For a classification of stars see The H-R Diagram.
Red Giants are big stars that have left main sequence. They change fast and will, at some point, go supernova or will decay into White dwarfs. However, because human lifetime is so short compared to a star, they can be a good destination for settlers. They have a certain Habitable Zone.
Supergiant Stars are aged stars with enough mass to support nuclear fusion of heavier elements then helium and carbon and will eventually go supernova. A special page, dedicated to them, are Red Giants Approaching Supernova.
The Stars[]
Giant stars are very large, but their mass is small, comparable with our sun. In fact, the Sun is expected to become a red giant.
Lifetime[]
Depending on mass, when a star starts fusing helium, various things can happen.
- K - type stars are able to support helium fusion. However, when this happens, all helium in their core is highly compressed, like inside White dwarfs. These stars will fuse in a few seconds at least half of their helium. The released energy will not make them explode. Instead, this will make them heat and expand, becoming very large but red. They will remain as this for a while, then, as they will cool down, they will decrease in size. Later, they will decay into white dwarfs.
- G - type stars, like our Sun, will face a similar fate. The main difference is that once they cool, the remaining hydrogen and helium surrounding the core can be fused. Shell helium fusion occurs in flashes, at roughly 10 thousand years one after the other. Each flash expels part of the outer layers into cosmos, creating what is called a planetary nebula. It is possible that the most massive K - type stars can produce a few flashes.
- F - type stars seem to have a similar fate. The main difference is that core helium ignition heats the star, sending it on what is called the blue loop. During this phase, the star's outer layers heat and become blue-white. Then, shell helium flashes occur more often and last more.
- A - type stars show a bit different scenario. Their cores might not be so compressed and helium fusion might not occur in a powerful burst. Their blue loop is less detectable and their helium flashes are in a larger number.
- Some B - type stars face a different scenario. The blue loop is almost invisible for them. They don't fuse helium in an explosive manner. As they become giants, they are often considered intermediate size. Their mass is sufficient to fuse carbon, but not heavier elements. After core helium fusion, the core, composed mainly of carbon, is compressed. Carbon ignites in a few seconds and is fused into sodium, neon and magnesium. The released heat, combined with shell helium flashes, expel the outer layers into a planetary nebula, leaving a white dwarf behind.
Environment[]
Red giants have powerful solar winds. They lose matter in a very fast way. Such a strong wind is like a scourge to the poor nearby planets, so strong that it can blow a significant part of a gas giant's atmosphere away. However, at least in the initial stages of expansion, while the star is in the yellow or orange giant phase, a planet may still hold onto its atmosphere.
Not all red giants have the same luminosity, depending on what point they are on their lifetime. For the first part of their lives, changes are not fast, compared to a human lifetime. However, during flashes and while passing the blue loop, changes are short enough to be sensed in a human lifetime.
Hosted planet[]
There is a research about star Canopus and a possible hosted planet (since Frank Herbert's Dune is supposed to be there). For the moment, Canopus is an orange giant, so it is still safe. Other models are available from ISDB:
- Planet around Canopus: comfort distance = 310 AU, orbital period = 263 years
- Planet around Betelgeuse: comfort distance = 67 AU, orbital period = 123 years
Planets are confirmed to exist [1] around red giants.
How would look a world with a red giant? Well, not in the last phases, when it is losing mass, creating so violent solar winds. Let's imagine Solar System with a huge red sun. The Earth will be heated to 1000 K, low chances to find life. Mars will also be overheated and will lose all gasses. The asteroids will do so. Many of them contain water, so they will most likely become comets. So will do the satellites of Jupiter. The four large moons will have liquid water, so that they would be a good place for humans (?) still wondering around. The other smaller ones will become comets, spinning around Jupiter, who will look completely different. Its ammonia clouds will be replaced by water.
During helium flashes, things will change. In pauses between flashes, ice will melt on the moons of Saturn. However, when flashes will occur, ice will melt even on the moons of Neptune.
Extended habitable zone includes all planets that could be terraformed with some use of technology. By adding greenhouse gasses, as long as plants have enough light, new planets can be suitable for life. However, red giants, like any red star (including M - type stars, generate most of their light in infrared, while the visible light is mostly in red. Plants can survive with a light of 0.1% Earth intensity, in both red and blue spectra. Orange giants have more blue, but still red is the dominant color, while yellow giants have the same kind of light our sun has. In the case of Solar System, plants can survive 32 times further away from comfort zone (Earth), so they can live as far as the orbit of Neptune, if they have correct temperatures. If solar light were more red, the outer distance would be the orbit of Uranus or even Saturn. Also, remember that sub-giants and stars that are just leaving main sequence have a light thousands of times dimmer then a supergiant. The following list shows some stars and their habitable zones:
- Pollux (orange subgiant)
- Comfort zone: 5.6 AU
- Outer limit for Earth-like plants: 120 AU
- Aged Sun (red giant)
- Comfort zone: 10 AU
- Outer limit for Earth-like plants: 175 AU
Because distance is so big, orbiting period is long. A planet in comfort zone around Pollux will have an year of 8 Earth years. This means that, if that planet is tilted, seasons will last two years each. Will plants survive such long winters? Will humans wait so long or will prefer to migrate? It will be very hard for many species to adapt. This is similar to what we will find on a planet orbiting A - type stars.
About the aged Sun? Since the year will be so long, a season will take a lifetime to complete. No plant will survive such a long time. Ice will gradually cover one hemisphere, while the other will experience a long summer. Some people will live all their life in moderate climate (spring or autumn). Still, seasons are shorter then what we see on planets orbiting B - type stars. Ice will not have time to grow so thick, so the oceans will not decrease dramatically. In summer, endorheic lakes will not dry completely, so the climate will not be so dangerously affected. Equatorial land would be less affected (while in a planet orbiting a B - type star, it will be affected, because a lot of ocean water is moving into polar ice.
Given the huge extended habitable zone, many planets can be terraformed. For Solar System, for the moment when Jovian moons will have liquid water, the outer border of habitable zone will extend to 150 AU. This means that all large Kuiper Belt Objects could be terraformed and could sustain Earth-like plants. At 150 AU, we are in the interstellar environment. As for large giants like Canopus, outer range is 8000 AU or 0.13 LY. That might be considered out of a stellar system's border. So, a free-floating planet that came too close to a planet (but is moving too fast to become a satellite) can be terraformed.