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Helium holds on to its
electrons the strongest. Neutral metals, and even more so, molecules are easily ionized.
O stars exhibit ionized Helium lines because they are VERY HOT. M stars, which
have molecules in their atmosphere, are VERY COLD. O, B, A, F, G, K, M, is
a TEMPERATURE sequence:
A popular activity in most college astronomy classes is to come up with a mnemonic to remember the sequence. One of the first, which has stuck since the middle of the last century is:
Recall, we already had a method to estimate temperature of a star by studying its spectrum: Applying Wein's Law. The identification of atomic and molecular lines in the star is an even more powerful estimate for stellar temperature.
What about Composition?
But, does this mean that HOT stars have no metals (atoms heavier than Hydrogen and Helium)? Do COLD stars have no Helium??Not necessarily. A very careful analysis, taking into account the physical condition of the stars atmosphere, must be included, when determining the composition of a star. If Iron lines are seen to be very weak in the spectrum of a star, it could be the temperature is not right for the formation of that line, OR that the amount of iron in the star's atmosphere is low.
We know, for instance, that about 90% of all atoms in nearly all stars are Hydrogen atoms. But clearly, that is not always the strongest lines in a given star!