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How do astronomers distinguish hot stars, reddened by the interstellar light absorption, from the actual cold red stars?

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Reddening does not change the details of the spectrum of a star. The differences in absorption lines that are present in a hot star vs a cool star are still there after reddening. So if you have a spectrum then distinguishing the two scenarios is easy.

If you don't have a spectrum but have two colours then it is still possible to distinguish the scenarios by tracing the star back along an extinction vector in a colour 1 vs colour 2 diagram until it intercepts an intrinsic locus for (e.g.) main sequence stars. This can be definitive, but can be ambiguous depending on the types of star and particular colours.

If you just have a colour, you can't tell.

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