![]() albedo A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely reflected away from the body. A galaxy hosting an AGN is called an active galaxy. active galactic nucleus (AGN) A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object. accretion disk A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years. absolute magnitude A measure of a star's absolute brightness. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
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