Fundamental Plane
Luminosities, surface brightness, central
velocity distribution, (and others), are
correlated, hence the term “fundamental
plane.”  Ellipticals populate a plane in
parameter space.  BIG area of research – very
useful tool and helps us understand galaxies.
Faber & Jackson (1976) is a classic in this area
(you might want to look up and read this one):
L ~ σx where x ≈4
So, get dispersion from spectrum, get
luminosity, and with magnitude get distance!