A Luminous Infrared Galaxy (LIRG) is a Galaxy emitting more than 10¹¹ times solar Luminosity in the Far Infrared.

They are often a Starburst Galaxy and/or have an Active Galactic Nucleus or a Quasar. The relatively high infrared stems from Dust heated by the Electromagnetic Radiation (EMR) of the AGN or Star Formation (SF), re-emitting the EMR in a cooler (infrared-intensive) Black Body spectrum, that can exceed the directly-viewed Emission from the star formation or AGN. Generally, the higher the IR, the higher the Star Formation Rate (SFR). LIRGs are among the brightest objects in the universe. Variations:

• Ultraluminous Infrared Galaxy (ULIRG): 10¹² times solar luminosity in far infrared.
• Hyperluminous Infrared Galaxy (HLIRG): 10¹³ times solar luminosity in far infrared.
• Extremely Luminous Infrared Galaxy (ELIRG): 10¹⁴ times solar luminosity in far infrared.