Red dwarf

The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars with hotter "blue" dwarf stars. It became established use, although the definition remained vague. In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5 or "later than K5". Dwarf M star, abbreviated dM, was also used, but sometimes it also included stars of spectral type K. In modern usage, the definition of a red dwarf still varies. When explicitly defined, it typically includes late K- and early to mid-M-class stars, but in many cases it is restricted to M-class stars. In some cases all K stars are included as red dwarfs, and occasionally even earlier stars. The most recent surveys place the coolest true main-sequence stars into spectral types L2 or L3. At the same time, many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain hydrogen-1 fusion. This gives a significant overlap in spectral types for red and brown dwarfs. Objects in that spectral range can be difficult to categorize. ==Description and characteristics==

Red dwarfs are very-low-mass stars. As a result, they have relatively low pressures, a low fusion rate, and hence, a low temperature. The energy generated is the product of nuclear fusion of hydrogen into helium by way of the proton–proton (PP) chain. Hence, these stars emit relatively little light, sometimes as little as that of the Sun, although this would still imply a power output on the order of (10 trillion gigawatts or 10 ZW). Even the largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of the Sun's luminosity. In general, red dwarfs less than transport energy from the core to the surface by convection. Convection occurs because of the opacity of the interior, which has a high density compared with the temperature. As a result, energy transfer by radiation is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, a red dwarf will have a region around its core where convection does not occur. Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared with larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespans of these stars exceed the expected 10-billion-year lifespan of the Sun by the third or fourth power of the ratio of the solar mass to their masses; thus, a red dwarf may continue burning for 10 trillion years. As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract. The gravitational energy released by this size reduction is converted into heat, which is carried throughout the star by convection. According to computer simulations, the minimum mass a red dwarf must have to eventually evolve into a red giant is ; less massive objects, as they age, would increase their surface temperatures and luminosities, becoming blue dwarfs and finally white dwarfs. The boundary between the least massive red dwarfs and the most massive brown dwarfs depends strongly on the metallicity. At solar metallicity the boundary occurs at about , while at zero metallicity the boundary is around . At solar metallicity, the least massive red dwarfs theoretically have temperatures around , while measurements of red dwarfs in the solar neighbourhood suggest the coolest stars have temperatures of about and spectral classes of about L2. Theory predicts that the coolest red dwarfs at zero metallicity would have temperatures of about . The least massive red dwarfs have radii of about , while both more massive red dwarfs and less massive brown dwarfs are larger. ==Subdwarfs==

Some M-class dwarfs are categorized as subdwarfs, rather than red dwarfs, because of their lower luminosity class of VI. They fuse hydrogen in their cores like normal main-sequence stars, but due to their low metallicity, they are less opaque, permitting more radiation, especially ultraviolet, to escape. They are therefore hotter but dimmer, falling about two magnitudes below the main sequence. They are much less common than regular main-sequence stars. Red dwarfs, to contrast, are common main-sequence stars. ==Spectral standard stars==

is a pair of red dwarfs, with GJ 623a on the left and the fainter GJ 623b to the right of center. The spectral standards for M type stars have changed slightly over the years, but settled down somewhat since the early 1990s. Part of this is due to the fact that even the nearest red dwarfs are fairly faint, and their colors do not register well on photographic emulsions used in the early to mid 20th century. The study of mid- to late-M dwarfs has significantly advanced only in the past few decades, primarily due to development of new astrographic and spectroscopic techniques, dispensing with photographic plates and progressing to charged-couple devices (CCDs) and infrared-sensitive arrays. The revised Yerkes Atlas system (Johnson & Morgan, 1953) listed only two M type spectral standard stars: HD 147379 (M0V) and HD 95735/Lalande 21185 (M2V). While HD 147379 was not considered a standard by expert classifiers in later compendia of standards, Lalande 21185 is still a primary standard for M2V. Robert Garrison does not list any "anchor" standards among the red dwarfs, but Lalande 21185 has survived as a M2V standard through many compendia. The review on MK classification by Morgan & Keenan (1973) did not contain red dwarf standards. In the mid-1970s, red dwarf standard stars were published by Keenan & McNeil (1976) and Boeshaar (1976), but there was little agreement among the standards. As later cooler stars were identified through the 1980s, it was clear that an overhaul of the red dwarf standards was needed. Building primarily upon the Boeshaar standards, a group at Steward Observatory (Kirkpatrick, Henry, & McCarthy, 1991) and D. Kirkpatrick has recently reviewed the classification of red dwarfs and standard stars in Gray & Corbally's 2009 monograph. The M dwarf primary spectral standards are: GJ 270 (M0V), GJ 229A (M1V), Lalande 21185 (M2V), Gliese 581 (M3V), Gliese 402 (M4V), GJ 51 (M5V), Wolf 359 (M6V), van Biesbroeck 8 (M7V), VB 10 (M8V), LHS 2924 (M9V). == Planet formation ==

