30 Bolometric Magnitude and Bolometric Correction

V. B. Bhatia

 

1.  Learning Outcomes

 

After studying this module, you should be able to

  • explain the meaning and significance of bolometric magnitude
  • understand how to compute bolometric magnitude for a black body
  • state the zero of the bolometric scale
  • carry out computation of absolute visual and blue magnitudes of stars as blackbodies
  • relate temperature to the colour index
  • grasp the meaning and importance of bolometric correction
  • appreciate the problems with the computation of bolometric correction and how these problems are being tackled

 

2. Introduction

 

In the last module we discussed the UBV system of photometry for stars. We explained how the magnitude of a star could be different at different wavelengths.  We discussed Johnson and Morgan UBV system and its extensions to other wavelength bands. We defined colour index and explained how it could be related to the surface temperature of a star. We then discussed the colour – colour diagrams and emphasized their use as tools for recognizing the various categories of stars and other objects.  It was explained that starlight is differentially absorbed in the interstellar region and therefore stars appear redder than they actually are, a phenomenon called reddening or extinction. The difference between the observed colour and the actual colour of a star, the colour excess, is indicative of the chemical composition of the interstellar medium through which its light propagates. Lastly, the distinction between reddening and redshift was explained.

 

 

References 

  • https://en.wikipedia.org/wiki/Absolute_magnitude
  • https://en.wikipedia.org/wiki/Bolometric_correction
  • http://www.astro.princeton.edu/~gk/A403/constants.pdf
  • https://www.cfa.harvard.edu/~dfabricant/huchra/ay145/magnitudes.pdf
  • http://home.strw.leidenuniv.nl/~brandl/OBSTECH/Handouts_3sep.pdf
  • ftp://ftp.osupytheas.fr/pub/gcomte/LECTURES_Beijing_NAOC/Exercises_in_Photometry/exercise_bolometric_magnitudes.pdf
  • https://sites.google.com/site/mamajeksstarnotes/bc-scale
  • http://spiff.rit.edu/classes/phys440/lectures/color/color.html
  • https://ned.ipac.caltech.edu/level5/Sept11/Buta/Buta15.html