Wednesday, July 22, 2015

Aliens from Hot Stars

Let’s talk about star types to set the context for this discussion. Stars are categorized in many ways, but the two which make the most difference to aliens living around one are the mass and the spectral type. Mass is self-explanatory, but the spectral type is determined by the composition of the star. Most stars which are not already burned down to white dwarfs or some other end state are on the main sequence, meaning stars with about the average spectral content. Note that the elemental distribution in a star, where elements are being transmuted on a daily basis, is not like the elemental distribution on a planet. Here on Earth, there is just a very tiny amount of gas escaping and only a very tiny amount of mass accretion via meteors, so the elemental distribution of the planet is fixed in time. Some uranium is decaying, so there is intrinsic change, but it’s not much as uranium is rare and has a long lifetime. Stars are different and change their composition as they burn.

The bottom line is that stars pretty much burn according to an understood process, and the mass and spectral type are correlated closely, closely enough so that spectral type is indicative of mass. And mass is indicative of lifetime. So we can say, ball-park-wise, an O star’s lifetime is in the millions of years, a B star’s is tens of millions of years, an A star’s is around a hundred million years, and an F star’s is about a billion or two or three. Down here where we are, on a G star, lifetimes are around ten billion years and on a K star, something like thirty billion. The longest lived are red dwarfs, M’s, coming it at hundreds of billions or more.

There is a continuous distribution, and the discrete nomenclature is just an indicator. So there are stars, near the boundary of F and G stars, with four billion years, or six billion. The distribution is smooth, with there being much fewer at the upper end and much more at the lower end. Brightness is also strongly correlated with spectral class, with the upper ones much more visible. Most of the stars we see in the night sky are at the upper end of the spectrum, O’s and B’s and A’s.

Nobody lives there. These bright stars have such short lifetimes that a planet in the habitable zone would not have the necessary time to evolve intelligent life. The time it takes rubble around a newly formed star to condense might be about three hundred million years, and then there is some time for the tectonics to stabilize, another hundred million or two. Stellar death has already happened before the planet is stable. F stars are the highest in the spectral classification of stars to have planets. There is no point to looking for them higher up. It is true that everything happens in a distribution, so somewhere there might be a planet or two that formed quite quickly around a hotter star, and so it would be fun to find one, but besides the accolades, it would not be of much use for alien hunting. Maybe a rogue planet could be captured, even more fun to detect and examine, but it wouldn’t have aliens.

If the Earth is a good guide, it takes four billion years or so to form intelligent life, assuming we are it. This means that F stars may have planets, but before they get through the evolutionary cycle, the star burns up and it’s game over. Again, there is a distribution, so maybe around an F star that’s just barely an F, with a lifetime of four billion years, there is a planet which just barely develops life and intelligent life to boot, all just in time to watch the sun turn into a red giant and envelop them.

Stars don’t just turn on like electric lights and stay at a constant temperature. They evolve, and higher stars evolve much faster. This means the habitable zone moves around. Planets have their angular momentum and just keep orbiting the star at about the same radius, while the sun evolves and moves the habitable zone. What was habitable is no longer habitable and what wasn’t habitable might have become so. Either way, that is no place to find an alien world. G stars are the ticket to fame and fortune.

K stars certainly have enough lifetime to generate life, and if they can get past the chlorophyll Great Filter, and maybe other filters, intelligent life. Red dwarfs, probably not. So G stars and K stars are where we look for alien civilizations. Fortunately, there are many of them, over 10% together. The celebrity stars, O’s and B’s and A’s are less than 1% of the total, but their visibility makes up for their numbers, and they are what we navigate by.

The main sequence classification is divided further, using a single digit after the letter to denote the finer divisions, with the 0 meaning hotter. The sun is a G2 star with about 10 billion years of lifetime, of which almost half are gone. This means we have something less than five billion years to figure out alien civilizations, as the tail end of that sequence is where the sun will be evolving the most and messing up our environment.

The upper end of the potential solar systems with alien civilizations is near the F8 or F9 level, where they have just enough time to evolve intelligent life. Aliens on a planet in this situation face unique problems. They will soon realize that their sun is evolving too fast for them to stay on the planet. They can adapt as much as possible, choosing where to live on the planet, but other choices will need to be made. One thing about solar systems may help them. There is no reason to think that hotter stars, being more massive, will not have more mass in their solar system, meaning perhaps more planets. There may be a chance they can migrate to another planet within the same solar system. This will not buy much time on a stellar lifetime scale, but lots on an alien civilization timescale. Their other option for survival is to migrate to another star.

F stars with a lifetime of three or so billion years cannot have been formed when the galaxy was formed, it is too old, thirteen billion years or so. They are formed continuously, many in the densest part of the spiral waves that form the galaxy. This means that if colonization is possible, it will have been done. If there were no category 1 civilizations, just category 2, each existing civilization would be occupying only one planet. If there is a Great Filter operating that keeps all habitable planets from having life already, up to intelligent life given enough time, there will be a place for them to go, and they can devote all their talents and resources to getting there. If there was a category 1 civilization that started a billion years before them, it would have finished the galaxy off, and the F8 guys would have no vacant planets to go to. There are other options, becoming a nomadic civilization for example, or being brutal and taking over some other planet with life already there, perhaps battling some other civilization, likely a category 1, for it. War is expensive and resources are short, so this is a last resort. Overcoming the defenses of a colonized planet with only the resources than can be transported on interstellar ships is quite a formidable problem.

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