with the James Webb Space Telescope. Gas-rich disks (protoplanetary disks) have been detected around low-mass stars and brown dwarfs with ages as high as around 45 Myrs. This is unusual as more massive stars usually don't show primordial disks beyond 10 Myrs. These old disks have been dubbed Peter Pan disks, with J0808 being the prototype. The long presence of gas in the disk could enable the formation of resonant chains, such as seen in TRAPPIST-1. It is thought that only some will reach this high age and most will dissipate after 5 Myrs. The environment can play a role in the disk lifetime, such as stellar flybys and external photoevaporation, which can result in ionized proplyds. Some edge-on protoplanetary disks around early M-stars are resolved, such as Tau 042021 and HH 30. These show jets and more recently disk winds in NIRCam and NIRSpec observations. The disk wind is an important part in removal of mass from the disk and accretion of material onto the surface of stars. Observations with the Mid-Infrared Instrument has advanced the study of the composition of the inner part of primordial disks around late M-dwarfs. Studies found either hydrocarbon-rich composition (e.g. 2MASS J1605–1933, ISO-ChaI 147, J0446B) or water-rich composition (e.g. Sz 114). The disks show a trend from oxygen-rich in younger disks to carbon-rich in older disks. Silicates are also detected for some disks. This is explained with a model of inwards drifting material. At first water-ice-rich pebbles drift inwards, increasing the amount of oxygen in the inner disk. Then carbon-rich vapour drifts inwards and increases the amount of carbon in the inner disk. This process is more efficient in very low-mass stars because the icy outer part is closer to the inner disk. This trend of carbon-rich disks is also present in brown dwarfs and planetary-mass objects. The brown dwarf 2M1207 has a disk rich in hydrocarbons, This composition could influence the composition of the planets formed within these disks, especially their atmospheres. If close-in planets accrete their atmospheres early, they could have a low C/O ratio (low amounts of carbon, high amounts of oxygen). If they accrete their atmospheres late, their atmospheres could have a high C/O ratio (similar to Titan). The removal of carbon from the solids could also result in carbon-poor composition of the soldis (core/mantle/crust) in rocky planets. After the primordial gas is removed, the system is left with a debris disk. Examples of debris disks around red dwarfs are AU Microscopii, CE Antliae and Fomalhaut C. ==Planets==

, an M-type (spectral class M1Ve) red dwarf star less than 0.7% the age of the Sun. The dark areas represent huge sunspot-like regions. Many red dwarfs are orbited by exoplanets, but large Jupiter-sized planets are comparatively rare. Doppler surveys of a wide variety of stars indicate about 1 in 6 stars with twice the mass of the Sun are orbited by one or more of Jupiter-sized planets, versus 1 in 16 for Sun-like stars and the frequency of close-in giant planets (Jupiter size or larger) orbiting red dwarfs is only 1 in 40. On the other hand, microlensing surveys indicate that long-orbital-period Neptune-mass planets are found around one in three red dwarfs. Observations with HARPS further indicate 40% of red dwarfs have a "super-Earth" class planet orbiting in the habitable zone where liquid water can exist on the surface. Computer simulations of the formation of planets around low-mass stars predict that Earth-sized planets are most abundant, but more than 90% of the simulated planets are at least 10% water by mass, suggesting that many Earth-sized planets orbiting red dwarf stars are covered in deep oceans. At least four and possibly up to six exoplanets were discovered orbiting within the Gliese 581 planetary system between 2005 and 2010. One planet has about the mass of Neptune, or 16 Earth masses (). It orbits just from its star, and is estimated to have a surface temperature of , despite the dimness of its star. In 2006, an even smaller exoplanet (only ) was found orbiting the red dwarf OGLE-2005-BLG-390L; it lies from the star and its surface temperature is . In 2007, a new, potentially habitable exoplanet, , was found, orbiting Gliese 581. The minimum mass estimated by its discoverers (a team led by Stephane Udry) is . The discoverers estimate its radius to be 1.5 times that of Earth (). Since then Gliese 581d, which is also potentially habitable, was discovered. Gliese 581c and d are within the habitable zone of the host star, and are two of the most likely candidates for habitability of any exoplanets discovered so far. Gliese 581g, detected September 2010, has a near-circular orbit in the middle of the star's habitable zone. However, the planet's existence is contested. On 23 February 2017 NASA announced the discovery of seven Earth-sized planets orbiting the red dwarf star TRAPPIST-1 approximately 39 light-years away in the constellation Aquarius. The planets were discovered through the transit method, meaning we have mass and radius information for all of them. TRAPPIST-1e, f, and g appear to be within the habitable zone and may have liquid water on the surface. ==Habitability==

s orbiting in the habitable zone of a red dwarf. Modern evidence suggests that planets in red dwarf systems are extremely unlikely to be habitable. In spite of their great numbers and long lifespans, there are several factors which may make life difficult on planets around a red dwarf. First, planets in the habitable zone of a red dwarf would be so close to the parent star that they would likely be tidally locked. For a nearly circular orbit, this would mean that one side would be in perpetual daylight and the other in eternal night. This could create enormous temperature variations from one side of the planet to the other. Such conditions would appear to make it difficult for forms of life similar to those on Earth to evolve. And it appears there is a great problem with the atmosphere of such tidally locked planets: the perpetual night zone would be cold enough to freeze the main gases of their atmospheres, leaving the daylight zone bare and dry. On the other hand, a theory proposes that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet. Furthermore, even if a red dwarf's characteristics render most of its planet's surface uninhabitable, there is a chance for life to exist around a limited region, such as the planet's terminator. Variability in stellar energy output may also have negative impacts on the development of life. Red dwarfs are often flare stars, which can emit gigantic flares, doubling their brightness in minutes. This variability makes it difficult for life to develop and persist near a red dwarf. While it may be possible for a planet orbiting close to a red dwarf to keep its atmosphere even if the star flares, more-recent research suggests that these stars may be the source of constant high-energy flares and very large magnetic fields, diminishing the possibility of life as we know it. == Notes ==