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27498 results in Astrophysics

Page 54 of 1375

  • 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.

  • 10 - Analysis of Stellar Continua

  • David F. Gray
  • Book:
    The Observation and Analysis of Stellar Photospheres
    Published online:
    05 January 2022
    Print publication:
    16 December 2021, pp 182-209

  • Summary

    The energy distribution, or how the starlight is distributed in wavelength across wide wavelength spans, depends strongly on the temperature of the star.In the visible spectral region, we see the Paschen continuum and a short portion of the Balmer continuum.How the flux in these regions is measured and calibrated is a central theme of this chapter.We look at both observed and calculated energy distributions with the aim of deriving stellar temperatures and surface gravities, and eventually radii.

  • 7 - Energy Transport 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 115-132

  • Summary

    The main mode of energy transport through the photosphere is via radiation, i.e., photons.We consider how these photons are created and destroyed and how the energy flows outward.We build on the material from the previous two chapters and formulate the mathematical integral needed to calculate the spectrum of a star.Convection is considered briefly, mainly because it introduces velocity fields into the photosphere.

Page 54 of 1375