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About 10% of stars are found NOT to lie on the Main Sequence (luminosity class V shown below).
Stars not on the Main Sequence (not luminosity class V) are NOT currently burning Hydrogen in their cores. They exist in an advanced stage of stellar evolution.
Spectroscopic Parallax
This is not a parallax at all, but
is a means of estimating the distance to a star from
its spectral type (OBAFGKM) and luminosity class (V,VI,III,II,I).
If we know a stars spectral type and luminosity class, then we know in principle (see above) what their absolute Visual magnitude should be. Comparing that to their apparent visual magnitude tells us the stars distance!