Final answer:
A star's radius can be determined from its spectrum as it indicates the atmospheric pressure, which affects the width of spectral lines and the ionization state of atoms in the star's photosphere.
Step-by-step explanation:
A star's radius can be inferred by looking at its spectrum because the spectrum is affected by the star's atmospheric pressure. Stars with a large radius will have low atmospheric pressure in their photosphere, resulting in narrower spectral lines, due to reduced collisions between particles. Conversely, stars with a small radius will exhibit broader spectral lines indicating higher atmospheric pressure. Furthermore, the low pressure in a giant star's photosphere leads to more ionized atoms, altering the spectral signature in a way that allows astronomers to differentiate between stars of the same temperature but different sizes.
Stellar spectra also provide information on a star's chemical composition, the presence of elements like hydrogen and helium. By examining line shifts through the Doppler effect, astronomers can determine the radial velocity of a star, and by analyzing the broadening of spectral lines, they can measure the star's rotational velocity.