Hence, the spectral features of the supergiant are different from those of the Sun-in accordance with the Saha equation- even though both stars are essentially at the same temperature. The pressure effect is somewhat less important than the temperature effect for it appears in the equation only linearly, whereas the temperature enters exponentially.

A giant will exhibit almost the same spectrum as does a main-sequence star of the same spectral type, as long as the giant's surface temperature is lowered slightly to compensate for its lower electron density. Even in this case, however, the spectral of the giant will be sharper than those of main sequence star because the giant’s features suffer much less pressure broadening.

According to Zeilik's astronomy and astrophysics book, spectrum features vary by electron density even at the same temperature. And even if it has the same spectral type, the spectral lines of giant stars come out narrower.

  1. If the spectral type is the same, does that mean that the spectral features are the same? Then, why is there a difference in the strength of the line? Does it mean that even if the spectral type is the same, the features are different?

  2. What do spectral features include? I don't know the exact meaning.

How can red supergiants be cooler than dwarfs of the same spectral type?

I looked up what features mean in the link above, but I'm confused.

  • 4
    $\begingroup$ 'Strength' of line does not mean so much width as it means depth. Width is determined by rotation and pressure, depth by composition $\endgroup$ Commented Mar 14, 2022 at 22:32
  • 1
    $\begingroup$ Can you please clarify what spectral features really mean, else it is very tricky for one to answer $\endgroup$
    – DialFrost
    Commented Sep 24, 2022 at 10:16
  • 1
    $\begingroup$ @DialFrost The textbook does not clearly define features, but it seems to mean one of the types of intensity and absorption lines in the context. $\endgroup$ Commented Sep 24, 2022 at 10:55


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