What basis is used for Color Index? Discrepancy on Betelgeuse
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Betelgeuse (https://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+39801) has a B-V color index of 1.85 according to the databases SIMBAD references. In the HYG v3 database, this color index is 1.5.
Is this a one-off error? Is it a matter of different bands being used for B and/or V?
The color indices are generally the HIPPARCOS values for the B-V color index. The value for Betelgeuse is 1.500 as in the following results directly from HIPPARCOS: https://hipparcos-tools.cosmos.esa.int/cgi-bin/HIPcatalogueSearch.pl?hipId=27989&hdId=&tyc1=&tyc2=&tyc3= .
Having said that, since Betelgeuse is intrinsically quite variable, its B-V color index also varies quite a bit. Here's an example of a fluctuation during its deep minimum in 2019-2020: https://astronomy.stackexchange.com/questions/35095/color-index-b-v-of-the-dimming-betelgeuse
So ... not really sure this is an actual error - any chosen value of B-V for a star like Betelgeuse is likely only valid for the specific time of observation (or is possibly an averaged value over a long time) and should be treated as a rough estimate only.