I'm working on a project with stars where, given their spectral type (such as B0IV), I want to be able to know its approximate absolute magnitude.

I have searched online and found data sources like this one from the University of Northern Iowa or a similar table at the back of a bright orange textbook called An Introduction to Modern Astrophysics by Carroll and Ostlie.

However, these data sources are missing data values for spectral types like B4V, and they don't have any data for luminosity classes II and IV.

I was wondering if anyone knows of any other data sources where I could enter a spectral type and get an absolute magnitude, or if there was any way I could fill in the missing values myself given the data that's already there. I understand that these data values will be approximate, and that's okay for the scope of my project.


  • $\begingroup$ If I understand you correctly, you want a Hertzsprung-Russel diagram, but as a table indexed by spectral type rather than as a diagram? $\endgroup$ Mar 8 at 15:30
  • $\begingroup$ Indeed! I have been searching for this online, but have not seen any. $\endgroup$
    – J minding
    Mar 9 at 1:20

1 Answer 1


Finding a unique solution is not so easy. In principle this information is encoded in the Hertzsprung-Russell diagram which shows the absolute magnitude of stars over their colour index. And you then can attribute certain regions to certain star types (main sequence, white dwarfs, giants etc), which is somewhat encoded in the spectral type as well.

I found a tabular way to describe this in these lecture notes (which seems the same you found). One may find papers which discuss the colour-luminosity relation for certain star types empirically, but I'm not aware of a comprehensive overview with approximate equations. I found in the (meanwhile slightly dated) book (Unsöld: Neuer Kosmos) an overview diagram.HRD

It's evident that the relation between spectral type and luminosity class on the one hand and the colour on the other hand is not a simple one - so going by a look-up table as linked with the lecture notes seems the easiest approach. You could possibly get approximate data from interpolating from the shown diagram.

That diagram seems an author-generated diagram, but some references given in that chapter are e.g. Houk (1976) or Yamashita, Nariai and Norimoto "An Atlas of Representative Stellar Spectra". I'm reasonably sure there exist something more comprehensive based on Gaia data (but I didn't find it quickly).

  • $\begingroup$ Thanks so much for this answer! The graphs on the Hertzsprung-Russell diagram are definitely not a direct relationship, which I guess sort of does explain why no one's created a 1-to-1 table. I suppose I'll be using the look-up table then. $\endgroup$
    – J minding
    Mar 9 at 1:29
  • $\begingroup$ For dwarfs only (luminosity class V), there's Eric Mamajek's "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence" which has a host of global properties (such as absolute magnitude) and photometric colors (e.g. Bp-Rp from Gaia data) as a function of spectral type. I don't know of an equivalent for giants or supergiants though. $\endgroup$ Mar 10 at 2:04

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