How did this unusually large nebula form? One of the largest nebulas yet detected is actually a complex ring of emission nebulas connected by glowing filaments. The unusual network, known as N11, spans over 1000 light years and is a prominent structure of the Large Magellanic Cloud, the largest satellite galaxy of our Milky Way Galaxy. In the center of the above image is open star cluster LH9, also known as NGC 1760, composed of about 50 bright blue stars that emit radiation that has eroded a whole in their surroundings. A leading hypothesis for the formation of N11 is shells of successive generations of stars being formed further out from the center. The bright region just above center is N11B, an explosive domain where stars are being formed even today. http://antwrp.gsfc.nasa.gov/apod/astropix.html Please Register or Log in to view the hidden image!
I've read that N11 does, indeed, house at least 3 successive generations of star formation. In this case, it will contain a number of recent supernova remnants from massive-short-lived stars of earlier generations; rapidly-speading overlapping wavefronts of ionised gas, laced with radioactive nickel, could form the overall structure we see in your picture. And we've still got several distinct regions of ongoing starbirth, of course - compression associated with supernova shockwaves will encourage this.
