Tuesday, February 23, 2016

Stars Evolve But So What

Like most things, if you keep digging deeper into the details of stars and how they change with time, it gets complicated. Lots of interesting things happen inside a star, as well as outside it. Is there anything here which affects the likelihood of alien civilizations arising?

In other posts, we have already mentioned the short lifetime of hot stars, and how, if life takes the same time on a planet around one of these, the star wouldn't last long enough for a civilization to form. There is some possibility that life might originate faster on such a planet than on ours, or perhaps our planet, Earth, was subject to some difficulties which might not occur in other planets. If this is so, we need to rethink the lack of attention given to planets around hot stars. Once a bit more understanding is obtained about the pathway or pathways by which life can originate, this question can be revisited and answered. For now, the best guess is that hot stars don't host alien civilizations.

As you go down the scale from the largest stars, O class, through B and A, you get to the F class stars. They live long enough for life to originate and do some evolution, but it might be too coarse a measure to say they last long enough. These stars evolve, and change their output flux. What this means is that a planet that was at a nice comfortable radius, where life might evolve if things worked out well, wouldn't be at a nice comfortable radius after the star evolved a billion years or so.

Astrophysicists like to expound on the final stages of a star's life, because it is so interesting and variable. During those stages, which amount to only about 10% of the star's life, it can expand by a huge fraction, explode, collapse, change color, and other things. However, for an alien civilization on a planet orbiting such a star, it is game over at this point. That isn't really very important for figuring out any implications of stellar evolution on an alien civilization. The important part is what happens during the first 90% of its life, when life might be originating and trying to evolve into smart aliens with starships.

There is an initial interval, before planets get formed, when things are very interesting as well. Gas is flowing in toward the central core of a giant cloud of gas, the center of the core is getting hotter from all that kinetic energy of the infalling gas, and pressure is increasing from the increased gravity of the ever denser core. Then fusion igntion starts, and the star changes internally. Exactly how depends on the attributes of the cloud that formed it, but again, this isn't during the period when alien civilizations would give a damn about it. Their planet hasn't even coalesced yet. What does matter is the middle 80% of the life.

Fortunately, it is calm, but unfortunately, not absolutely unchanging. Nowadays, the universe and specifically our galaxy is mostly hydrogen, but with a lot of helium as well. Inside a star, hydrogen is the first element that stars to fuse, and it makes helium. This hydrogen burning goes on, but one slight effect is that the hydrogen fuel gets used up in the core. A little less energy is produced, and it gets a little cooler on the planets. The core becomes more and more helium, and the star compresses some more, and this heats things up and hydrogen in a shell outside the core starts to burn. This is sort of like a charcoal fire that you start at the bottom of a pile of fuel. The bottom gets burning and then the fire moves upwards. This makes more fuel available, and the output increases.

For larger stars, like F class, the 'ash' of the hydrogen burning, helium, can start to burn itself, which is something you wouldn't see in your BBQ pit. But the helium can fuse into carbon, and this opens up a new source of energy, so things can get hotter still.

Small stars evolve differently than mid-size stars, which evolve differently than large stars, even in this middle part of their life, as the types of processes that occur in the different layers of the stars can differ as to which one is more dominant. This is all fascinating, but what matters to the alien civilization on some planet around a star is the output energy, and to a lesser extent, the color.

Stars can grow cooler or warmer, depending on their age, and on the various attributes of the star itself and its prior history. Cooler is not good and warmer might be okay.

If what we guess is the way life originates is accurate, it originates not based on solar heating, but in an environment that is fairly insulated from any stellar effects. A deep sea vent is about the most remote place, other than underground, that you can get from the effects of stellar variation. The temperature there is controlled by the heat from the vent or volcano, and there is a large gradient of temperature. If something is trying to get started as life and it needs to be at 80 degrees C, and the sea vent is at 120 and the ocean is at 20, there is going to be somewhere in between that is 80. If the sun heats up and the ocean temperature goes up to 25, there is still going to be somewhere in the rocks around the vent where things are 80. If the sun cools down and large ice sheets cover the ocean, the ocean water temperature might go down to 0. There is still somewhere on the vent periphery where it is 80. So stellar variation doesn't have much effect on the origination of life as we now envision it.

It takes evolution a really long time to get to chemotrophs, or so we think, and they can happily live around the different sea vents and volcanic areas on the planet, fairly independently of what the star is doing. To make the big switch over to solar power, when chlorophyll is evolved, there has to be a benign surface climate. Somewhere on the surface there has to be open water. This allows free-floating photosynthetic organisms to exist and further evolve.

If the stellar output diminishes at this point, and the open water disappears, this branch of life is going to go extinct. As long as the chemotrophs keep their line of business going however, it can come back for a second try if things warm up again. Warming may be by the star growing warmer or by volcanic activity affecting the atmosphere and generating a greenhouse effect. Color change of the star's output can also have an effect, as the effective albedo of the planet might change, or the transmissibility of the atmosphere could be slightly different with the changed color spectrum.

If the planet was mostly frozen over, with some portion of the tropic area not frozen, and photosynthetic life evolved, and then the sun grew warmer, this would be an open invitation for further evolution of the photosynthetic life, as well as the organisms that would certainly evolve to eat them. A little more warming and some crawling out onto the land surface might happen, and after that, the sky is the limit.

Let's summarize. If some astronomer sees a planet which is in the liquid water zone, LWZ, or even better, in the temperate water zone, TWZ, where water is likely to be between 0 and 33 C, he might think this is a good bet for life, at first glance. If the star has been growing colder, this means the planet was formerly warmer, and possibly not a very good chance for life to have evolved during the last few billion years, on a too hot planet. On the other hand, if the star has been growing warmer, the scenario discussed above, with some virtually independent regions under the ocean evolving life, and then the environment becoming just great for more evolution, he probably has a good bet that life is on his new planet. Do we know how stars evolve? Yes, fairly well, although it is only recently that this has been worked out, and details are still emerging. Then we can use this information to further refine our expectations on all the potential alien solo worlds.

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