Irregular but Important Galaxy NGC 5408

Astronomers had long mistaken this faint object in Centaurus, NGC 5408, for a planetary nebula — an expelled cloud of material from an aging star. Upon closer examination, however, NGC 5408 was found to be an entire galaxy located about 16 million light-years away. Unlike the 75 percent of galaxies with a recognizable spiral or elliptical shape, however, NGC 5408 is among the 25 percent classified as irregular galaxies, ones lacking a particularly definable outline. As unremarkable as it is visually, the galaxy is associated with a rare object known as an ultraluminous X-ray source dubbed NGC 5408 X-1, which is one of the best studied of its class. Astrophysicists believe these sources are strong candidates for intermediate-mass black holes. This hypothetical type of black hole has significantly less mass than the supermassive black holes found in galactic centers, which can have billions of times the mass of the Sun but have a good deal more mass than those formed when giant stars collapse.