#astronomy 

The Yerkes spectral classification, also called the MK, or Morgan-Keenan, is a system of stellar spectral classification introduced in 1943 by William Morgan, Phillip C. Keenan, and Edith Kellman from Yerkes Observatory.

## Scale
| **Luminosity class** | **Description**                               |
| -------------------- | --------------------------------------------- |
| 0 or Ia+             | hypergiants or extremely luminous supergiants |
| Ia                   | luminous supergiants                          |
| Iab                  | intermediate-size luminous supergiants        |
| Ib                   | less luminous supergiants                     |
| II                   | bright giants                                 |
| III                  | normal giants                                 |
| IV                   | subgiants                                     |
| V                    | Main-sequence stars (dwarfs)                  |
| sd (prefix) or VI    | subdwarfs                                     |
| D (prefix) or VII    | white dwarfs                                  |