Spectral Types

Looking at the spectra of stars, we can classify the stars based on the appearance of different spectral lines in the star.

Pickering & Fleming (1890s) developed a system based on the strength of the Balmer lines of hydrogens, with A stars being the having the strongest Balmer lines.

Annie Jump Cannon (1901) rearranged the sequence (into what we now recognize as a temperature sequence), and added subdivisions (0-9). This is called the Harvard classification scheme, and from hottest to coolest stars it runs

OBAFGKM

also known as "Oh Be A Fine Girl/Guy, Kiss Me" (yeah, that's right, I'm teaching the politically incorrect version...).

How would the color of stars change along this sequence?

The line strengths change due to the temperature effects on

the number of electrons in different energy levels in the atom
the fraction of atoms which are ionized

For example, Balmer lines (lines from the first excited state of hydrogen atoms) are strong in A stars. O stars are too hot -- all the hydrogen is ionized. G stars are too cool -- all the hydrogen atoms are in the ground state.

Here are examples of how spectral lines change along the sequence from F through G stars. (See also Figs 8.2 & 8.3 from your text.)

(spectra courtesy Dave Silva)

We can also determine the luminosity class based on the spectral lines:

Luminosity Class	Star
I	Supergiant
II	Bright Giant
III	Giant
IV	Subgiant
V	Dwarf (Main Sequence)
VI	Subdwarf

Under this scheme, the sun is a G2V star.