A star's luminosity (the amount of energy it emits in a given time) is determined by its brightness, size and distance. Astronomers use a photometer or a charge-coupled device to measure the amount of light emitted from a star. Once they know the brightness of the star and its distance from Earth, they can calculate its luminosity; closer stars appear brighter than those farther away. The phenomenon called the Doppler effect occurs when the wavelengths of stars are affected by their movements. For example, a star moving away from Earth develops a slower wavelength (i.e., a red color), whereas a star moving toward Earth becomes brighter. By measuring the particular star over a period of time and comparing its spectrum to the standard, we can observe the Doppler shift, detecting changes in the star's direction and speed. The Doppler shift can also detect changes in a star's rate of rotation as it spins on its axis.
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