NGC 2403 is a luminous member of the M81 group of galaxies. Similar in morphology to M33, NGC 2403 is noted for its exceptionally bright HII regions and high rate of star formation. A subset of its HII regions are so exceptionally bright that each are comparable to the most massive HII regions in our local group of galaxies, the 30 Doradus complex within the Large Magellenic cloud and NGC 604 in M33. These immense and exceptionally luminous emission clouds are called Giant HII regions. In a recent survey NGC 2403 was found to possess at least six of these monster clouds, an unusually high number for a galaxy. In fact the most luminous and extended of all the giant HII clouds in NGC 2403 covers an area 2000 light years across, almost 50 Orion Nebulas! These giant HII regions are powered by large OB associations and often contain up to dozens of Wolf-Rayet candidate stars, soon to become supernovas.
Recent observations of NGC2403 at radio wavelengths have detected anomalous clouds of neutral molecular hydrogen (HI) extending beyond the disk plane of the galaxy. The ejection of gas out of the disk, called a galactic fountain, is thought to be powered by stellar winds from massive stars and supernova explosions. After cooling, this gas most likely falls back into the disk. This process may exist in many galaxies and may be responsible for some of the neutral gas detected at high latitudes within our own galaxy.