What do these letters represent O B A F G K M N?

Answer 1

OBAFGKMN represents the spectral types of stars in order as seen on the Hertzsprung Russell Diagram.

A common mnemonic device for this is: Oh Be A Fine Girl Kiss Me Now.

They stand for the diffrent classes of stars. They are put in these classes depending on what color they are and how hot they are.

The letters refer to how prominently hydrogen is visible in a stars spectrum. It originally existed on a full scale: ABCDEFGHIJKLMNO

As the physics of stars became better understood, many of the letters (CDEL) were rendered obsolete and it was realised that very hot stars have less prominent hydrogen lines, so that the order was re-arranged with O (no lines) then B before AFGKMN.

The reasons for O and B at the top is that in hot stars the Hydrogen becomes ionised which gives a different set of characteristic lines.

As such, they are now in order of overall surface temperature (and color), from white-blue stars more than 33,000K, down to red N class, at less than 3,500K.

Also, as long as we talk about main sequence stars, stars in their main stage of life, it generally indicates size as well, with O class stars being the largest (50+ solar masses) to N being the smallest (0.5 solar masses or less)