Unlike most other classes of stars that have clear and unambiguous classification standards, red dwarf stars fall into a wide variety of masses, luminosities, and spectra that have thus far eluded proper classification. In fact, the term “red dwarf” can include most, if not all K-type and earlier stars, with the class usually referring to dwarf stars whose spectral type ranges from K5V to M5V.

Nevertheless, red dwarfs, the most abundant type of star in the Universe, have masses that vary between 7.5 and 50 percent that of the Sun, with those stars less massive referred to as brown dwarfs, and the next more massive variety of stars including yellow dwarfs, like our sun. The reduced size of red dwarfs means that they are fairly dim and burn at relatively low temperatures of less than 6,380 F (3,500 C), compared to the Sun, for instance, which has an effective temperature of 9,900 F (5,500 C). Below are ten more interesting facts about red dwarf stars you may not have known you may not have known.

Red dwarfs will outlive the Universe

All red dwarf stars are fully convective, which means that instead of helium accumulating at the stars’ core, the helium in these stars is continually being mixed throughout the star. As a result, nuclear synthesis by way of the proton-proton chain reaction is slowed down, which in turn means that red dwarfs have exceedingly long life spans. In fact, studies show that low mass red dwarfs will outlive the Sun by at least the third or fourth power of the ratio between their mass and that of the Sun, which means that most red dwarfs will live for at least 10 trillion years, making them the longest lived of all the star types.

The lighter a red dwarf, the longer it lives

As an example of how long red dwarf stars are expected to live, consider the following: A typical 0.16 solar mass red dwarf, which is approximately the mass of Barnard’s Star, will typically spend about 2.5 trillion years on the main sequence, before evolving into a blue dwarf. As the Universe is estimated to be just 13.8 billion years old, blue dwarf stars are therefore based upon theoretical models, and are predicted to live for at least another 5 billion years, during which time it will have about 30% of the Sun’s luminosity, and an effective temperature of between 6,500K and 8,500K.

All red dwarf stars contain metals

All known red dwarfs contain metals, which in astronomical jargon means that they contain elements that are heavier than hydrogen and helium. This is rather strange considering how long these red dwarfs have lived, especially as current theory holds that red dwarfs that may have formed during the first burst of star formation (Population III stars) would be metal-poor, but still alive. Nevertheless, no metal-poor red dwarfs have been found thus far, and a viable explanation for why this is the case still continues to elude investigators.

There are no “standard” red dwarf stars

Despite the fact that as a class, red dwarf stars account for the bulk of the Milky Way’s stellar population, there is no uniform standard to describe the class, as is the case with all other stellar classes. Although there have been many attempts to devise a standard classification model, there appears to be little agreement between the various classification schemes in use today. Nonetheless, there is at least some agreement on the primary spectral classes for red dwarfs. Some star examples are listed here, with their stellar classification in brackets: GJ 270 (M0 V), GJ 229A (M1 V), Lalande 21185 (M2 V), Gliese 581 (M3 V), GJ 402 (M4 V), GJ 51 (M5 V), Wolf 359 (M6 V), Van Biesbroeck 8 (M7 V), VB 10 (M8 V), LHS 2924 (M9 V).

Most stars in the Milky Way are red dwarfs

Based on studies of the stars in the Sun’s neighbourhood, it appears that red dwarf stars may account for up to 75% of the total stellar population of the Milky Way galaxy, even though without optical aid, not a single red dwarf is visible from Earth. Nonetheless, out of sixty of the stars closest to Sun, fifty are red dwarfs, with the closest being Proxima Centauri, a M5-type red dwarf with an apparent magnitude of 11.05.

All red dwarf stars are low-mass, low-luminosity stars

The masses and luminosities of red dwarfs vary greatly, with M9V-type stars weighing in about 7.5% of the Sun’s mass and a luminosity of 0.015% that of the Sun, while stars of the M0V class are the biggest and most massive red dwarfs, even though they weigh in at just 60% of the Sun’s mass, and only shine about 7.2% as brightly.

Red dwarfs are used to calculate the age of star clusters

Since red dwarfs are so long-lived, and remain on the main sequence for as long as they do, their masses can be used to estimate the mass at which more massive stars evolve off of the main sequence, which point is used to estimate the age of star clusters of all types, as well as determining the lower limit on the age of other structures such as the Galactic disc and Galactic halo.

About 40% of red dwarf stars host “super-earth” planets

Detailed investigations and studies have shown that up to 40% of red dwarfs are hosting “super-earth” planets, and particularly in the habitable zones around the stars, where liquid water is expected to exist. Moreover, computer models have revealed that at least 90% of the “super-earth” planets that orbit red dwarf stars contain at least 10% water (by mass) which suggests that planets of this class around red dwarfs are covered by deep liquid water oceans.

Super-earth planets orbiting red dwarfs are likely not habitable

Unfortunately, earth-type planets around red dwarfs are almost certainly tidally locked to they stars they orbit, given their close proximity to their host stars. In some cases, planets are orbiting at distance of only 6 million km, meaning that one side of the planet will be roasted, while the other side is likely to be permanently frozen over. However, new studies suggest that in at least a few cases, the presence of a liquid ocean might drive heat and other climatic features to the cold, dark side of such planets.

One red dwarf has seven Earth-sized planets

While computer modelling suggests that earth-type planets are far more likely to form around red dwarf stars than any other type of planet, one red dwarf, designated TRAPPIST-1, is known to host seven earth-sized planets, with two planets orbiting the star in the habitable zone. The system is located about 39 light years away, in the constellation Aquarius.