# Equivalent Width: Strong and weak lines

Equivalent width of lines are known to characterize the overall strength of lines. However, if there are two absorption troughs, with the same equivalent width but different depth, which is possible if one is less spread out w.r.t the other, then which profile will be stronger? The one with larger depth?

• What do you mean by "strength"? – Rob Jeffries Aug 7 at 7:26
• Assuming that you are plotting intensity in a wavelength range $\lambda$ to $\lambda + \delta \lambda$ against wavelength $\lambda$ then you need to compare the areas under the graph. – Farcher Aug 7 at 8:01
• @RobJeffries E.g. I have the spectra of two stars, and the equivalent width of Fe II lines in the spectra of the two stars are the same. But the lines have different depths. Then can I infer something about the abundance of Fe in the stars, as in which one is Fe-rich out of the two? – Lekha Aug 7 at 9:42
• Well, are the stars otherwise identical? If so, why are the widths of the lines different? If they are not identical then it is possible to have the same equivalent width but different abundances. The only scenario I can think of is two stars that are identical except for different rotation rates. In that case they have similar EWs, different depths but the same abundance. – Rob Jeffries Aug 7 at 9:50