Origin Of Spectral Class Characteristics

Atomic theory explains why hot blue (O-type) stars and cool red (M-type) stars produce spectra that look so different even though all stars are made of practically the same ingredients.

Every chemical element has a characteristic temperature and density at which it is most effective in producing visible absorption lines.

At extremely high temperatures, as in O stars, gas atoms are ionized, or broken up. Only the most tightly bound atoms such as singly ionized helium survive, and the lines of ionized atoms dominate the spectrum. When the temperature is around 5800 K, as in G stars such as our Sun, metal atoms such as iron and nickel remain neutral without being disrupted. At temperatures below 3500 K, as in M stars, even molecules such as titanium oxide can exist.

Does the absence of the characteristic absorption lines of a particular element like hydrogen in a star's spectrum necessarily mean that the star does not contain that element? Explain._

Answer: No. The star's temperature determines which kinds of atoms can produce visible absorption lines.

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