An Etymological Dictionary of Astronomy and Astrophysics
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The division of galaxies into a "red sequence" and a "blue sequence" in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

→ galaxy; → bimodality.