In astrophysics elements heavier than helium are termed “metals” collectively. These elements are more opaque and radiate more efficiently than hydrogen and helium. As a result, metallicity has a profound effect on stellar evolution. For example, high metallicity enables the protostars to contract faster and ignite the nuclear fusion sooner, which prevents the formation of extremely massive stars. High metallicity stars also more easily lose the outer envelope at the late phase of stellar evolution.
But I don’t understand why heavier elements are more opaque. It’s not hard to understand that helium is very transparent because helium has the highest 1st ionization energy. However, the 1st ionization energy of hydrogen is lower than nitrogen, oxygen and neon, which are among the most abundant metals. Does that mean these elements should not be regarded “metals”?