The 30 Doradus region within the Large Magellenic Cloud (LMC) is the nearest extragalactic HII region to earth. Because of its proximity, the favorable inclination of the LMC, and the absence of intervening dust, 30 Doradus is a virtual laboratory of massive star formation. Large enough to be classified as a Giant HII region it is not only a site of recent star formation but is the largest and brightest HII region among all the galaxies in our local group. Its staggering size of 3000 light years and mass of one million suns dwarfs many of the largest HII regions in our Milky Way. It could hold 75 Orion Nebulas!
The spectacular nebula is energized by an extraordinary OB association NGC 2070. Giant HII regions like 30 Doradus have an extraordinary luminosity and are powered by star clusters with an enormous ionization flux requiring the equivalent of 100 to even 1000 O-type giants. Some clusters like NGC 2070 have such an exceptionally high concentration of massive stars they are referred to as super star clusters (SSC). SSC's typically have a cumulative stellar mass of 100,000 to 1,000,000 suns. The central cluster of NGC 2070, and source of almost half the illuminating energy for 30 Doradus, is the remarkable core cluster R136. Once thought to represent a single massive star, R136 is now known to represent a compact core of very massive stars. The ionizing radiation from R136 is so powerful, it distinguishes 30 Doradus (the nebula and core cluster together are known as 30 Doradus) as the most luminous HII region in our local galactic group.
R136 is an exceptional cluster by any standard. Although thousands of massive OB type stars are scattered throughout 30 Doradus, R136 represents the core of the OB association and contains an unusual number of very massive O and B type giants including several powerful Wolf-Rayet stars. Some of the most massive stars (O3 types) have an incredible mass of 120 suns. The brightest star in R136, a Wolf-Rayet type designated Melnick 34, has a mass of 133 suns! R136 is also remarkable for its compact nature. The center of the cluster contains 121 stars concentrated within a diameter of only 15 light years. This extraordinary stellar density is several hundred times the normal concentration of OB stellar associations in the Milky Way. It is possible that R136 is at a stage similar to what Globular Clusters experienced very early in their evolution.
R136 and its surroundings represent the nearest example of a massive starburst region in another galaxy. R136 is not the sole producer of massive stars in 30 Doradus. There appears to be a second wave of triggered star formation occurring in the perimeter of the cloud where the expanding ionization front of the visible nebula contacts the cold gases of the adjacent molecular cloud. Essentially 30 Doradus has undergone a recent two stage starburst where R136 represents the initial massive starburst occurring some 2 million years ago, ultimately triggering the outer starbursts began one million years later, with the expansion of the HII cloud. To make the story even more complicated, older populations of stars some 10 to 20 million years old exist in the cloud suggesting that the star forming history of 30 Doradus is more complex than we realized and strongly suggests multiple generations of starbursts.
The striking honeycombed appearance of the 30 Doradus nebula is without a clear counterpart in our Milky Way. The unique form of the nebula suggests a series of interlocking giant shells surrounding hollow cavities. The inner shells are tightly arranged and are expanding at some 35 kilometers per second while the diameter of the outer shells becomes greater towards the outer periphery of the nebula. The shells are believed to have formed from the collective stellar winds of successive generations of powerful OB stars and their supernovae. It is estimated that the complex has experienced at least 40 supernovae within the last 10,000 years including the most recent one, SN 1987a.