Wednesday, August 5, 2015

How Bad are Supernovae? Part I

One of the perils of the galaxy would be a nearby supernova. There are two very important questions that arise from supernovae that relate to the presence and age of alien civilizations in the Milky Way galaxy. One of the questions relates to the interference of supernovae with the rise of a planet from uncompacted rock to a home for star-traveling aliens. Would supernovae kill off many such civilizations, long before they took the first trip to another star? The other question relates to the interference of supernovae with the continued residence of an alien civilization on a planet, either their home planet or a colony they were already inhabiting. These will be answered in two posts.

One simple way to think about supernovas is to imagine a star exploding over a period of a day. They happen because gravitational energy is released in a collapse event inside the star. The quantity is so huge that the outer shells of the star are blown outward. Furthermore, the intensity of nuclear reactions jumps up leading to radiation being generated and sent outward, mostly neutrinos and gamma rays. As an extreme, 10% or more of the star’s mass might be converted into radiation within that day. It propagates directly outward.

Supernova explosions happen to two kinds of stars. One kind is large stars, roughly 10 to 100 times the mass of the sun. They simply burn up their hydrogen fuel, forming cores of the next heavier elements, which are eventually ignited and continue fusing, making up heavier elements. Unfortunately, there are some nuclear mechanisms that can reduce the pressure that the core exerts against the gravitational force created by its mass, and the core starts to collapse, which heats it up and makes these pressure-reducing nuclear mechanisms work even better. Soon the core is collapsing at a good fraction of the speed of light, leading to the gravitational energy release that powers the supernova’s explosion. These have the largest radius of effect.

The other kind are white dwarfs, which are already the residual cores of burnt-out stars. If something untoward happens to them, such as a binary star companion loses mass to the white dwarf, it also becomes too heavy for the pressure of the core to keep the star from further collapsing, and again, a huge amount of gravitational energy is released. As before, radiation is generated and blasted outward.

What happens to a nearby planet of the habitable variety? Radiation hits the atmosphere, where it creates chemical changes. A planet which has already undergone a shift to oxygen, through photosynthetic plants, is largely shielded from solar radiation by its ozone layer. The supernova radiation creates chemical reactions which eliminate the ozone, and life is now subjected to ultraviolet radiation from the star, which can be lethal to organisms on land or just under the surface of bodies of water. On Earth, one of the major extinction events recorded in the fossil record is attributed to such an event.

A large star supernova might have a lethal radius of 200 or more light years, a white dwarf supernova, 20. If you take the current expected rate of supernovas in the galaxy, 0.03 per year, along with the smaller of these two volumes, and run this rate for a billion years, you cover half or more of the volume of the galaxy. In other words, running it for two billion or three billion years eliminates much of the life on many of the planets which might have generated space tourists. The ozone layer doesn’t restore itself quickly, and the planet’s own star just keeps pelting it with UV. Bad news for evolved life.

Back in the earlier days of the galaxy, before the large spiral arms formed, the gas making up the galaxy was moving in a less organized pattern. If the galaxy formed from gas alone, it was converging. If the galaxy was formed from multiple smaller galaxies colliding, then the original motions of these smaller galaxies would keep the gas churning for some time. What this means is that there would be regions forming with higher gas concentrations, meaning that larger stars would be forming. These large stars are all candidates for supernovas. To be succinct, the chaotic gas movements of the formation of the galaxy and the motions by which it stabilizes into a spiral and bar are likely to lead to more large stars and more supernovas that the rate we see now. To be explicit, it means that sterilization of more planets would be happening in the earlier part of the galaxy’s history. With the rate that is going on now, there is a good chance that any solar system will see a nearby supernova every billion years or so.

The saving grace here is that the galaxy is not uniform now, and there are regions in the suburbs which have less supernova action. We live in a suburb called the Sagittarius Arm. But the suburbs are new developments, and those solar systems which formed significantly earlier than we did found themselves in a time of higher density of supernova explosions. This certainly does not mean that no alien civilization could develop in that window of time up to two billion years before now, and start doing colonization. It does mean that the numbers of such civilizations would be smaller than one would otherwise calculate, due to the cutting back on surface life by supernova radiation of the planet’s atmosphere.

It is interesting to ask about the effect of a nearby supernova on a planet which had not passed the chlorophyll transition, when free oxygen replaces carbon dioxide in the atmosphere. Ozone is simply O3 and is in equilibrium with normal oxygen. No free oxygen, no ozone. No ozone, no UV barrier. So the chlorophyll transition, conducted by underwater plants living on the red and blue photons from the sun, had to be in deep enough water so that the UV photons would be absorbed by the water before reaching them. Nature is just wonderful, and has arranged that UV photons are absorbed much more than the visible photons that chlorophyll loves to get. This way with no ozone, any photon can reach the upper surface of the water, but only the desirable photons in the visible range get through the water to the chlorophyll plants. Then the chlorophyll plants make the oxygen, which transforms into ozone, and now the land surface is shielded from the hard UV photons. When the supernova explodes, radiation comes and still impacts the upper surface of the atmosphere, but there is nothing there to be damaged that would impact the subsurface life.

The impact of supernovas on alien life in the galaxy is that it can destroy land plants, eliminating whole ecologies, and any evolutionary processes going on might come to a halt. Life does not disappear on a planet which is hit by a supernova, but it is set back in the evolutionary timescale and has to redo some steps. This means it is less likely to have category 0 or 1 aliens around in that window of time before Earth developed land lifeforms, but it certainly is possible. It also means that colonies trying to transform a planet might have much more difficulty. But it does not mean that seeding a planet stuck at the chlorophyll Great Filter would not work, assuming the seeding was done with organisms that live in deep water.

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