NGC 3603 (left) and NGC 3576 (right) are two of the most luminous HII regions in our galaxy but their juxtaposition is really an illusion. The objects are physically unrelated, NGC 3603 being twice as far as NGC 3576. Although they appear side by side NGC 3603 is located in the carina arm of our galaxy while NGC 3576 resides in the Sagittarius arm. There are some similarities as both are undergoing a high rate of sequential star formation.
NGC 3603, a Giant HII region is an extraordinary object. It is likely the most massive visible HII region in our galaxy extending at least 1000 light years across and containing the overall mass of 10,000 suns. At its center is the extremely compact and bright cluster core, HD 97950 which contains at least 50 massive O type stars including several Wolf-Rayet types. Both NGC 3603 and its cluster core have been compared to the extragalactic Giant HII region 30 Doradus in the LMC and its massive cluster core R136. 30 Doradus represents the largest known HII region in the local group of galaxies and exceeds NGC 3603 in stars by a factor of 3 and in gas by a factor of 50 times. That said, the crowded cluster core HD97950 shows an equivalent stellar density to R136 making it the most compact and densest concentration of massive stars in our galaxy. HD97950 produces an extraordinary ionizing flux about 100 times greater than the trapezium cluster in the Orion Nebula. Aside from the bright O and B type giants, many low mass stars have been detected within HD97950 proving that sub-solar mass stars do form in violent starbursts.
Notable within HD97950 is the presence of three immensely hot and massive Wolf-Rayet stars (WR) within the center of the cluster. Wolf-Rayet stars are potentially important age indicators for Giant HII regions as the Wolf-Rayet phase occurs at the end of the short lifetime of very massive stars. The energy of their stellar wind is far greater than the winds of O type giants and dominates the energy output of starburst regions for several million years. The three WR stars designated A1, B, and C are located only a tenth of a light year apart in the center of the cluster. Each possesses over 100 solar masses and together they contribute an amazing 20% of the ionizing energy plus 60% of the kinetic energy driving the entire Giant HII region NGC 3603. The three stars formed some 2 million years ago, which probably defines the age of the cluster HD97950.
NGC 3576 contains several imbedded clusters as star propagation has progressed sequentially east to west during the last several million years. The bright emission component extends some 100 light years across and contains scattered small dark nebulae known as Bok Globules. Bok Globules are dense well defined concentrations of gas and dust named after the Dutch astronomer Bart Bok (1906-1983) who first defined them. Typically they possess 10 to 50 solar masses within a space of a single light year. Most commonly found within HII regions they are known to be cocoons of star formation, particularly the formation of double or multiple star systems.