I haven't finished talking about spectral classes and absolute luminosities. Consider A-type stars Vega, Sirius, Alioth and Fomalhaut.
Vega is classified as A0V (the "V" means luminosity class V, but it also means a main sequence, hydrogen-fusing star). Vega's (well-measured) absolute V luminosity is 47 times solar. Its Johnson B-V index is -0.001. The distance to Vega is 25 light-years. Jim Kaler estimates is mass as 2.3 solar
Sirius is classified as A1V, although the classification of A0V is also seen. Its absolute V luminosity is 22 times solar, about half that of Vega. Its Johnson B-V index is +0.009. The distance to Sirius is 8.6 light-years. Jim Kaler estimates its mass as 2.12 solar
Alioth, the brightest-looking star in the Big Dipper, is classified as A0p ("p"for peculiar). Jim Kaler, however, lists it as an unevolved star of type A0, one that is still fusing hydrogen in its core. Its V luminosity is 105 times solar, double that of Vega. Its Johnson B-V index is -0.022. The distance to Alioth is 83 light-years. Jim Kaler estimates its mass to "close to triple that of the Sun"
. Kaler adds:
Large and luminous for its class, Alioth is probably ageing, and is nearing the end of its main sequence hydrogen-fusing lifetime. Of greater significance, Alioth is the brightest of the "peculiar A (Ap) stars," magnetic stars in which a variety of chemical elements are either depleted or enhanced
Fomalhaut, finally, is classified as A3V, so it is smaller, cooler and less massive than the other three A-type stars. Fascinatingly, Jim Kaler lists it as belonging to spectral class A8V! I think Kaler is definitely wrong here. Anyway, Fomalhaut's V luminosity is almost 17 times solar, so it is not much fainter than Sirius at 22 times solar. Its Johnson B-V index is +0.145. Kaler estimates its mass as 2.0 solar
, not much less than that of Sirius. The distance to Fomalhaut is 25 light-years.
Let's mention one more A-type star, Altair, one of the Summer Triangle stars (the other two are Vega and Deneb). My software classifies it as A7IV-V, suggesting that it is on the verge of using up the hydrogen in its core. The classification of IV-V also suggests that the star may have expanded somewhat and become brighter than it was before.
Jim Kaler classifies it as A8V, the same as Fomalhaut!
The V luminosity of Altair is 11 times solar, considerably fainter than that of Fomalhaut and about half the luminosity of Sirius. On the other hand, it is also considerably cooler than Sirius. Its Johnson B-V index is +0.221. The distance to Altair is 17 light-years. Jim Kaler estimates its mass as between 1.7 and 1.8 solar
, significantly less than Sirius.
Let's now compare these A-type stars and say something about their absolute luminosities. Alioth appears overluminous, while Sirius seems to be slightly underluminous. A word of caution is needed here, however. The bright and obvious stars in the sky, the ones that have proper names, are rarely underluminous when compared with a large sample of stars of the same spectral class.
As for Fomalhaut, it seems perhaps slightly cool for its mass.
To return to Alioth, we might perhaps compare it with IQ Aurigae, another star classified as A0p. IQ Aur is probably more or less as bright as Alioth. Hipparcos says that its V luminosity is 94.2 ± 8.8 light-years at a distance of 413 ± 19 light-years. But IQ Aur is really strikingly blue, with a Johnson B-V index of -0.167. Bright Star Catalog says that IQ Aur is one of the hottest Ap stars known, about 17,000 Kelvin, comparable to a star of spectral class B4.
Anyway, if you tried to draw a straight line between spectral class and luminosity of these four, five or six A-type stars, you would certainly fail.