Spectral Types (star classification)

[Enola_Straight]

Oh, be a fine girl, kiss me like that.

Black dwarf

What is the logic behind OBAFGKMLT?

(as opposed to ABCDEFGHI)

[MonkeyMensch]

It was originally in alphabetical order but as more was learned about stars a number of letters were dropped and the remaining letters reordered in a more logical order and more was learned about stellar compositions and spectra.

[markn_1]

The Wikipedia areticle says:

The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved.

[dtilque]

@Enola_Straight

Oh, be a fine girl, kiss me like that.

There’s a yet cooler spectral type Y, used for sub-brown dwarfs. So let’s modify this:

Oh, be a fine girl, kiss me like that, Yowza!

More specific answer to the OP. The letters indicate how strong the hydrogen lines in the spectra were. A being the strongest and whatever the highest letter (not sure if they went past O or not) being non-existent. They rearranged them to correspond with temperature when the Hertzsprung-Russell diagram was invented.

[md2000]

IIRC from Astronomy 120 in 1973, the spectral types were based on the strength of certain spectral lines (implying certain relative amounts of elements).

Then it was realized the distribution was Bell-Curve-shaped, So O was the largest stars, as was B, not one of the weakest, and the number of categories was reduced. OBAFGKM

After M I learned of Q (“…Quick”), not L and T or Y. So the list has been in flux over time, as they figure out things like brown dwarfs and such.

Stellar classification

Draper System
In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue. It used a scheme in which the previously used Secchi classes (I to IV) were divided into more specific classes, given letters from A to N. Also, the letters O, P and Q were used, O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class.[43][44]

Harvard system

In 1897, another worker at Harvard, Antonia Maury, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of twenty-two types numbered from I to XXII.[45][46]

In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, and M, used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one-fifth of the way from F to G, and so forth.[47][48] Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.[49][50] This is essentially the modern form of the Harvard classification system. A common mnemonic for remembering the spectral type letters is "Oh, Be A Fine Guy/Girl, Kiss Me

[jtur88]

When I lerned them, it was OBAFGKMRNS . . .kiss me right now, smack!.

But another thought just occurred to me: Are there any stars known that are unique in some way that they do not fall into any of these categories?

[watchwolf49]

Oh … that’s a mnemonic … that explains all the times I’ve had my face slapped …