What is the descriptive observation that defines a galaxy as a "red galaxy" in astronomy?

Answer

Light appearing shifted toward the longer, redder wavelengths when observed from Earth.

A red galaxy is defined by the observation that its light has shifted toward the longer, redder wavelengths when viewed from Earth. This shift can stem from stellar age, distance (redshift), or dust obscuration.

What is the descriptive observation that defines a galaxy as a "red galaxy" in astronomy?

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