On the 4th of July 1054, a bright new star appeared in the sky. Astronomers is China, Japan and across the Arab world took notice and of course it was heralded as an omen of great importance. This new star was a supernova explosion and while the initial glow faded long ago, the aftermath of this explosion is easily seen with a telescope: the crab nebula.
|A BVR+Halpha image of the Crab Nebula|
Supernova explosions occur at the end of the life of a massive star (at least 8 times the mass of our sun). Stars of this size are much hotter at their core and burn through their supply of hydrogen very quickly (in perhaps 10 million years). Once they run out of hydrogen in their core, they begin to fuse helium into carbon and oxygen. When helium runs out, carbon and oxygen is fused into heavier elements and so it continues. This is so that the star can generate the heat necessary to counteract the compressing force of the star's own gravity.
This process continues, producing heavier and heavier elements until eventually the core of the star is made of iron. Only elements smaller than iron can be fused to create energy. Fusing elements heavier than iron actually uses up energy. Fusion then halts within the core of the star. Eventually, the inert iron core of the can no longer support the weight of the outer layers of the star and collapses into a ball of neutrons. It does this at up to 23% of the speed of light. The outer layers of the star collapse down on top of it, rebounding outwards in a huge explosion we call a supernova. In this one explosion, a star can outshine all the 100 billion stars in its host galaxy combined.
The Crab Nebula
The crab nebula is an excellent example of what remains after this explosion. All of the ejecta thrown out from this explosion (as well as prior to it, as the last years of a star can be very violent) forms a cloud around the remnant core of the star. (The remnant in this case is thought to be a neutron star).
Supernova remnants are small and compact so use galaxy cam to zoom right in.
As always with nebulae, there are many filter options. Using BVR to capture a simple colour image works very well for this object. Using the narrowband filters also yields good results. H-alpha gives the clearest image, but since the star that exploded produced a fair amount of oxygen prior to exploding, using OIII also gives you something interesting to look at.