Why are small red dwarf stars considered by scientists to be the cradle of life?

Stars are the main body of visible matter in the universe, accounting for 99% of the visible matter in the universe. Why is it visible matter? Because modern scientific research believes that the real dominant force in the universe is invisible matter, namely dark energy and dark matter. These two substances account for about 95% of the mass and energy of the entire universe.

Today we will not talk about these invisible and intangible things, but only talk about the stars that are active in our field of vision.

The life of a star can be roughly divided into three stages, namely the birth period, the mature and stable period, and the decay period. The birth period and the decay period account for a very small proportion of the total life of a star, and the mature and stable period accounts for more than 90% of the star's life cycle. The stars during this period are called the main sequence stage, and the main active stars are red dwarfs, orange dwarfs, yellow dwarfs, blue dwarfs, etc.

Some people believe that there are brown dwarfs, but these stars are called failed stars because they are too small and the core pressure and temperature cannot ignite hydrogen nuclear fusion. I don’t think they should be counted among the stars.

In this way, the stars that really emit light, heat and radiate into space mainly refer to red dwarfs, orange dwarfs, yellow dwarfs, blue dwarfs, etc. The classification of these star types is mainly based on the mass and spectrum of the stars. Generally speaking, the larger the mass of the star, the brighter it is. Therefore, the spectrum type of the star is closely related to the mass of the star.

Astronomers divide the stellar spectrum types into seven categories, namely: O, B, A, F, G, K, M. Of course, each spectrum type has a range, so scientists divide each of these major categories into 10 sub-types, labeled with Arabic numerals 0 to 9, such as A1 or G2, etc.

This forms the relationship between stellar mass and spectrum with multiples of the solar mass, arranged from small to large: stars with M-type spectra refer to the smallest red dwarfs, whose mass spans between 0.08 and 0.5 times that of the sun, with a surface temperature of about 2000~3500K, red in color, and about 4% of the sun's brightness; stars with K-type spectra are orange dwarfs, with a mass between 0.5 and 0.8 times that of the sun, a surface temperature of about 3500~5000K, orange in color, and about 40% of the sun's brightness.

Above that is the spectrum type to which our sun belongs, i.e. a yellow dwarf star with a spectrum of type G, with a mass between 0.8 and 1.7 times that of the sun, a surface temperature of about 5000-6000K, a pale yellowish white color, and a brightness between 0.8 and 6 times that of the sun. Our sun belongs to the spectrum of type G2.

Next up are blue dwarfs, which are stars with a larger mass than yellow dwarfs. They are divided into four spectral levels, namely F, A, B, and O. These stars are medium-to-large-mass stars, ranging from several times the mass of the sun to dozens or even 200 times, with surface temperatures ranging from 7500K to 60000K, and colors ranging from white to bluish white and even blue, with brightness ranging from tens of thousands to 1.4 million times that of the sun.

It can be seen that the more massive a star is, the higher its temperature and the brighter its brightness. However, the life span of a star is exactly the opposite of its mass: the more massive a star is, the shorter its life span is.

As a result, the life spans of stars vary widely, with the shortest lasting only millions of years and the longest lasting trillions of years. This is because the greater the mass of a star, the higher the core pressure and temperature, the more intense and wilder the hydrogen nuclear fusion reaction, and the faster the fuel is consumed. Once the core fuel of a star is exhausted, it reaches the final stage of evolution and decay.

The larger the mass of a star, the more violent its death. Generally, stars with a mass more than 0.5 times that of the sun may trigger helium fusion at the end of their lives, and eventually react to carbon, which will cause the star to expand and become a red giant. After the outer gas dissipates into space, a small white dwarf will be left in the core.

For stars with a mass greater than 8 times that of the sun, thermonuclear runaway will occur at the end of their evolution, triggering a supernova explosion, and eventually either being blown to pieces and scattered in space, or leaving behind a neutron star. For stars with a mass greater than 30 to 40 times that of the sun, a black hole will be left at the core after a supernova explosion.

Today we will not elaborate on the heroic death of stars, but mainly talk about why scientists are so optimistic about red dwarfs and believe that red dwarfs are the cradle of life, or the final destination of life.

In fact, to put it simply, there are three reasons.

First, red dwarfs have the longest lifespan.

Stars are the basic condition for life to obtain energy. It can be said that without stars, there would be no life, let alone the emergence of wisdom and civilization. Life and civilization need time to be nurtured. Just like the sun and the earth are 4.6 billion years old, although life has also experienced billions of years of development and evolution, wisdom and civilization are still very immature. The scope of human activities is still mainly on the earth that gave birth to us, and wandering around the earth. Flying out of the solar system is even more of a dream.

