"Blue shift" refers to the phenomenon where light from an object moving towards an observer shifts to shorter wavelengths, making it appear bluer. This effect is commonly observed in astronomy, where it indicates that a star or galaxy is moving closer to Earth. The change in wavelength occurs due to the Doppler effect, which affects all types of waves, including sound. In contrast to "red shift," which occurs when objects move away and light shifts to longer wavelengths, blue shift provides important information about the motion of celestial bodies. For example, the Andromeda Galaxy is known to be blue-shifted as it approaches the Milky Way, suggesting a future collision between the two galaxies.