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27944 results in Astrophysics

Page 76 of 1398

  • 3 - Spectroscopic Tools

  • David F. Gray
  • Book:
    The Observation and Analysis of Stellar Photospheres
    Published online:
    05 January 2022
    Print publication:
    16 December 2021, pp 50-80

  • Summary

    The main tool for studying stars is the spectrograph.Here we look at the astronomical aspects of spectrographs, how they work, and how to optimize them for stellar work.The characteristics of diffraction gratings are a central theme.The roles of the collimator and camera are then discussed, as is the resolving power of the final unit.Spectrographs using gratings in low orders are contrasted to echelle spectrographs used in high orders.Interferometers and Fourier spectroscopy isdiscussed briefly, as are some aspects of telescopes.

  • 17 - Velocity Fields in Stellar Photospheres

  • David F. Gray
  • Book:
    The Observation and Analysis of Stellar Photospheres
    Published online:
    05 January 2022
    Print publication:
    16 December 2021, pp 392-438

  • Summary

    Stellar photospheres, particularly in F, G, and K spectral types, are full of motions driven by convection.Hot rising flows with cooler falling lanes in between give a mottled surface, seen as granulation on the Sun.Such motions introduce Doppler shifts that re-shape spectral-line profiles.One of the tasks of this chapter is to extract information about these velocities from the line profiles.Detailed explanations are presented showing applications of the analysis tools and the results.Three signatures of granulation can be identified in stellar spectra: non-thermal line broadening, asymmetric line profiles, and differential blueward velocity shifts that depend on line strength.Velocity fields vary with stellar temperature and surface gravity, with particularly large changes occurring toward high luminosities.

  • 11 - The Line Absorption Coefficient

  • David F. Gray
  • Book:
    The Observation and Analysis of Stellar Photospheres
    Published online:
    05 January 2022
    Print publication:
    16 December 2021, pp 210-242

  • Summary

    Spectral lines tell us a great deal about stars.On our quest to extract this information, we need to understand the basic physics that shapes the line absorption process.This chapter is where it starts.We look into the natural atomic broadening associated with the intrinsic widths of the atomic levels, various types of pressure broadening, and the ever-present thermal broadening.All these processes are put together in the line absorption coefficient, described by the Hjerting function.We are then armed to calculate theoretical line profiles.

  • 9 - The Model Photosphere

  • David F. Gray
  • Book:
    The Observation and Analysis of Stellar Photospheres
    Published online:
    05 January 2022
    Print publication:
    16 December 2021, pp 153-181

  • Summary

    The model photosphere is the core of the theoretical side of our studies.It is a numerical theory from which we compute the theoretical spectrum to be compared to the observations.Our model photospheres are based on hydrostatic equilibrium.To compute such models, we need the temperature distribution, a value for the surface gravity, and the chemical abundances of the electron donors.This chapter reveals how this process is done.

Page 76 of 1398