Definition: The word "achromaticity" in astronomy refers to the lack of color in stars due to their very high luminosity, which causes a star's colors to be completely black or white. This phenomenon can happen because stars are made up of tiny pieces of hydrogen and helium that orbit at extremely high speeds. When light is reflected from these tiny pieces, it does not have the opportunity to change its intensity based on the amount of matter surrounding it, so it reflects at a uniform speed regardless of color. The term "achromaticity" was coined by British astronomer Sir John Flamsteed in 1607, and it describes this phenomenon as "an absence of any change or alteration whatever in the colour of a star, which is black or white." The lack of color allows astronomers to study stars at an unprecedented scale without having to worry about interference from other wavelengths.

achromaticity