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30 Doradus (also known as the Tarantula Nebula and as NGC 2070) in the Large Magellanic Cloud is the closest and most active starburst region in the nearby universe. At the center of 30 Doradus is the spectacular R136 cluster, whose massive stars shine brightly in the far ultraviolet, ionizing the surrounding gas to produce the visible 30 Doradus nebula. 30 Doradus is luminous in X-rays as well, indicating the existence of gas shock-heated to millions degrees Kelvin by supernova remnants. The R136 cluster, at a tender young age of 2-3 Myr, still has all of its massive stars intact. Who are responsible for the generation of X-rays in 30 Doradus?

To find the responsible party, we need not look too far from R136. Projected at 140 light years from R136 is the cluster Hodge 301. Hodge 301 and R136 show sharp contrast between each other both in stellar content and interstellar environment. Hodge 301 is a sparsely populated cluster of which the most luminous stars are early B supergiants, while R136 is a highly concentrated dense cluster which contains a large number of the earliest O stars. Hodge 301 is bathed in an X-ray halo and surrounded by delicate filaments in visible light, as opposed to the rustic ionized clouds and wild elephant trunks around R136.

To study the cluster Hodge 301 and to show the contrast between Hodge 301 and R136, You-Hua Chu (University of Illinois), Eva Grebel (University of Washington) and collaborators have obtained HST WFPC2 images of Hodge 301 with exactly the same filters and camera orientation as those of R136. They find that the contrast between Hodge 301 and R136 can be explained entirely by the age difference between the two clusters. Hodge 301 is about 20 Myr old. Its most massive stars have ended their lives in supernova explosions. The supernova ejecta have shocked the ambient gas and compressed it into thin sheets and filaments. Hidden amidst these filaments are numerous supernova remnants, including one expanding at 300 km/s, the fastest in 30 Doradus!

30 Doradus reveals that a starburst is a complex, multi-generation process. The Generation-X cluster R136 contain the most luminous stars, but does not generate the bulk X-ray emission from 30 Doradus. The older, baby-boomer cluster Hodge 301 has a thinning population, but it still holds the shocking power!

Rancho Del Sol Observatory Image of the Tarantula Nebula

Image courtesy of K. Crawford (Camino, CA)