An Etymological Dictionary of Astronomy and Astrophysics
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A member of a particularly faint population of galaxies with a central → surface brightness below the brightness of the background sky. The central regions of many of them resemble a → dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large → spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey → Freeman's law.

→ low; → surface; → brightness; → galaxy.

The brightness of an extended object, such as a planet, nebula, galaxy, or the sky background, expressed as magnitudes per unit area (usually square arc second). Surface brightness is calculated by dividing the object's magnitude by its size (→ isophotal radius).

→ surface; → brightness.