low surface brightness galaxy
A low surface brightness galaxy is a galaxy that is at most a few percent brighter than the sky background, making it very difficult to see. The first low-surface-brightness (LSB) galaxy to be found, Malin-1, was discovered as recently as 1987 and it now appears that up to half the galaxy population is of this type and had been missed in earlier surveys. LSB galaxies have fewer stars per unit volume than normal galaxies, possibly because they are more isolated in space and have not undergone tidal interactions with other galaxies that stimulate bursts of star formation. Recent photographic and CCD surveys have uncovered large numbers of new small and medium-sized, moderate-to-low surface brightness spiral galaxies; however, LSB giants such as Malin-1 have remained relatively rare.