A galaxy's color reveals its age: blue galaxies are young and active, while red galaxies are old and dormant.

Astronomers use the color-magnitude diagram to classify galaxies into two main groups: the 'blue cloud' and the 'red sequence.' Blue galaxies are dominated by massive O and B-type stars. These stars are extremely hot, reaching temperatures over 30,000 Kelvin, but they have short lifespans of only a few million years. Their presence proves that the galaxy is currently forming new stars from cold gas reservoirs.In contrast, red galaxies have exhausted their supply of interstellar gas and dust. Without this raw material, star formation ceases, a process known as 'quenching.' Over billions of years, the short-lived blue stars die off, leaving behind smaller, cooler, and longer-lived K and M-type dwarf stars. These older stars emit light primarily in the red and infrared spectrum, giving the galaxy its distinct crimson hue.This transition often occurs when galaxies collide or enter dense galaxy clusters. Observations from the Hubble Space Telescope and the Sloan Digital Sky Survey (SDSS) have confirmed this evolutionary path. Data from the SDSS, which has mapped over 930,000 galaxies, shows a clear gap between the blue and red populations called the 'green valley.' This represents a transitional phase where galaxies are rapidly shutting down their star-making factories.