In the numerical subdivision (0 to 9) within a primary spectral class, what does a lower number indicate?
Answer
A hotter star within that primary class
The classes are subdivided using digits 0 to 9, where a lower number denotes a star that is hotter within its specific primary class; for example, an O0 star is hotter than an O5 star.

Related Questions
What color hue indicates the stars possessing the absolute highest surface temperatures?What criterion is used by astronomers to systematically categorize stars based on the Harvard spectral classification system?Which spectral class holds the undisputed champions regarding stellar temperature?What is the general lower limit for the surface temperatures of O-type stars in Kelvin?In the numerical subdivision (0 to 9) within a primary spectral class, what does a lower number indicate?What is the spectral classification assigned to our Sun?What is the dominant color associated with M-type stars, the coolest in the standard classification?How does the extremely high surface temperature of O-class stars affect their expected lifespan?What specific characteristic dictates the definitive placement of a star into its spectral class?What are the L, T, and Y brown dwarfs, which are cooler than M-type stars, sometimes called?