Thursday, August 6, 2015

How Bad Are Supernovae? Part II

From the previous post on supernovae it was clear that a close supernova squelches evolution on a planet that has surface organisms or ones living shallowly in water. Things living deeper would survive, even ones that used chlorophyll, provided they lived deep enough to allow the star’s UV radiation to be attenuated by the water. Evolution could start again from undersea photosynthesis, just as it had done before. Supernovae delay evolution, but do not stop it.

F and upper level G stars have UV radiation that is objectionable to living cells, providing too much of a punch to maintain them intact. But K stars have much less. All stars produce a full spectrum of radiation, but the distributions are different. It will be worth discussing elsewhere if K stars are more immune to nearby supernovae than are G stars. Red dwarfs, M’s, would almost certainly be, but they have their own issues with providing the energy levels needed for anything more than chemotrophs.

The second question on supernovae asks what happens to an existing intelligent alien civilization when one goes off. Obviously, if a hot star, O or B, explodes a couple of light years away, the shock wave hitting the solar system will affect the alien planet. This has been cited as a cause for migration. It would require some detailed calculations to figure out just how close a home star could be to a supernova of different kinds to avoid destroying the habitability of the planet. Some curve exists relating damage radii to spectral class, and we will not need it. For this post, it is sufficient to know that such a curve exists.

A second curve exists, which shows a larger radius for each spectral class, and the radius would denote civilization damage, not loss of habitability. There could actually be a separate damage radius curve for each type of damage. What might go wrong?

First of all, there is a shock wave propagating out from the supernova, and when it hits the solar system with the aliens’ home planet in it, an electromagnetic pulse might be generated. Magnetic storms on a star, such as ours, can also generate these pulses, and in our solar system, they are carried by the solar wind out and away from the generating sunspot. Electromagnetic pulses are a peril of a solar system, and even we know about them. One was experienced here on Earth in the nineteenth century, and some electrical disruption was noted. There was little electrical equipment in those days, but an alien civilization past asymptotic technology would have lots. However, as noted elsewhere, in order to last for many millennia, even thousands of them, the civilization would have experienced and recovered from all the perils a planet and a star can bestow upon a civilization. They would know how to build equipment that was sufficiently protected from the electromagnetic impulse the supernova’s shock wave would produce, except if they were too close.

We on Earth understand the beginnings of protection of equipment from electromagnetic impulses, and we know it is harder and harder as the pulse gets stronger and stronger. In other words, protection is good to a degree, and then it fails. When it fails, it fails seriously. So, we can expect the alien civilization to be well protected against EMI from a supernova, but only if the supernova is not very close. How close might the supernova be? One of the galactic perils noted previously showed that, since stars in the galaxy have random motion, every once in a rare while, they get close to one another. One bad effect of getting too close is the disturbance of planetary orbits, but here we are noticing another. Getting too close, and not necessarily too close for orbital disturbance, might mean EMI is too strong.

There are other effects, certainly. Gamma radiation is spewed out in copious amounts from the supernova, and there are two sources. One is the nuclear reactions that go crazy during the supernova explosion, but these are low energy nuclear gammas, and are readily shielded. Other gammas are made in the explosion and shock wave, and these can be of higher energy and therefore more penetrating. Thus, besides the EMI damage radius for each star type, there is a gamma damage radius. Staying inside is good against gammas, and almost all aliens will be living in large enclosed cities. Perhaps they just go to the basement to avoid personal injury.

To abstract what has been hinted upon, there is some distance that the supernova can be without damage to the civilization and the planet as well, and some distance at which a migration is necessary. Either the planet’s habitability or some aspect of the civilization will be terminated. Will there be enough warning time?

For the white dwarf supernovas, a monitoring would have to be done of all the sufficiently close white dwarfs to see if there is infalling matter that would lead to a supernova explosion. The telescopic ability of the alien civilization would certainly be capable of monitoring all the white dwarfs to the outer damage radius of a white dwarf supernova. They are extremely numerous in the galaxy, but they do not change rapidly with time, so monitoring should be a piece of cake.

For large, hot star supernovas, we can almost do that now, based on a good determination of spectral lines, which tell us what elements exist in the star, and a good mass determination. There are some caveats. Large stars are like the earth, having different layers, which do not mix very much. Knowing what the layering is inside a hot star would mean more extensive measurements, and a good theoretical knowledge of the nuclear reactions inside. We almost have that now, and certainly aliens would have it early in their approach to asymptotic astrophysics. So the answer is yes, they would be able to generate a window of time during which the hot star would turn into a supernova.

They also would have the ability to predict the motions of stars in the galaxy, so they could predict if a certain hot star was approaching them and how close would it come. We do this with asteroids and the principle is the same, except that there are gas clouds in the galaxy to be concerned with, but that is also a solvable problem. So, they can predict the location of the stars near them, and they can predict when one might blow up. Is there an OOPS?

The answer is no and not if they try hard. Stars in the galaxy have random motions of the order of 10 km/s, meaning it takes about 30,000 years to move a light year. Consider that a moving star, which was also priming itself to become a supernova, might have to move 10 light years to get close enough to render the planet uninhabitable. This gives the alien civilization 300,000 years to emigrate, which seems, even considering all the problems of interstellar emigration, to be enough time. Now comes the slippery part. It would be a real faux pas to have the alien scientists call for an emigration to another planet, and the aliens do this, only to find out that the potential supernova went off later than expected, after it had already passed out past the damage radius from their old star. Recall that there are limits to omniscience created by unobservable things, such as the composition of the inner core of an A class star. If the time to explode is a sensitive variable to something unobservable, in other words, if the scientists, no matter how much they look at the surface of that A star, they can’t detect what is in the core, and, what’s in the core makes a lot of difference in how long the A star lasts, then there might be a false alarm.

We don’t have a good picture of how they might attack this problem, perhaps with an orbiting observatory around the A star, gathering information to finer precision, looking at the sunspots where material might migrate upward, taking samples of the solar wind, or whatever. Certainly, with an unneeded emigration at stake, there would be no shortage of funds for a fast probe to the approaching A star. So, the civilization would probably try very hard, and solve this problem, and there would be no OOPS. Maybe.

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