OBAFGKMLTY
The Stellar Spectral Classification Sequence
Mnemonics are easily remembered phrases designed to aid your
memory. They are especially useful for remembering the orders of
things. Geology students, for example, have mnemonics for remembering
the order of geological time periods (Cambrian, Ordovician, ...), and
medical students have many colorful (and sometimes downright filthy)
mnemonics for remembering the names of bones or organs.
In astronomy, we have only a few sequences of things that are
important to memorize, like the order of planets from the Sun, the
Galilean Moons of Jupiter, or the order of stellar spectral types.
For example, a mnemonic for remembering the order of the planets from
the Sun is
- My Very Educated Mother Just Showed Us Nine Planets
Which is left over from when Pluto was still counted among the major planets.
The original Harvard stellar spectral classification sequence we learned
about in class is
- O B A F G K M
running from the hottest/bluest stars to the coolest/reddest stars.
The traditional mnemonic for the stellar spectral classes was
- Oh Be A Fine Girl/Guy Kiss Me!
Not the greatest, but it was what many of us learned for decades. When
I was in college, a popular counter-culture mnemonic was
- Oh Buy A Fine Green Kilo, Man!
Allegedly made up at UC Berkeley.
In the 1990s and early 2000s, three more spectral classes were added to
the cold/red end of the sequence by the discovery of faint cool stars:
L, T, and Y.
We need a new Mnemonic for a new century:
- O B A F G K M L T Y
Can you come up with one?