See the The H-R Diagram for a classification of stars.
Supergiant stars are aged massive stars that have exhausted hydrogen in their cores and are fusing helium or heavier elements. A good example is Betelgeuse.
Classification[]
On The H-R Diagram, these stars share two distinct luminosity classes: I and II. The only difference between these classes is their luminosity. So, they can be analyzed on a common page.
They share different colors: red, orange, yellow, white or blue. They share common spectral characteristics with main sequence M-Type Stars, K-Type Stars, G-Type Stars, B-Type Stars and O-Type Stars. However, they are far more massive, larger and more luminous.
Some members belong to more rare groups, like C - type stars (carbon stars) and S - type stars (stars with an equal concentration of carbon and oxygen), which are rather similar to M - type stars based on their color and energy output.
Lifetime[]
An aged star spends less then 10%, sometimes less then 1% of its total life in supergiant state. Given the fact that massive stars don't live much (less then 10 million years usually), they will last less then a million years as so.
While the supergiant stage is short on a cosmic scale, for a human civilization it can be long enough. Some supergiant stars are stable and can support a terraformed planet for long enough. Other stars suffer significant transformations as they age, which render terraforming not feasible.
It appears that supergiant stars are not safe when their surface temperature is at certain values (e.g. when they are yellow or white in color). Because of this, they will often make left-right excursions on The H-R Diagram. As they accumulate much heat in their mantle, they will shift fast from red and orange (K spectral class) to blue and white (B spectral class). As their outer envelopes cool down, they shift back to orange and red.
Stars with a mass less then 25 times that of the Sun will remain as red supergiants. The best example is Betelgeuse.
Stars with a mass higher then 30 Solar masses, will change from red to blue-white close to the end of their lives and will explode in this stage.
Higher stars, weighting around 70 Solar masses, will shift many times between red and blue, in alternating cycles.
Even higher stars, with a mass over 100 Solar masses, will remain blue all their lives. As they heat-up, they become Super-Supergiant Stars and then end-up as Wolf-Rayet Stars, expelling huge amounts of matter into space until they end-up in a violent supernova.
Environment[]
Supergiant stars are characterized by a dense Stellar Wind. However, in case of red and orange supergiants, the winds are moving slow. By opposite, in case of blue and white supergiants, winds achieve high velocities. This is very important for a terraformed planet, because a strong magnetosphere can deflect a slow-moving wind.
Another interesting phenomena is that red supergiants are variable. Their light output vary, usually after a cycle. Their luminosity output shows short-term variations (less then an year), medium-term variations (a few years) and long-term variations (hundreds of years). This has a significant effect on a terraformed planet. Blue supergiants are also variable, but on a smaller scale.
Habitable Planet[]
It is known that around supergiant stars planet exist. A planet in the habitable zone is, most likely, one that was in the Kuiper Belt when the star was on the main sequence. Because this, there is a high chance that the planet will be mostly made of ices. As they melt, it will become an Oceanic Planet.
Other planets that orbit closer, might once had been in the Habitable Zone. As the extreme heat and powerful solar winds affect them, they might look somehow similar to Venus or (when they lost their atmospheres) like Mercury. There is also a chance that, if they lose their atmospheres fast, they will look like giant comets.
Still, it is possible that other planets can orbit far enough to have their water frozen.
Feasibility of Terraforming[]
compared to a star, a human life is much shorter. Therefore, the life of a human civilization does not exceed many thousands of years. Terrraforming should be considered feasible if the planet will be habitable after 2000 years.
There are a few key problems we need to answer:
- The first question one needs to answer is how long will the star last. This subject is analyzed at Red Supergiants Approaching Supernova.
- Then, we must avoid stars that often make left-right excursions on The H-R Diagram. Such stars might change significantly within a human lifetime.
- The third problem is if a planet can hold an atmosphere for long enough and if stellar radiation is not too dangerous. Because of this, blue supergiants should be avoided. They have strong solar winds and powerful UV and X radiations.
Climate[]
A habitable planet will be very far away from the star and will require thousands of years to complete an orbit. Because of this, after terraforming, the target planet will have an Equivalent Earth Climate. There will be different average temperatures in different Geographic locations, but there will not be seasons detectable for a human lifetime. Seasons will still exist, but they can be noticed only after thousands of years.
If the planet has a tilted axis, polar days and polar nights will last for thousands of Earth years. Ice will accumulate at the poles during polar night and will melt during polar day. This will impact climate and ocean level.
The sky of such a planet will be completely different from what we see on Earth. The strong stellar winds will cause endless auroras that will be visible even around the equator, all night long and sometimes also during day. The star will also be different. Supergiants are not spherical and have brighter and darker regions. Around them, one can see massive jets of plasma escaping into space. Some of them might have a halo of gas surrounding.
Most habitable zone planets are presumed to be former Kuiper Belt objects. At their distances, they are expected to have large Hill spheres, which means there is a high chance they host satellites. The moons, like most objects in the Kuiper Belt, contain high amounts of water ice and frozen ices. Having a too small gravity to hold an atmosphere, most moons will behave like giant comets. Around a terraformed planet, the comet-like moons will be clearly visible during day and during night. Their comas might create a halo around a planet or might be sometimes absorbed by the planet's own gravity.
Supergiants are rare in the Universe. The closest one to us is Betelgeuse, which is 600 light years away. Still, they offer surprising conditions and sometimes their planets can be habitable.