What type of star is Acrux?

Answer 1

Acrux (Alpha Crucis) is not a single star, what we know of it makes two stars visible (A1 and A2). Both stars are Class B, pushing Class O (blue-white to blue). A1 is a B0.5IV (B=blue-white class, 0.5=temperature range {0 being hottest, 9 being coolest}, IV puts A1 in the sub-giant range. A2 is a B1V, the same B class, however it is cooler than A1 and much smaller, V-making it a dwarf.

So, B is the color from the Harvard Spectral Classification. The number is a subdivision of the Harvard Spectral Classification and addresses temperature and luminosity. The Roman numeral is the Yerkes Spectral Classification and addresses the size and luminosity (broken into 8 divisions from hypergiants to white dwarfs).