Electrons can be used to determine how quickly a star is moving away from us, and even how far away it is.Electrons in an atom can move from one energy level to another. For example, when an atom absorbs a photon (a small packet of light energy), an electron goes from a low energy level to a higher level (Figure 1). This is excitation and the atom is in an excited state.
A photon has a specific energy value, which is related to a particular wavelength of light. We can detect the wavelength of light absorbed using spectroscopy. An absorption spectrum has black lines that indicate the wavelengths of light that have been absorbed. Each element has a characteristic absorption pattern. Figure 2 shows the absorption spectrum for hydrogen.
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