Astrophysics is a difficult observational science.
Stars are distant which makes ordinary experiments impossible. Optical light will be defined in a second. For now, take it to mean the type of light to which our eyes are sensitive. Until recently the complete EM spectrum was not utilized because that is, most types of EM radiation cannot penetrate the Earth's atmosphere. Today, because we can place telescopes into orbit about the Earth, we are able to study stars across many portions of the EM spectrum.