Science for Thought

Red Dwarfs

Red Dwarfs

No, this is not about something from the Ring Cycle. This is about the most common star in the universe.

Three quarters of the stars in our galaxy are red dwarfs, and they last for not billions of years but rather, trillions of years. They will outlast everything else in the universe.

Red dwarf stars are not very large and are of low mass compared to our Sun. They have masses from 5% to 60% of the mass of our Sun. Their luminosities vary from 0.0015 to 7.5 %. Since they have surface temperatures below 4000 K, they appear to be glowing red. In many cases we could stare at them without any damage to our eyes.

The reason why a red dwarf star lasts so long is because they fuse hydrogen very slowly and the helium that's produce doesn't accumulate in their cores. A red dwarf star is completely convective so the helium that's created is distributed throughout the star. This allows a red dwarf to burn most of its hydrogen before it goes off the main sequence. This is important because it means that a red dwarf star remains constant on the main sequence for a very long time.

However, the early life of a red dwarf is not very constant. They tend to flare a lot, and sometimes the flares can be very powerful, much more powerful than the flares from our Sun. Flares from red dwarf stars can outshine any other star in our galaxy. Eventually, these red dwarf stars settle down and quit throwing tantrums like a naughty child.

Most red dwarf stars have lots of exo-planets orbiting around them, and most of these planets are rocky instead of gas giants. This makes them prime candidates for the formation of life.

However, there are some difficulties with the idea of life forming on planets orbiting in the habitable zone of red dwarfs. One is the fact that the habitable zone is very close to the star, which means that a planet would be tidally locked to its star. That would make life complicated on a planet with one side always facing the star and the other side in perpetual darkness.

Another problem is that most of the electromagnetic radiation from a red dwarf is in the infrared, not the visible and ultraviolet part of the spectrum. Plant life that forms under these conditions would be different than our plants. They would be dark, possibly black, in order to absorb any light for photosynthesis. Aquatic life would find it difficult because the light would not penetrate very far into water.
For some reason, sunspots on a red dwarf are prone to cover the star up to 40% and these dark spots can last for many months. This could require life to hibernate and oceans could freeze over.

Since red dwarfs produce lots of flares, a planet with life would require a very strong magnetic field, one that could deflect very powerful coronal mass ejections that could bow the atmosphere of a planet all the way to the ground.

The best chance for a planet to be habitable around a red dwarf would be if it formed beyond the habitable zone during the 1.6 to 2 billion years that the dwarf star is unstable and flare prone and then drifted back into a closer area of habitability. Life that existed underwater would also have a better chance of surviving if this happened.

The good news is that these red dwarf stars last a very long time, allowing life to develop before they go off the main sequence. The other good news is that Earth-sized planets seem to form around these red dwarf stars, which makes it more likely that life could form on them, and since the stars last so long, a planet orbiting them could develop intelligent life, possibly even a technological society. I find that idea very fascinating.

Thanks for reading.