21.6 Emission and Absorption Spectra from Different Stars
Stars emit a continuous spectrum of light because of their extremely hot, dense surfaces that behave like near-perfect blackbodies. As this light passes through the cooler outer atmosphere of a star, atoms in the gaseous layers absorb specific wavelengths corresponding to electronic transitions between energy levels. This creates dark lines in the continuous spectrum where specific colours of light have been absorbed. It is known as the absorption spectrum. Although the absorbed energy is re-emitted at the same wavelengths, the emitted photons are scattered in various directions making them unlikely to reach observers on Earth. Therefore, we detect a characteristic absorption spectrum with distinct missing wavelengths. In emission and absorption spectra from distant stars and galaxies, the lines in the spectra show an increase in wavelength from their known laboratory values (Fig. 21.5). This phenomenon is called redshift which occurs due to stretching of photons when they travel through the universe.
(Fig. 21.5: Light spectra from Sun and distant galaxies, showing a redshift in wavelength)