Astronomical Spectroscopy
Astronomical spectroscopy disperses the light of celestial objects into its component wavelengths to reveal composition, temperature, motion, and physical conditions through spectral lines.
Definition
Astronomical spectroscopy is the measurement and analysis of the distribution of an object's radiation as a function of wavelength, from which physical and chemical properties are inferred.
Scope
This area covers the acquisition and interpretation of astronomical spectra: the instrumentation that disperses light and the reduction of the resulting data, the classification of stars by spectral appearance, the measurement of Doppler shifts to infer line-of-sight motion, and the use of emission and absorption lines to diagnose physical conditions. It excludes broadband flux measurement (photometry) and positional measurement (astrometry).
Sub-topics
Core questions
- How is starlight dispersed into a spectrum and recorded for analysis?
- What do the pattern and strength of spectral lines reveal about temperature, composition, and pressure?
- How are line shifts and widths used to measure motion, rotation, and turbulence?
- How are emission and absorption lines used to diagnose the conditions of gas in stars and nebulae?
Key theories
- Spectral line formation
- Atoms and molecules absorb and emit at characteristic wavelengths, so the lines in a spectrum identify the elements present and, through their strengths, the temperature and density of the emitting gas.
- Doppler effect
- Motion of a source along the line of sight shifts spectral lines toward shorter or longer wavelengths in proportion to velocity, enabling measurement of radial velocities and redshifts.
Clinical relevance
Spectroscopy provides the chemical abundances of stars and galaxies, the radial velocities used to find exoplanets and weigh galaxies, the redshifts that map cosmic expansion, and the temperatures and densities of interstellar and intergalactic gas.
History
Spectroscopy began with Fraunhofer's mapping of dark solar lines and Kirchhoff and Bunsen's identification of elements by their spectra, grew through photographic spectral classification at Harvard, and matured with CCD detectors and echelle spectrographs delivering high resolution and precision.
Related topics
Seminal works
- gray2005
- tennyson2019
- chromey2016
Frequently asked questions
- What can a spectrum tell us that a brightness measurement cannot?
- A spectrum reveals which elements are present, the temperature and pressure of the gas, and the line-of-sight velocity, information encoded in the positions, shapes, and strengths of spectral lines rather than in total brightness.
- Why do stars show absorption lines and nebulae emission lines?
- A cool stellar atmosphere absorbs continuum light from below, producing dark lines, while a hot, thin nebula re-emits at discrete wavelengths against a dark background, producing bright lines.