Our sun is a yellow dwarf star with a lifespan of about 10 billion years. It is now middle-aged, at 4.6 billion years old, and will die in more than 5 billion years. At that time, the sun will become a red giant, with its diameter expanding to about 200 times its current size, and its edge will be right near the Earth's orbit. The Earth will most likely be vaporized and swallowed by the sun's fiery flames.

At this time, the entire solar system has been in tatters, and life has long ceased to exist. Research suggests that the sun can only support life for 1 billion years at most from now on. After 1 billion years, the sun's brightness will increase by 10%, and the earth will be devastated and uninhabitable.

No one knows whether humans can develop into an interstellar civilization and escape the solar system to survive in other star systems or interstellar space. But one thing is very important, that is, the gestation of life and the development of civilization require a stable space environment, so the life span and stability of stars are necessary conditions for the development of life and civilization.

Red dwarfs are the smallest stars and have the longest lifespans according to the laws of stellar lifespans. According to mass and spectrum, the largest red dwarf is capped at 0.5 times the mass of the sun and can live up to 50 billion years; smaller red dwarfs have lifespans of more than 100 billion years; and even smaller red dwarfs can live up to trillions or even tens of trillions of years.

The extremely long lifespan of red dwarfs provides a long-term opportunity for the birth and development of life and intelligence. When humans can no longer stay in the solar system in the future, they will inevitably seek other interstellar places to settle down. The most likely option is to find a red dwarf and rely on the stable energy there to continue to reproduce.

The closest star to us is called Proxima Centauri, which is a red dwarf. Proxima Centauri has a mass of about one-eighth of the Sun and a lifespan of more than 100 billion years. Scientific exploration has found that this weak star, 4.22 light-years away from us, has three planets, two of which are in the habitable zone and have the conditions for the existence of life.

Second, red dwarfs are the most common stars in the universe.

There are about 400 billion stars in our Milky Way, of which yellow dwarfs like our sun account for about 10%, the total number of stars with a mass larger than the sun is only about 3%, orange dwarfs with a smaller mass than the sun account for about 12%, and the remaining 75% are red dwarfs.

Observations have found that among the 10 groups of stars closest to the sun, 9 are red dwarfs; among the 50 stars orbiting the sun relatively close, more than 80% are red dwarfs.

Further scientific investigations have found that within 1,500 light-years centered on the Sun, the number of stars of different spectral types is approximately: B-type stars account for 1%, A-type stars account for 1.5%, G-type stars account for 13%, K-type stars account for 20%, and M-type stars account for 56%. Based on the relationship between spectrum and mass, red dwarfs account for the vast majority, which is consistent with the above estimate.

There are so many red dwarf stars in the universe, and their lifespans are extremely long. It seems unreasonable that life and civilization cannot be nurtured and developed in such places.

Third, red dwarfs have long periods of stability.

The gestation of life requires a relatively comfortable environment, but early scientific research believed that red dwarfs are not suitable for the gestation and reproduction of life, and of course are not suitable for the gestation and development of civilization.

The reason is that red dwarfs have low mass and low temperature. If a planet that can nurture life wants to be in the habitable zone of such a low-mass star, it must be very close to the star. The so-called habitable zone is the heat radiated by the star to the planet, which is just suitable for the existence of liquid water, that is, the temperature of the planet must be in a suitable range around 0 degrees.

For example, the closest star to us, Proxima Centauri, has a planet in the habitable zone called Proxima b, which is only about 7 million kilometers away from the main star, only one twenty-first of the distance from the Earth to the Sun. Due to this distance, two phenomena will inevitably occur. First, the planet's orbit speed must be very fast so as not to be pulled into the embrace of the star's gravity; second, it will be tidally locked by the star's gravity, that is, one side will always face the star.

This is the case with Proxima b. It has an orbital period of only 11.2 days and is tidally locked to Proxima Centauri. One side is baked by the star, while the other side is always facing away from the star, immersed in darkness and cold.

Another reason is that in the early stages of their life cycle, red dwarfs usually appear as flare stars, which means their surfaces are extremely unstable and they often emit flares of enormous energy. Such huge energy outbursts occur only once every ten to twenty years on the sun, but occur every few weeks on red dwarfs.

In this way, this huge energy radiation attacks planets at a very close distance, making it impossible for life and civilization to be nurtured.

But after decades of follow-up research, scientists have made new discoveries and understandings: Even if a red dwarf planet is tidally locked, as long as the planet has an atmosphere and the flow of the atmosphere can transfer heat, even the eternally dark side will be warm; and after a red dwarf passes through its early wild stage, it has plenty of time in a stable mature period, which is much longer than that of a solar-like star and is long enough for the gestation of life and the development of civilization.

Therefore, in comparison, the current scientific community, especially astronomers and cosmologists, generally believe that the most suitable cradle for the birth and development of life and civilization in the universe, or the final destination of cosmic civilization, is red dwarf stars. What do you think about this? Welcome to discuss.

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