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If I look up a star on Simbad, (for instance this one), a list of fluxes in different bands might be given.

Star classes (O, B, A, F, G, K or M) depend on the color of the star. The color of a star can be inferred by looking at the spectrum of the star, or by comparing its flux in different bands.

Is there a way of converting this list of fluxes from Simbad into a star class? How can I deduce the star class based on the info given on the Simbad page?


I know that stellar classes have precise subcategories based on the spectra (e.g. our own Sun is a G2V star), but I'm only interested in knowing if it is possible to obtain the most general star class (e.g. a G star)

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    $\begingroup$ You might be able to map the fluxes onto a blackbody curve to find the $T_{eff}$ then use temperature to determine spectral class. $\endgroup$ Oct 14 '21 at 12:37
  • $\begingroup$ Simbad already gives the spectral type for most (all?) stars under "Spectral type". No need to do this. $\endgroup$
    – Kyle
    Nov 12 '21 at 1:04
  • $\begingroup$ @Kyle In the star given as an example in this question, where can I find the Spectral type? $\endgroup$
    – usernumber
    Nov 13 '21 at 8:36
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There is no direct conversion from a flux to a spectral type, but the ratio of fluxes, or the equivalently, the difference in magnitudes in different bands (e.g. $B-V$, $J-K$ etc.), known as a colour is spectral type-dependent.

The conversion from colour(s) to spectral type can be done using a calibration table.

There are a number of these in use. A very respected one is that compiled by Eric Mamajek and which contains most of the colours you can think of.

Note that the conversions do also depend on surface gravity. The Mamajek tables referred to above apply to main sequence stars and if you know the object is a main sequence star then precise colours should give the spectral type to one or two subclasses. However, spectral type depends on both temperature and gravity. I know of some cruder calibration tables for lower gravity giants that can be found online in the Handbook of Space Astronomy and Astrophysics by Martin Zombeck. The conversion from colour to spectral types for giants tends to be a little less precise, but should still easily be sufficient to distinguish a G-type giant from a K-type giant for example.

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While color and spectral type are definitely heavily related by temperatures, the way that spectral types originally came to be is (by the name) according to spectra, and so without the spectra it could be difficult to classify the star completely (e.g. into something like G5 with its luminosity class) but it seems like you’re looking more for a general idea, which could be obtained.

One way that isn’t necessarily a flux approach but could be used if you just really want spectral types is by going to MAST and seeing if there are other observations done (like by a spectrograph) that could more precisely narrow down the spectral type.

Another option if either you can’t find anything on MAST or are curious about the flux approach is to just find a table of some well defined spectral types and their corresponding B-V’s or some other related color mechanism (I imagine the type of color indicator might vary depending on what kind of stars you’re looking at). I’ve seen these tables before, most notably in the back of a textbook I have I believe, and so if you can’t easily find them somewhere let me know and I can find an exact source for the one I’ve seen. This is obviously filter dependent, so care would need to be taken to make sure you’re comparing measurements from the same types of filters.

This last method isn’t very precise, but neither is determining spectral type by color alone; it’s a good indicator for general classification, but harder for more specific classifications that require spectra.

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