Final answer:
Stars have dark lines in their spectra due to absorption of certain wavelengths of light by different elements in the star's outer layers, reflecting the star's composition.
Step-by-step explanation:
Stars have dark lines in their spectra because the atoms at the star's surface absorb certain wavelengths of light. This absorption leads to the appearance of dark lines at those specific wavelengths. The atoms in the star's interior emit light, but parts of the spectrum may appear dark due to these absorption lines. The dark lines in stellar spectra are evidence of certain chemical elements within the star absorbing specific wavelengths of sunlight. This is how we can determine the composition of stars.
It's important to note that while all gases show these characteristic absorption lines, each element absorbs a different set of colors, hence showing various dark lines. Understanding the connection between spectral lines and elements was a significant advancement in astronomy. The presence or absence of these lines, combined with knowledge of the star's temperature and pressure, allows astronomers to determine the abundance of elements within the star.