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M - type star.jpeg
MB - type star HZ.jpeg

Blue dwarfs are a hypothetical type of star. It is estimated that M - type stars, also known as red dwarfs, at the end of their lifetime, will become for a short period of time blue dwarfs.

Theory

No blue dwarf has been detected yet. Given the extreme lifetime of a red dwarf, which exceeds all estimations for the age of the known Universe, no blue dwarf had time to form. However, in the far future, they are predicted to exist.

Red dwarfs have a very long lifetime, sometimes about trillions of years for two reasons. Firstly, they fuse their hydrogen at a very slow rate. Secondly, they have convective currents, that allow all matter to go to the core, where hydrogen is fused. So, they will fuse virtually all their hydrogen into helium.

At the end of their lives, as they will have less hydrogen, fusion rate should decrease and the core will contract. To prevent the star from collapsing, the core will contract until temperature and pressure increases, so that hydrogen fusion will accelerate. With a higher temperature in the core, heat will radiate towards the outer layers and the star will heat-up, losing heat and forcing the star to fuse hydrogen at a higher rate. The process continues until there is no more hydrogen left in the core. At that point, the star starts to contract and becomes a low-mass White Dwarf, composed of helium.

In case of larger stars, the increased heat will cause the star to expand. However, in case of red dwarfs, given their strong surface gravity, the star will not increase in size. What will happen is that surface layers will become hotter and the star will change color from red to blue, as its surface temperature will change from 3000 K to 20000 K.

As hydrogen gets exhausted, helium is not exposed to high enough pressure and temperature to fuse into carbon. So, the star contracts as it cools down. This process generates further heat, until the entire star becomes a white dwarf with the size of Mercury.

It is not known for how long the blue dwarf stage will last, but estimates are of a few billion years. This is very short compared to the lifetime of a red dwarf, but long enough for a civilization to develop on any planet orbiting them.

Using formulas from Math And Terraforming, we can see that Barnard's Star, a typical M - type star, will have, as a blue dwarf, a maximum Solar Constant about half of our Sun's. As this will happen, its Habitable Zone will change from around 0.05 AU to 0.67 AU, which is around the orbit of Venus. As this will happen, many ice-covered planets will become Oceanic Planets.

Planets

It is known that red dwarfs have planets, but because of their dim light, only planets in a torch orbit are in the habitable zone. As the star becomes a blue dwarf, extra heat will allow outer planets to be heated enough to have liquid water, while planets from the former habitable zone will become a desert or will be affected by a runaway greenhouse effect.

Given the extreme lifetime of a red dwarfs, it is questionable what planets will be in orbit when they become blue dwarfs. Former planets might have long been ejected into interstellar space and other planets might have been captured. Also, former planets might have migrated to other orbits over time.

Compared to other latter stages of hotter stars, which become Red giants, blue dwarfs are the most moderate form of all. They are expected not to blow matter into space and will not explode in a supernova. They will still have flares, UV and X rays.

All theoretical models show planets around blue dwarfs, placed in the habitable zone, are potentially habitable. They might be one of the last resorts for late civilizations in the Universe.

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