What does it mean the number after the stellar class? Magnitude?

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SUMMARY

The discussion clarifies the meaning of the number following the stellar class designation, specifically in the context of the Sun being classified as a G2 V star. The number indicates the subclass of the spectral type, where a lower digit signifies a cooler star. Additionally, the Roman numeral following the spectral class denotes the luminosity class, which categorizes stars from hyper giants (0) to white dwarfs (VII). The Sun's classification as G2 V indicates it is a main sequence star within the G spectral class.

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What does it mean the number after the stellar class? Magnitude?
 
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Like in G2 for Sun? Subclass. The lower the digit, the colder the star.
 
If instead, you mean the roman numeral sometimes placed after the spectral class, this is the luminosity class as follows:

0 Hyper giant
I Super giant
II Bright giant
III Normal giant
IV Sub giant
V Main sequence (dwarf)
VI Sub dwarf
VII White dwarf

Thus the Sun is a G2 V star.

Sub classes of the luminosity class are labled a,ab & b
 
Great.
 

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