An infrared magnitude with effective wavelength 21.0 μm and bandwidth of 5.5 μm. There is also an index termed Q, formed by combining the optical magnitudes U, B, V:
Q = (U − B) − 0.72(B − V),
which is independent of interstellar reddening (see interstellar absorption) and is strongly correlated with spectral type on the upper main sequence.
Subjects: Astronomy and Astrophysics.