Final answer:
Helium absorption lines are not seen in stars like the Sun because the helium atoms in the Sun's atmosphere are mostly in the ground state and can only absorb photons in the ultraviolet part of the spectrum, which is lacking in the Sun's radiation.
Step-by-step explanation:
Helium absorption lines are not seen in stars like the Sun because of the energy state of helium atoms at the temperature of the Sun. In the Sun's atmosphere, the helium atoms are mostly in the ground state (unexcited) and can only absorb photons that have enough energy to excite the electrons from the ground state to higher energy levels. However, these photons lie in the ultraviolet part of the electromagnetic spectrum, and there are very few ultraviolet photons in the Sun's radiation. Therefore, the helium atoms in the Sun's atmosphere do not absorb photons in the visible part of the spectrum, resulting in the absence of helium absorption lines.