Stellar spectral classification

Stars are grouped into spectral classes according to the absorption features in their spectra. These features appear where atoms, ions or molecules in a star’s atmosphere absorb particular wavelengths of light.

The main sequence of spectral classes is O, B, A, F, G, K, M, from hotter to cooler stellar surface temperatures. Intrinsic colour and prominent absorption features change along this sequence. The temperature ranges below are approximate.

• O: Blue; $$25{,}000–50{,}000\ \mathrm{K}$$. Prominent lines of $$\ce{He+}$$, $$\ce{He}$$ and $$\ce{H}$$.
• B: Blue; $$11{,}000–25{,}000\ \mathrm{K}$$. Prominent $$\ce{He}$$ and $$\ce{H}$$ lines.
• A: Blue-white; $$7{,}500–11{,}000\ \mathrm{K}$$. Hydrogen lines are strongest, and ionized-metal lines are also prominent.
• F: White; $$6{,}000–7{,}500\ \mathrm{K}$$. Prominent ionized-metal lines.
• G: Yellow-white; $$5{,}000–6{,}000\ \mathrm{K}$$. Prominent ionized- and neutral-metal lines.
• K: Orange; $$3{,}500–5{,}000\ \mathrm{K}$$. Prominent neutral-metal lines.
• M: Red; below $$3{,}500\ \mathrm{K}$$. Prominent absorption from neutral atoms and titanium oxide, $$\ce{TiO}$$.

A neutral atom has no overall electric charge; an ionized atom has lost one or more electrons. The prominence of an absorption feature depends on conditions in the stellar atmosphere, including temperature. A strong line therefore does not by itself mean that the element producing it is the star’s most abundant element.

Temperature dependence of Balmer absorption

A hydrogen atom can absorb a photon if the photon has exactly the energy needed to move its electron from one allowed energy state to a higher one. Balmer absorption occurs when the electron starts in the $$n = 2$$ state. Because only particular energy differences are allowed, absorption occurs at specific wavelengths, producing Balmer absorption lines.

The strength of these lines depends on how many neutral hydrogen atoms have electrons in the $$n = 2$$ state and are therefore available to absorb Balmer photons. Temperature changes this population:

• At low temperatures, relatively few hydrogen atoms have enough energy to be in the $$n = 2$$ state, so Balmer absorption is weak.
• At intermediate temperatures, a larger proportion of atoms occupy $$n = 2$$, so more photons at Balmer wavelengths can be absorbed. The lines are strong.
• At very high temperatures, much of the hydrogen is ionized: its electrons have been removed from the atoms. Fewer neutral hydrogen atoms remain available to produce Balmer absorption, so the lines weaken.

Strong Balmer absorption therefore does not simply mean that a star contains more hydrogen. A star can contain abundant hydrogen but have weak Balmer lines if its temperature leaves relatively few neutral atoms in the required $$n = 2$$ state.