB.
How is radial velocity measured?
Doppler shift of spectral lines. Compare observed to laboratory wavelengths. Δλ/λ = v/c. Modern instruments (ESPRESSO, HARPS): precision <1 m/s. Used for: stellar binaries, galactic dynamics, exoplanet detection. Most fundamental observable in spectroscopy.
What about temperature?
Multiple methods. (1) Wien's law: λ_max = 2.898×10⁻³/T. Peak in spectrum gives T. (2) Color: blue stars hot; red stars cool. (3) Specific spectral lines — different lines optimal at different T. (4) Hα profile width. (5) Iron line strengths. Combined: T to ~50 K precision.
How are massive surveys done?
SDSS, LAMOST (China), 4MOST (Europe), DESI (US), APOGEE — millions of stellar spectra. Robotic operation. Multi-fiber spectrographs simultaneously observe many stars. Computer analysis. Statistical studies of galactic populations, stellar physics, evolution. Driving most modern stellar astronomy.