A few months ago this story caught my attention on the BBC news website. It is an image of part of the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way galaxy, and shows both star birth and star death.
The circular looking “shell” of gas towards the left of the image is a supernova remnant, part of the gases which have been blown off by an exploding massive star. To the right of the image is an area called the Dragon Nebula, which is an area of current star formation.
The LMC is about 170,000 light years from our Milky Way, visible to the naked eye from the Southern Hemisphere. It is a small irregular galaxy which is actually in orbit about our much larger Milky Way, and is so named because the first European to note it was Ferdinand Magellan, who was also captain of the first expedition to circumnavigate the Earth. It is called the Large Magellanic Cloud because near to it (and also visible to the naked eye) is a smaller irregular galaxy, which is called the Small Magellanic Cloud (astronomers are nothing if not inventive in their naming 😉 ).
In 1987 a massive star exploded (a supernova) in the LMC, SN1987A, which was the first naked-eye supernova visible since Kepler’s supernova in 1604. However, it is not the SN1987A supernova which is shown in this image, the supernova here is older than that, something we can determine from the size of the remnant (the expanding shell of gases which the supernova blows off when it explodes).
It may seem to be quite a coincidence to capture a supernova remnant and a region of star formation in the same image, but in fact it is not so surprising. The shock waves caused by a supernova explosion are thought to often trigger the collapse of giant molecular clouds which lead to new star formation, so stellar death leads to stellar re-birth. Not only that, but the new generation of stars will contain heavier elements which have been created in the massive star and its supernova. Elements up to iron can be created in the massive star itself, but the many elements beyond iron in the periodic table are mostly created in the supernova explosion itself.
The wispy (mainly) red areas in the image are due to the gases fluorescing, just as I have described in previous blogs, for example here and here. The red colour is due to the dominant line-emission process, the h-alpha emission in hydrogen. You may also notice some dark areas (lanes) in the right hand part of the image where the star formation is going on. This is not the absence of stars, but is due to stars being hidden by dust, the same kind of dust I mentioned in my blog on BICEP2 last week. To see through the dust and to see the stars which are currently still forming, we need to look in the infrared and millimetre, where we can see through the dust and see the much cooler “proto-stars”, I will blog about this in the near future.