An absorption spectrum is a spectrum that shows the specific wavelengths of light absorbed by a substance, typically a gas or a liquid. When light passes through a cooler gas, certain wavelengths are absorbed by the atoms or molecules in the gas, resulting in dark lines or bands in the spectrum. These lines correspond to the energy levels of the electrons in the atoms, indicating which wavelengths are absorbed. Scientists use absorption spectra to identify the composition of distant stars and galaxies. By comparing the absorption lines in the spectrum of a celestial object with known wavelengths from elements like hydrogen or helium, they can determine what elements are present and their relative abundances.