Throughout the cosmos, there are many variable stars. Terraforming their hosted planets will be a higher challenge then terraforming planets that orbit regular stars.
This article is not a classification of variable stars, but a quick handguide for future terraformers.
Binary and multiple stars Edit
In many cases, when a binary system orbits in the same plane as the Solar System, an observer from Earth will see that, when a star passes in front of the other, the amount of light diminishes significantly. However, if the observer is placed outside from the rotation plane, it will not see the same thing.
Around a binary star, a planet would orbit around star A, around star B or around both stars at a higher distance. As distances between the planet and both stars change, so will the amount of energy received. This will cause a variation in climate patterns, not because one star would be variable, but because distances are always changing. However, all climate changes will be predictable.
Regular variable stars Edit
Many stars have a regular variability in energy output. In majority of cases, they are pulsating stars. Their radius is increasing and then decreasing, following a certain pattern. When they contract, their energy output can rise from a moderate 10% to over 1000%, becoming hotter and more blue. When they expand, by opposite, they become redder, cooler and darker. For each star, the pulsation takes the same amount of time, varying from less then an hour to over an Earth year.
If the pulsations are fast, lasting for a few hours, then the planets will have a more stable climate. The atmosphere will absorb excess of heat. Settlers will not have any problems at all.
If the star is pulsating at every a few hours, then there will be some temperature changes between each day, depending on which part of the day an increase of light occurs. There will be hotter and colder days. When the stellar surface is hotter, the amount of UV and X radiation is higher, so setters will have to take this into consideration when walking on the planet surface.
Long term variable star Edit
A long-term variable star will certainly influence the host planet. Adding an increase of heat can have the power to transform a winter into a long spring, but also can make a summer very hot. By opposite, a decrease of luminosity can make a summer look like an autumn, but will make a winter much colder. It is almost impossible that stellar seasons will occur at the same time with planetary seasons, induced by tilted axis or elliptical orbits. If in one year, the star made a summer colder, the next year it might be different.
On a planet orbiting a long-term variable star, the climate will be in a permanent change. Floods and droughts will not occur at the same rate and at the same time in an year. It is possible that in an year, the snow will not completely melt in summer, while in another year, water will not freeze in winter. However, if we have extreme heat in one hemisphere, in the other hemisphere, climate might be just good.
All human activities will be affected. Farmers will have good years and very bad years. Probably it will not be feasible to grow crops in a single place. Based on weather predictions, farmers will plant their crops each year in a different region of the planet.
Cities will also be affected. Each city will need to have heating and cooling facilities. Or, people will relocate to a different town each season. Both adaptation and relocation are expensive solutions, but are the only possible ways to deal with the extreme climate. Probably some people will prefer to move and others will prefer to stay.
Transportation will be challenging and vital. As shown above, farmers will have to change the place for crops each season. This might include moving from one hemisphere to the other one. Each farm will require machinery, seeds, fertilizers and other staff. Then, crops need to be transported to factories and food to be sent to settlements. On Earth, this kind of transportation is usually done on a local scale and usually from certain producers to certain countries. On a terraformed planet orbiting a variable star, this kind of transport will change direction every season and the volume of freight transported will be much larger. It is possible that a whole hemisphere will be unable to produce any food for an year. Roads and railways will be affected by floods, snowfall and strong winds. Waterways will also be unsafe. A tropical river can freeze in a cold season, while in a hot season, extreme hurricanes can form. Also, air transport can be difficult in these conditions.
Changing climate seasons will affect the hydrology of the planet. It is possible to see large rivers running dry, while in other seasons, massive floods will feed large rivers that will produce erosion to a level that it will cover cities and farmland with layers of mud. This will affect every manmade structure, unless massive work is done to prevent the damage.
Irregular variable stars Edit
Some stars don't change their luminosity after a certain pattern. A terraformed planet will be exposed to unpredictable changes.
Some large giant stars are semi-irregular or irregular. It is not clearly known why this happens, but what is known is that their energy output variation is about 2.5 times. Giant stars are usually dying and they use the last resources of energy to keep them alive. They also have strong solar winds.
Flare stars Edit
The M - type stars (also known as red dwarfs) are small and red. Many of them are flare stars. Their energy output can increase up to 100 times in a few seconds, then drop back to normal or even lower. Some of them have very rare flares (Barnard's Star is a good example). Others, have frequent flares. Proxima Centauri has new flares about every two hours. The size of a flare is comparable with the Solar explosions. The main difference is that since M - type stars produce less then 10000 times less energy then our sun, a habitable planet must be very close to them. A flare might have catastrophic effects on the surface.
If flares are rare, settlers can live with them. There is a high chance the planet will not be hit by the flare, but still, the massive increase of heat and luminosity will be felt on the entire planet.
When a flare hits the planet, it has devastating effects. It heats temperature above water boiling point on the surface. Grass and leaves can be instantly dried. All humans and animals will have severe burns on the skin. Its ionizing effect on the atmosphere can be seen, as the sky will change color and huge auroras might form. However, this will not last long, only a few seconds or minutes. Those lucky enough to find a shelter or jump into water will be safe.
Settlers might be able to predict when the next flare will occur, by constantly monitoring the star. This will be important to warn the population to take cover and to protect all electronics by shutting them down.
If the flares occur too often, like around the star UV Ceti, then terraforming might not be possible. Flares will erode the atmosphere in time, but for a human lifetime this will not be a problem. The main challenge is that frequent flares will constantly add heat to the planet. Since flares frequency is rather chaotic, there is no way to know exactly when the next flare will occur and how strong it will be. So, we will not know the amount of heat the planet will receive. The planet will risk ending as a Snowball Earth or as a Fireball Earth.
There are many types of variable stars, but, even if the cause of these variations is not the same, the effects of a terraformed planet are similar.