The luminosity class that refers to stars on the main sequence is V.

Here's a breakdown of the luminosity classes:

* I: Supergiants

* II: Bright giants

* III: Giants

* IV: Subgiants

* V: Main sequence (dwarfs)

* VI: Subdwarfs

* VII: White dwarfs

The main sequence is where stars spend the majority of their lives, fusing hydrogen into helium in their cores.