D4000

D4000, or the 4000 Angstrom break, is a feature in a galaxy's spectrum caused by the accumulation of many absorption lines from older stars' atmospheres. It appears as a sudden increase in the galaxy’s light absorption at a specific wavelength, around 400 nanometers. The size of this break helps astronomers determine the age of a galaxy’s stellar population: a larger D4000 indicates older stars and less recent star formation, while a smaller D4000 suggests younger stars and active star formation. It’s a useful tool for studying galaxy evolution and understanding the history of star formation in the universe.