Astronomical Spectroscopy

the branch of astrophysics that includes the study of the spectra of celestial bodies in order to discover the physical nature of the sun, stars, planets, nebulae, interstellar matter, and so on, and also their motions in space. In the narrow sense of the word, astronomical spectroscopy is the branch of practical astrophysics that deals only with the motion of celestial bodies or their components along the line of sight on the basis of measurements of the displacement of spectral lines, in accordance with the Doppler effect. Astronomical spectroscopy seeks to obtain spectra by means of astronomical spectrographs, to measure accurate values of the wavelengths of spectral lines, and also to estimate and measure the intensities of various formations in the spectra. The energy distribution in spectra constitutes the subject matter of astronomical spectrophotometry. Diverse physical phenomena that take place in celestial bodies can be considered by the analysis of their spectral properties. The internal motion of gaseous masses and the axial rotation of the sun, planets, nebulae, and galaxies determine the variety of radial velocities in different parts of their visible images. In stars that yield a point image, axial rotation results in the broadening of the spectral lines, which in this case become photometrically shallow. Strong turbulence in the atmosphere of a star leads to the broadening of spectral lines without significant weakening of their intensities. Periodic oscillations of spectral lines near their average position in the spectrum of a star indicate that this star is a close binary system.

The analysis of the intensity and photometric profile of the spectral lines permits the determination of the ionization state of the chemical elements in stellar atmospheres, the chemical composition and temperature in the atmospheres of stars, and the pressures, in particular electronic, in them. The differences in behavior of the diverse elements at various stages of ionization permit the extension of the spectral classification by taking into account the gaseous pressure in the atmospheres of stars, which is inseparably connected with their sizes and luminosities; by these means a two-dimensional spectral classification of stars can be developed. The application of polarization devices to the spectral analysis of the sun and stars affords the possibility of studying the magnetic fields of stars, which are usually variable.

By means of astronomical spectroscopy one can also determine the chemical composition (including isotopic) of the atmospheres of planets. The analysis of molecular absorption bands permits the determination of the temperature and pressure in planetary atmospheres.