Galaxy Color Magnitude Diagram
A scatter graph of galaxies showing the relationship between each galaxy’s absolute magnitude and its estimated temperature, or between optical and perceptual proxies for these quantities. The construction of such a diagram is similar to that of the Hertzsprung-Russell diagram, but the interpretation in terms of physical properties is not as precise.
Absolute Magnitude and Temperature Scales
To compute a galaxy’s absolute magnitude, it is treated as a point-like object, whereupon its radiation is corrected (by the inverse-square law) to a distance of 10 pc, and the absolute magnitude is computed as the number of 10−0.4 attenuations of similarly compensated power from a reference to achieve a match.
The temperature is also the same “effective temperature” as is used in the H-R diagram. However, galaxies are visible from much farther away than individual stars, so a galaxy’s recessional redshift exerts an appreciable influence on its effective temperature. Because light from...