English: As a planet orbits a star, the star wobbles. This causes a change in the appearance of the star’s spectrum called Doppler shift. While the star is moving toward us, its spectrum is blueshifted (top spectrum): All of the wavelengths look compressed and absorption lines appear at shorter (bluer) wavelengths than when the star is not moving toward or away from us (middle spectrum). While the star is moving away from us, the light is redshifted: All of the wavelengths look stretched out and the absorption lines appear at longer (redder) wavelengths (bottom spectrum). Because the change in wavelength is directly related to relative speed, astronomers can use Doppler shift to calculate exactly how fast an object is moving toward or away from us. Astronomers can also track the Doppler shift of a star over time to estimate the mass of the planet orbiting it.