24 July 2011
My wife grabbed a video of some students having fun at a nearby pool today, and it immediately struck me as a good illustration of what happens in a supernova. You might remember that I have several times HIGHLY recommended the online lectures from Alex Filippenko’s Introduction to Astronomy course at U.C. Berkeley (sadly no longer available online*). I think learned more astronomy from those lectures than the two college courses I had in the subject.
In one lecture on supernovas, Fillippenko describes what happens to a star as it runs out of hydrogen to fuse to helium. He explains how the star contracts, gets hotter, and begins to fuse other elements all the way up the periodic table to iron. The energy resulting from iron fusion is less than the energy needed to do it, so that’s the end of the line (literally and figuratively). Every element in your body, (and every element in the universe) other than hydrogen, helium and a little lithium, was formed in stars this way.
Small stars, (like our sun) turn into smoldering white dwarfs when they reach this point, but big stars do something much more grand, they violently explode! At the end of its life, the stars outer envelope collapses and it falls in and meets the expanding inner core. This causes a huge bounce that’s not something we intuitively expect and results in a supernova explosion! (The supernova explosion produces elements heavier than iron, like uranium etc.)
Fillipenko uses two balls to illustrate how this happens. He drops a large ball that begins to bounce, and then drops the second ball such that it falls on the lower ball as it is bouncing. The effect is stunning, the smaller ball shoots off across the room with unexpected force. (Try it at home if you do not believe me.)
You can see something perhaps similar in the video on the right (click the picture). The laws of motion have surprises in them! I also found a video illustrating the effect with a better explanation here.
* You can buy a whole set of lectures taught by Alex Filippenko on the Teaching Company website. They go on sale frequently for 70% off so wait until then. If you have not bought a teaching company course, you will thank me for the tip. I especially recommend the following profs: Fillipenko, Brier, Greenburg, and the particle physics lectures (quit laughing it’s fantastic-trust me!) by Dr. Stephen Pollock.