Red Dwarfs. The longest lived Stars.
Red dwarfs are one of the smallest stars in the main sequence that occur in the Universe. In general, they are slightly smaller than the Sun, but in space they have also been found in sizes which are comparable to those achieved by the largest planets of the solar system.
The artist's conception shows a hypothetical planet with two moons orbiting in the habitable zone of a red dwarf star.
In the Milky Way, about 80% of stars are red dwarfs, and their surface temperatures are lower than 4000 K. Due to the low brightness, none of these stars is visible to the naked eye in the Earth's sky, and because of the slow pace of hydrogen synthesis into helium they glow for a long time and evolving slowly, the estimated life time of red dwarves reaches about 10 billion years.
Red Dwarfs are one of the tiniest stars and reach only about from 7 to 50 % of total mass of our Sun and they are also very dim. Their radiation index reaching even the nearest planets is small. In addition, in the case of this type of stars there is a high risk of synchronous rotation, the globe circling the red dwarf would be constantly returned to its star with only one side, just like in the case of the Earth - Moon system. The phenomenon of synchronous rotation appears in tight double systems, as well as in the case of bodies connected with each other by gravitation, between which distances are small. Chances that the planetary system around red the dwarfs will resemble tightly arranged circles, are really big.
These stars have a much weaker gravity, which is why the globes formed on their orbits are often closer to their central stars, than in the case of Mercury, which is about 58 million kilometers away from the Sun. As it is not difficult to guess, in such conditions the width of the enabling zone also significantly decreases development of life. In the case of the Sun, it extends between the orbits of Venus and Mars and is actually slightly less than the distance of 120 million kilometers that divides both planets. In the case of red dwarfs, the zone enabling the existence of conditions for the development of life is at least halved.
As if that was not enough, these small, hidden in the darkness stars, are not the friendlier ones. The instability of red dwarfs is due to their high activity. The flares of red dwarfs are observed much more often than in the case of the Sun, because of their irregularity, they are very difficult to predict. You never know when these inconspicuous, pale dots will increase your brightness many times.
What is certain, however, is that the red dwarfs will remain the last shining stars in the Universe. And it is their longevity and ubiquity that make them objects a little more friendly to planets orbiting their orbits.
Like all stars, red dwarfs fuse hydrogen into helium, but while more massive stars accumulate all the fused helium in their cores, red dwarfs stay convective, meaning that the helum and hydgrogen constantly mix. So they use up their fuel incredibly slowly, before they are extinguished. According to current knowledge, none of these stars has gotten old enough to completely expire. Not only that, none of them has yet reached the stage allowing to exit the main sequence. Red dwarfs are the stars in the prime of life, even those that were born in the early stages of the Universe's existence. These inconspicuous trinkets are the oldest existing in outer space, and the globes in their orbits may be even more than twice as large as the Sun.
Billions of years is a sufficient amount of time for the development of life and its evolution from the simplest forms, for highly advanced, intelligent creatures. That is why astronomers are keen on watching a close neighborhood of red dwarfs.
Proxima Centauri is a Red Dwarf which is the closest one to our Sun. It is located about 4.24 light-years (40 billion km) from Earth, in the Centaur constellation. It was discovered in 1915 by Robert Innes, but its glow is too weak to be seen by the naked eye.
It is the unpredictable temperament of these small stars that may prove to be an obstacle in their search for life on the planets circling around them. A strong stellar wind, greatly intensified in the course of strong flares, in a few hours reaches the closest situated globes and certainly determines the environmental conditions prevailing on their surface. Astronomers assume that strong shock waves can effectively disrupt the formation of a thick atmosphere on the planets that makes life possible. Fortunately, this fatal scenario concerns exoplanets located very close to the red dwarfs. The lying globes may have a bit more luck in this respect. Especially that during the evolution the untamed activity of red dwarfs expires and becomes more life-friendly. Longevity of red dwarfs provides their planets with billions of years of evolution. Those of the globes, which are located in the zone of conditions conducive to the development of life, such a long period of time gives many a chance to develop on their surface various forms of existence.
References:
[1]http://www.ptma.szczecin.pl/prelekcje/2014_04_03_Ewolucja_gwiazd_Tadeusz_Smela.pdf
[2]http://www.astro.uni.wroc.pl/ludzie/steslicki/popularyzacja/ewolucja_Orle_2010.pdf
[3]http://www.if.pw.edu.pl/~wosinska/wyk8_gwiazdy.pdf
[4]https://pl.wikipedia.org/wiki/Czerwony_karze%C5%82
