rootofallworlds t1_j5man2a wrote
Spectroscopy.
The surface temperature can be determined by the relative intensity of different spectral lines (the letter and number in the spectral class) or by the colour index (bluer is hotter).
If the distance is known independently, then given the apparent magnitude, distance, and surface temperature, the radius can be determined.
The width of the absorption lines in the star's spectrum (the Roman numberals in the spectral class) allows to infer the surface gravity of a star. And if you know the radius and the surface gravity you can calculate the mass.
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