What is a feature of spectrum binaries?

Spectrum binaries are a type of binary star system in which the light from both stars can be resolved into distinct spectra, allowing astronomers to study their individual properties. When observing spectrum binaries, the concept of independent spectra is crucial because it enables the determination of the characteristics of each star, such as their temperature, composition, and velocity.

In this system, the distinct spectral lines of each star can be seen superimposed on each other, which reveals information about the motion, physical properties, and even the orbital dynamics of the binary system. This characteristic sets spectrum binaries apart from other types, where features might be blended or hardly discernible.

In contrast, other choices describe conditions that do not apply to spectrum binaries: showing a single spectral line would indicate there is only one star being observed, while the inability to detect both stars means that it is not a spectrum binary. Additionally, the absence of a Doppler effect would imply there is no movement detected between the stars, which is not consistent with the nature of spectrum binaries, where motion due to the gravity of the two stars does lead to measurable Doppler shifts in their spectral lines.