Academic journal article Journal of Geoscience Education

Life (Briefly) near a Supernova

Academic journal article Journal of Geoscience Education

Life (Briefly) near a Supernova

Article excerpt

ABSTRACT

Supernova explosions are so enormous that their scale is difficult to imagine. Thought experiments and simple calculations involving the Sun going supernova can help with visualization. The energy flux would be roughly equivalent to having the entire earth's nuclear arsenal detonated a kilometer away, and would be sufficient to boil away the surface at hundreds of meters per second. Even on the temporarily protected night side, scattered light in the atmosphere and light reflected from interplanetary dust would far exceed normal sunlight, and radiation reflected from the moon would heat the earth to lethal temperatures if the moon were near full. The earth would take at most a few days to vaporize. Fortunately, the sun is not massive enough to become a supernova. Supernova explosions occur only in short lived stars, so that the melancholy science fiction theme of a civilization being incinerated by its own sun is very unlikely to happen in reality.

INTRODUCTION

Before the Wisconsin ice advance, before the Sangamon interglacial, while the Illinoisan ice sheets still covered much of North America, an aged star in the Small Magellanic Cloud collapsed and exploded. It took until 1987 for the light to reach the Earth. Across 170,000 light years, this single star was as bright as the stars in the Big Dipper. It was the first supernova visible to the unaided eye since the invention of the telescope (Arnett and Others, 1989).

Supernovas are among the most violent events we witness in the Universe. For a few days, a single star puts as much energy as the rest of its galaxy combined. Most types of supernova occur in binary star systems when a companion star accretes matter from a larger neighbor, suddenly collapses gravitationally and begins a new cycle of fusion reactions (Arnett and others, 1989; Marschall, 1988; Murdin, 1990; Wheeler 2000). The sudden burst of energy blows off the outer layers of the star. Novas, a less violent type of stellar explosion, also always involve binary star systems. In novas, matter accreting onto a white dwarf from a neighboring star becomes dense and hot enough to undergo nuclear fusion on the surface of the white dwarf.

However, one variety of supernova, the type II supernova, occurs when massive single stars reach the end of their life cycles. After the hydrogen in their cores is exhausted, the core collapses until a new cycle of reactions fuses helium to carbon. Later cycles create oxygen, silicon, and finally iron. The core of the star assumes a shell structure with the residue of one fusion process providing the fuel for the next, until the end product, iron, results. It would seem that the shells could grow outward, each supplying fuel for the one inside it, until the star eventually consisted entirely of iron. However, the iron core soon becomes too massive to withstand its own gravity, and fusion of heavier elements absorbs energy, shutting off the radiation pressure that resists collapse. In milliseconds the core collapses to a neutron star (or in extreme cases, a black hole). The rest of the star falls inward onto the neutron star, which may have the mass of the Sun but a radius of only a few kilometers. The gravitational attraction of the neutron star core accelerates the infalling matter to perhaps ten per cent of the speed of light. Up to several times the mass of the Sun plows into a neutron star at a tenth of the speed of light and the results, to put it mildly, are impressive. During the collapse, nuclear reactions run riot and the energy outburst blasts off the outer layers of the star. We look straight into the thermonuclear core of the star.

To get across to students the staggering violence of a supernova explosion, I have developed a few analogies that I present here. The purpose of this paper is to present a few simple analogies that I have found more effective and accessible than the standard textbook comparison that the supernova outshines the rest of its galaxy. …

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