A Star Explodes in Slow Motion

Posted by jns on 27 April 2005

I’ve thoroughly enjoyed reading this book by Peter Atkins (reference below), and I found his slow-motion description of the process that leads to the creation of a supernova uncommonly gripping and dramatic, as well as enlightening.

Stars bigger than about eight Suns have a violent future. The temperature in these giants can rise so much, to around 3 billion degrees, that “silicon burning” takes place, in which helium nuclei can merge with nuclei close to silicon and gradually build heavier elements, stepping through the periodic table and finally forming iron and nickel. These two elements have the most stable nuclei of all, and no futher nuclear fusion releases energy. At this stage, the star has an onion-like structure with the heaviest elements forming an iron core and the lighter elements in successive shells around it. The duration of each of these episodes depends critically on the mass of the star. For a star twenty times as massive as the Sun, the hydrogen-burning epoch lasts 10 million years, helium burning in the deep core then takes over and lasts a million years. Then fuels get burned seriously fast in the core. There, carbon burning is complete in 300 years, oxygen is gone in 200 days, and the silicon-burning phase that leads to iron is over in a weekend.

The temperature is now so high in the core, about 8 billion degrees, that the photons of radiation are sufficiently energetic and numerous that they can blast iron nuclei apart into protons and neutrons, so undoing the work of nucleosynthesis that has taken billions of years to achieve. This step removes energy from the core, which suddenly cools. The outer parts of the core are in free fall and their speed of collapse can reach nearly 70 thousand kilometres a second. Within a second, a volume the size of the Earth collapses to the size of London. That fantastically rapid collapse is too fast for the outer regions of the star to follow, so briefly the star is a hollow shell with the outer regions suspended high over the tiny collapsed core.

The collapsing inner core shrinks, then bounces out and sends a shockwave of neutrinos through the outer part of the core that is following it down. That shock heats the outer part of the core and loses energy by producing more shattering of the heavy nuclei that is passes through. Provided the outer core is not too thick, within 20 milliseconds of its beginning, the shock escapes to the outer parts of the star hanging in a great arch above the core, and drives the stellar material before it like a great spherical tsunami. As it reaches the surface the star shines with the brilliance of a billion Suns, outshining its galaxy as a Type II supernova, and stellar material is blasted off into space.

[Galileo's Finger: The Ten Great Ideas of Science, Peter Atkins (Oxford University Press, Oxford, 2003) pp. 256--257.]