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The classes used to categorize stars (O, B, A, F, G, K, M) are in a bizarre order for historical reasons. Stars were labeled based on the spectral lines that were visible, then the categories were put in a different order to account for their temperature. When discovering the temperature of stars, the categories were not renamed, so that the old catalogs could still be used, and everyone just learned "Oh be a fine guy/gal kiss me".

The first seven classes were updated as knowledge improved, but brown dwarfs were discovered more recently, so why have the letters L, T and Y been chosen, rather than the easier to remember P, Q and R (or some other set of letters), for instance?

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  • $\begingroup$ Have you tried looking at the Stellar Classification Wikipedia article? It explains, for example, that R and N have already been used for carbon stars (though these are now generally classed as "C" stars), and that P was used for the central stars of planetary nebulae. $\endgroup$ Jan 16 '20 at 9:09
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The choice of L and T is explained in Kirkpatrick et al. (1999) 'Dwarfs Cooler than "M": The Definition of Spectral Type "L" Using Discoveries from the 2 Micron All-Sky Survey (2MASS)'. The principles behind the choice are given at the start of section 5.1:

In choosing a letter designation for the new spectral class, three important points must be considered : (1) The letter must be unambiguous, having not been used for any currently recognized spectral type. For example, though "N" follows "M" in the alphabet, it would be a poor choice of letter since it is used for a class of carbon stars. (2) The letter must create a taxonomy that is clearly distinguished from other types of astronomical objects. In this case, the letter must be accepted by the entire community, both by researchers involved in low-mass star and brown dwarf science and by astronomers in general. Although stellar spectroscopists might find "E0," "E1," "E2," etc., perfectly acceptable as new spectral subclasses, extragalactic morphologists already recognize these designations as elliptical galaxy types. (3) The letter must stand the test of time. For example, choosing "D" to mean that these objects are "degenerate" brown dwarfs would be flawed reasoning. Some of these dwarfs certainly are substellar (see § 7), but such a designation cannot be tied uniquely to any particular spectroscopic trait. The designation should apply to spectral features alone and be free of physical interpretation. Our understanding of the underlying physics may change with time ; our choice of letter should be impervious to such changes.

Applying these, they find the letters H, L, T and Y to be ok: the rationale for excluding the other letters is given in their Table 5:

Letter Status Notes
A In use Standard spectral class
B In use Standard spectral class
C In use Standard carbon-star class
D Ambiguous Confusion with white dwarf classes DA, DB, DC, etc.
E Ambiguous Confusion with elliptical galaxy morphological types E0–E7
F In use Standard spectral class
G In use Standard spectral class
H OK
I Problematic Transcription problems with I0 (10, Io) and I1 (11, II, Il)
J In use Standard carbon-star class
K In use Standard spectral class
L OK
M In use Standard spectral class
N In use Standard carbon-star class
O In use Standard spectral class
P Problematic? Incorrect association with planets?
Q Problematic? Incorrect association with QSOs?
R In use Standard carbon-star class
S In use Standard spectral class for ZrO-rich stars
T OK
U Problematic? Incorrect association with ultraviolet sources?
V Problematic Confusion with vanadium oxide (V0 vs VO)
W Ambiguous Confusion with Wolf-Rayet WN and WR classes
X Problematic Incorrect association with X-ray sources
Y OK
Z Problematic? Incorrect implication that we have reached "the end"?

They go with "L" as it is the closest letter to "M" that is still available. They prefer it to "H" because of the occasional use of the term "hydride dwarf" to refer to M subdwarfs which are dominated by CaH and MgH bands.

After that, the sequence is going alphabetically through the remaining available letters, so they put Gl 229B, which has a notably different spectrum to the other (L-class) brown dwarfs known at the time, into spectral class T. Spectral type Y took a few more years to show up in observations, and was chosen by the same principle.

If observations get to the point where it is worth erecting a new spectral class beyond Y then things will get interesting.

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