Stellar Spectral Classes

Astrophysics | AQA A-Level Physics

The OBAFGKM classification

The spectral classes used today were categorised by the astronomer Annie Jump Cannon. She reordered the original alphabetical system into seven temperature classes:

O B A F G K M

Stars are classified based on their intrinsic colour, temperature, and prominent absorption lines.
The intrinsic colour of a star is related to its peak emission wavelength, which is determined by its surface temperature as described by Wien's displacement law.

Spectral class	Intrinsic colour	Temperature / K	Prominent absorption lines
O	Blue	25 000 - 50 000	He⁺, He, H
B	Blue	11 000 - 25 000	He, H
A	Blue-white	7 500 - 11 000	H (strongest), ionised metals
F	White	6 000 - 7 500	Ionised metals
G	Yellow-white	5 000 - 6 000	Ionised and neutral metals
K	Orange	3 500 - 5 000	Neutral metals
M	Red	< 3 500	Neutral atoms, TiO

Temperature and absorption spectra

The relationship between temperature and absorption spectra is related to the effect of thermal energy on the atoms or molecules in a star's atmosphere.
At low temperatures: there may not be enough energy to excite atoms or break molecular bonds. This results in TiO and neutral atoms, as seen in classes K and M.
At higher temperatures: atoms have too much energy to form molecules, so ionisation can take place. This is seen in classes F and G.
At the hottest temperatures: hydrogen and helium are found in higher abundance in the atmospheres of the hottest stars. Crucially, their spectral lines start to dominate, as seen in classes O, B and A.

The Balmer series of hydrogen

The absorption and emission spectra of hydrogen and helium are of particular importance to astronomers because of their abundance in the universe.
The most important spectral series is the Balmer seriesThe set of spectral lines corresponding to electron transitions to or from the second energy level (n = 2) of hydrogen. These lines fall in the visible region of the electromagnetic spectrum., which involves electron transitions either to or from the second energy level ($n = 2$). The key part is that the wavelengths of photons in this series are in the visible spectrum.
The prominence of Balmer lines varies with surface temperature:

Spectral class	Prominence of Balmer lines	Explanation
O	Weak	Atmosphere too hot; hydrogen likely to be ionised
B	Slightly stronger	Atmosphere too hot; hydrogen likely to be ionised
A	Strongest	High abundance of hydrogen in the $n = 2$ state
F	Weak	Atmosphere too cool; hydrogen unlikely to be excited
G	Very weak / none	Too little atomic hydrogen
K	None	Far too cool to be excited
M	None	Far too cool to be excited

Crucially, the Balmer lines are strongest in class A stars. In hotter stars (O, B) the hydrogen is too ionised, and in cooler stars (F and below) the hydrogen is not sufficiently excited to the $n = 2$ level.
A common mnemonic for the order of spectral classes, attributed to Annie Jump Cannon herself, is: "Oh be a fine girl, kiss me!"

Common Mistake

Students often assume that Balmer lines should be strongest in the hottest stars. This is wrong. The key part is that at extremely high temperatures, the hydrogen atoms are ionised (the electron has been stripped away), so there are no bound electrons to produce transitions. The "sweet spot" for Balmer absorption is class A, where the atmosphere is hot enough to excite electrons to $n = 2$ but not so hot that the hydrogen is fully ionised.