Atlas Image mosaic of NGC 3603. This nebula is the largest HII (ionized hydrogen) region in the Milky Way Galaxy and is considered a giant HII region, in the same class of objects as 30 Doradus in the Large Magellanic Cloud. NGC 3603 is along the Carina spiral arm of the Galaxy, at a distance of about 6000-7000 pc (20000-23000 light years) from us. The nebula is ionized by an enormous cluster of hot, young, massive stars (seen shining brightly toward the center of the 2MASS image); this bright core of stars resembles the truly spectacular stellar cluster, R136, which is the source of ionization for 30 Dor. Brandl et al. (1999, A&A, 352, L69) recently imaged the central cluster in the near-infrared with ESO's Very Large Telescope; their star-for-star analysis reveals pre-main sequence stars and low-mass main sequence stars, with ages of less than 1 million years, which shows for the first time that sub-solar mass stars do form along with the most massive stars in such giant starbursts.