Here's why:
* Temperature: The spectral sequence classifies stars based on their surface temperature. The hottest stars are classified as O and the coolest stars are classified as M. The sequence OBAFGKM represents a decreasing temperature.
* Absorption Lines: The letters correspond to the presence and strength of specific absorption lines in a star's spectrum. These lines are created when elements in the star's atmosphere absorb certain wavelengths of light. Hotter stars have more ionized elements, producing different absorption lines than cooler stars.
* Historical Basis: The spectral sequence was developed in the late 19th century, and the initial classification was based on the prominence of hydrogen lines in the spectrum. As our understanding of stellar spectra grew, the sequence evolved to reflect temperature more directly.
It's important to remember that the spectral sequence is a continuous spectrum and that there are subclasses within each letter (e.g., O5, B2, G8). This allows for a more nuanced description of a star's temperature and properties.
So, the spectral sequence is not alphabetical because it reflects the physics of stellar temperature and absorption lines, rather than a simple alphabetical arrangement.
