Sunday, August 9, 2015

Galactic Peril – Stellar Encounters

There are a number of perils that might beset an alien civilization. They can be divided into three categories. Any one of them might stop or set back any planet which was trying to develop an alien civilization equipped with starships.

One category is planetary perils, like massive volcanoes, ice ages, the opposite of ice ages on planets with too little land, very strong winds or other atmospheric disturbances like tornadoes and hurricanes, opaque atmospheres, toxic water or air, and so on. We are not even talking about scarcity problems here.

Another category are solar system perils, which include the migration of the habitable zone because the star decides to change its heat output, tidal disruption of orbits of large planets, which in turn move other better-suited-for-life planets in or out of the habitable zone, asteroid impact, satellite orbit tidal decay leading to eventual collision, and probably more. This does not include the presence of useful photons, which is a question for red dwarfs, which constitute 70% of the galactic population. Having your own star turn into a supernova is probably also best put into this category. If the planet is orbiting a binary star, this goes double.

The third category are galactic perils, which include nearby supernovae, bad neighborhoods like near the galactic central black hole, stellar encounters, rogue planet encounters, perhaps something to do with Bok gas clouds, globular clusters passing through your part of the galaxy, and who knows what else. This post is going to chat about stellar encounters.

What might happen if another star passes by your solar system? Gravity is the name of the game here. If the star is about the same size as the sun of your solar system, it doesn’t have to get too close to produce orbital changes. For the sake of argument, let’s assume that if a star passes within 10 times the orbit of Neptune, which is 30 AU, it gradually affects the orbits of the planets in the system, both individually and via interactions with one another. It might be moving slowly, so the interaction builds up over many orbits. It is not hard to figure out the relative differential motion of stars, which have both a galactic orbital speed and a random motion. You can think of it as a gas that is both spinning and with thermal motions. Each atom has both the motion from the spinning and from the heat of the gas.

The random motion of the galactic stars are three dimensional, and don’t take into account other stars. That means we can figure out the time between stellar encounters by simply evaluating how much volume the second star sweeps out, which is a cylinder with a radius of 300 AU and a length of the random speed of the star, which we take to be 10 km/s. Stars in the vicinity of our solar system have a density of 0.03 every cubic light year, and this leads to a stellar encounter rate for any star of about one every 14 billion years. This much time is plenty for the development of stellar travel, except perhaps on planets suffering from scarcity problems. So stellar encounters aren’t much of a galactic peril, are they? They are not if the assumptions we have made about the radius needed are correct and if one other thing holds.

If the density were 10 times as much, the encounter time drops to 1.4 billion years. This is shorter than life on Earth took to develop, and if we are an average place in the galaxy, getting thrown out of the habitable zone after something like 1.4 billion years could end life on Earth, long before it got to chlorophyll. So the point is that as you look at stars closer in to the galactic center, when you get to the point where there are 10 times as many as here, you won’t expect to see much life. Even three times as much density is in the ballpark of having a big effect.

The density of stars climbs as you approach the galactic center. It’s hard to see there, because you are looking through a lot of dust and gas along the way, but estimates are that the density is 100 times our density when you get to about 300 light years from the center. Measuring the density of stars in the direction of the galactic center is a bit touchy, as the galaxy is not just a set of spiral arms revolving around a large central mass, but it has a bulge at the center, with a larger perpendicular component. Just ten times the density might be in at a radius of 5000 light years to 10000 light years, depending on the direction you take leaving the galactic core.

By the way, exactly what do we mean by density around the galaxy? The galaxy is not uniform, and has, besides the bulge and a tenuous halo of stars, spiral waves. Stars get born in clusters where the spiral waves, composed of gas, are located, and then they drift away. This means that stars are located closer than average when they are born, and then they gradually develop random motions by pulling on each other. How long does it take before they spread out? The spread is gradual, meaning that for the first few billion years, the rate of stellar encounters is larger than would be calculated by looking at the current density of stars around Earth’s sun. The spreading of newly formed stars is also affected by the gravity of the gas cloud that led to their formation, which would be pulling them back toward the center of the cloud, increasing the density over what purely random motions would produce. There might also be an increase in the stellar encounter rate because the galaxy is not a sphere, but more like a thin plate, with stars confined to a band perpendicular to the plate, and with their motions preferentially in those directions.

To sum up all this rough estimation, stellar encounters might delay the formation of intelligent life because they are more frequent when a star is new, and might not just delay it, but eliminate it, closer in toward the galactic center. This means that any calculation of how many stars might have life much older than ours is likely to be an overestimate, and how many stars might be habitable long enough for intelligent life is also likely to be an overestimate. Stellar encounters may be one of the principal reasons we have not seen any aliens hanging around Earth.

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