The photosphere of a star is responsible for the dark lines in its spectrum.

Here's why:

* The photosphere is the visible surface of the star. It's the layer where light actually escapes from the star and reaches our telescopes.

* The photosphere is cooler than the layers below it. This cooler gas absorbs specific wavelengths of light from the hotter interior, creating the dark lines in the spectrum.

* These dark lines are called absorption lines. They represent wavelengths of light that have been absorbed by atoms in the photosphere. Each element has a unique set of wavelengths it absorbs, allowing astronomers to identify the composition of the star.

So, when we analyze the spectrum of a star, those dark lines tell us about the elements present in its photosphere, providing valuable insights into its composition and physical characteristics.