## Morgan-Keenan Luminosity Classification Scheme *(abbrev. **MK** or **MKK**)* | Class | Type | | --------------------- | --------------------------------------------------------- | | $0$ or $Ia^{+}$ | <nobr>Hypergiants (extremely luminous supergiants)</nobr> | | $Ia$ | Supergiants (luminous) | | $Iab$ | Supergiants (intermediate-size luminous) | | $Ib$ | Supergiants (less luminous) | | $II$ | Bright Giants | | $III$ | Giants | | $IV$ | Subgiants | | $V$ | Main Sequence ("Dwarfs") | | $VI$ or $sd$ (prefix) | Subdwarfs | | $VII$ or $D$ (prefix) | White Dwarf | ^luminosity-classes The **Morgan-Keenan Luminosity Classes** account for the luminosity/radial distance between stars. This classification scheme simply adds a Roman Numeral (or a few) to the existing letter-number [[Spectral Classes#Harvard Spectral Classification Scheme|Harvard Spectral Classification]]. Combining these systems creates a complete spectral classification system for stars. The [[Spectral Classes#Harvard Spectral Classification Scheme|Harvard Spectral Classification]] can distinguish between stars with the same color/temperature and apparent magnitude; however, to distinguish between different luminosities/distances, we use the *broadness of the spectral lines*. Denser objects have broader spectral lines due to increased pressure/collisional broadening and higher surface gravity (i.e. the inter-particle separation). $\Delta \lambda \; \propto \; n \; \propto \; R^{-3} \hspace{1cm} \Leftrightarrow \hspace{1cm} \Delta \nu \; \propto \; n^{-1} \; \propto \; R^{3}$ ![[HR_diagram4.png|align:center|500]] ![[Mnemonics#Spectral Classification - Sun]]