Final answer:
Dark bands in a star's light spectrum are due to elements in the star absorbing light at specific wavelengths. These absorption lines help determine the star's composition. Reflecting dust or interstellar gas can further modify the observed spectrum.
Step-by-step explanation:
Dark bands in the spectrum of light from a star show where elements in the star absorb light. These are known as absorption lines and are caused by atoms at the star's surface absorbing specific wavelengths of light. Each element has a unique pattern of absorption lines, which allows scientists to determine the composition of stars. In contrast, an emission spectrum would show bright lines at specific wavelengths against a darker background, representing the wavelengths of light emitted by elements.
When starlight is reflected by dust, the spectrum may show a reddening effect as shorter (bluer) wavelengths are more easily scattered by dust, causing the starlight that reaches us to be more dominated by longer (redder) wavelengths. If a star is behind invisible interstellar gas, the light we observe will show additional absorption lines corresponding to elements in the gas cloud, which is called an intervening absorption spectrum.