What is Luminosity Class?
Luminosity class labels a star’s intrinsic brightness, and roughly places it within the star’s lifetime. Each star spends the bulk of its life in the “main sequence.” Here it is relatively stable in its light output, while it burns its hydrogen fuel. This is the “dwarf” stage, marked by the luminosity class roman numeral “V.” Luminosity class “IV” is a bit more complicated. Some class IV stars are pre-main sequence. These stars have not yet fully compressed to ignite their nuclear furnaces. The glow from the heat of compression makes the infant star appear to be a sub-giant. Later, the star shrinks, its surface becoming brighter (and bluer), and enters the main sequence. Much later, when its main fuel has been spent, the star balloons out to become, once again, a sub-giant — this time at the beginning of old age. In star catalogs, labelling of spectral types may follow many varying conventions, some of them containing much more than color and luminosity class. Thus, a detailed discussi
