Red dwarf

Red dwarf

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According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

, of the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

, either late K or M spectral type.

They constitute the vast majority of stars and have a mass of less than half that of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 (down to about 0.075 solar masses, which are brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s) and a surface temperature of less than 4,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

.

Stellar models indicate that red dwarfs are fully convective
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 so that the nuclear fuel hydrogen is constantly remixed to be burned to helium. They therefore develop very slowly, having a constant luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 and spectral type for some hundreds of billions of years, until their fuel is depleted. Because of the comparatively short age of the universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

, no red dwarfs of advanced evolutionary stages exist in the current era.

Description and characteristics


Red dwarfs are very low-mass stars with no more than 40% of the
mass of the Sun
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

. Consequently they have relatively low temperatures in their cores and energy is generated at a slow rate through nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 into helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 by the proton-proton (PP) chain mechanism. Thus these stars emit little light, sometimes as little as 1/10,000th that of the Sun. Even the largest red dwarfs (for example HD 179930
HD 179930
HD 179930 is a binary star system located about 50 light-years away from Earth, in the constellation Sagittarius.This system consists of a pair of nearly identical red dwarf stars, separated by 3 AU....

, HIP 12961
HIP 12961
HIP 12961 is a dim red dwarf star located approximately 75 light-years away in the constellation of Eridanus. It is one of the largest and brightest M class red dwarf stars known. In 2009 an extrasolar planet orbiting this faint star has been announced....

 and Lacaille 8760) have only about 10% of the Sun's luminosity.

In general red dwarfs transport energy from the core to the surface by convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

. Convection occurs because of opacity
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

 of the interior, which has a high density compared to the temperature. As a result, energy transfer by radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 is decreased, and instead convection is the main form of energy transport to the surface of the star.

As red dwarfs are fully convective, helium does not accumulate at the core and, compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

. As a result, red dwarfs have estimated lifespans longer than the estimated age of the universe, and stars with less than 0.8 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es 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 lifespan of these stars exceeds the expected 10 billion year lifespan of our Sun by the third or fourth power of the ratio of their masses to the solar mass; thus a red dwarf with 0.1 solar mass 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 generated by this size reduction is converted into heat, which is carried throughout the star by convection.
The fact that red dwarfs and other low-mass stars remain on the main sequence while more massive stars have moved off the main sequence allows the age of star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...

s to be estimated by finding the mass at which the stars turn off the main sequence. This provides a lower, stellar, age limit to the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 and also allows formation timescales to be placed upon the structures within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 galaxy, namely the Galactic halo and Galactic disk
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...

.

One mystery which has not been solved as of 2009 is the absence of red dwarf stars with no metals
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

. (In astronomy, a metal is any element heavier than hydrogen or helium). The Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 model predicts the first generation of stars should have only hydrogen, helium, and trace amounts of lithium. If such stars included red dwarfs, they should still be observable today, but none have yet been identified. The preferred explanation is that without heavy elements only large and not yet observed population III stars can form, and these rapidly burn out, leaving heavy elements which then allow for the formation of red dwarfs. Alternative explanations, such as the idea that zero-metal red dwarfs are dim and could be few in number, are considered much less likely as they seem to conflict with stellar evolution models.

Red dwarfs are the most common star type in the Galaxy, at least in the neighborhood of the Sun. Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...

, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest. However, due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.

Planets


At least six extrasolar planets were discovered orbiting the red dwarf Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

 between 2005-2010. One planet has about the mass of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, or sixteen Earth masses. It orbits just 6 million kilometers (0.04 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

) from its star, and so is estimated to have a surface temperature of 150 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

, despite the dimness of the star. In 2006, an even smaller extrasolar planet (only 5.5 times the mass of Earth) was found orbiting the red dwarf OGLE-2005-BLG-390L
OGLE-2005-BLG-390L
OGLE-2005-BLG-390L is a star thought to be a spectral type M - red dwarf star OGLE-2005-BLG-390L is a star thought to be a spectral type M - red dwarf star OGLE-2005-BLG-390L is a star thought to be a spectral type M - red dwarf star (95% probability, 4% probability it is a white dwarf,...

; it lies 390 million km (2.6 AU) from the star and its surface temperature is −220 °C (56 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

).

In 2007, a new, potentially habitable
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 extrasolar planet, Gliese 581 c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

, was found, orbiting Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

. If the minimum mass estimated by its discoverers (a team led by Stephane Udry
Stéphane Udry
Stéphane Udry is an astronomer at the University of Geneva in Switzerland, whose current work is primarily the search for extra-solar planets. He and his team, in 2007, discovered a possibly terrestrial planet in the habitable zone of the red dwarf star Gliese 581, approximately 20 light years...

), namely 5.36 times that of the Earth, is correct, it is the smallest extrasolar planet revolving around a normal star discovered to date and since then Gliese 581 d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....

 was discovered which is also potentially habitable. (There are smaller planets known around a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

, named PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

.) The discoverers estimate its radius to be 1.5 times that of the Earth.

Gliese 581 c and Gliese 581 d are within the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of Gliese 581, and are the most likely candidates for habitability of any extrasolar planet discovered so far.

An announcement in PhysOrg September 29, 2010 describes the discovery of a remarkable new planet: Gliese 581 g
Gliese 581 g
Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth...

. It has a near-circular orbit in the middle of the star's habitable zone and liquid water could occur in some regions on its surface. If confirmed, this would be the most Earth-like exoplanet yet discovered, and the first strong case for a potentially habitable one. Gliese 581 g has a mass three to four times that of Earth and an orbit of about 37 days. It is probably a rocky planet with plenty of gravity to retain a more massive atmosphere than Earth. However, as to be expected for a planet in close orbit round a red dwarf, it is tidally locked, with one face perpetually in darkness and cold, probably covered in glaciers of frozen atmosphere. The implications for the possibility of life on the planet are complex and uncertain. The greatest point of uncertainty is not whether an environment as stable and varied as the surface of that planet could sustain life, but whether the radiation supplied by a red dwarf could generate life in the first place.

In August 2011, another new habitable exoplanet 3.6 times the mass of the Earth, Gliese 370 b
HD 85512 b
HD 85512 b is an extrasolar planet orbiting the K-type star Gliese 370 approximately 36 light-years away in the constellation of Vela ....

, was detected orbiting Gliese 370.

Habitability



Planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 of red dwarf star systems is subject to some debate. In spite of their great numbers and long lifespans, there are several factors which may make life difficult on planets around a red dwarf star. 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
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

. 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 nude and dry. On the other hand, recent theories propose that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet, or life could survive by migration. Alternatively, a moon in orbit around a gas giant planet may be habitable. It would circumvent the tidal lock problem by becoming tidally locked to its planet. This way there would be a day/night cycle as the moon orbited its primary, and there would be distribution of heat.

In addition, red dwarfs emit most of their radiation as infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light, while on Earth plants use energy mostly in the visible spectrum. Red dwarfs emit almost no ultraviolet light, which would be a problem, should this kind of light be required for life to exist. Variability in stellar energy output may also have negative impacts on development of life. Red dwarfs are often covered by starspots
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

, reducing stellar output by as much as 40% for months at a time. At other times, some red dwarfs, called flare star
Flare star
A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection in the atmospheres of the stars. The brightness increase is...

s, can emit gigantic flares, doubling their brightness in minutes. This variability may also make it difficult for life to develop and persist near a red dwarf star. Gibor Basri
Gibor Basri
Gibor Basri is an American astrophysicist, born in New York on May 3, 1951.The son of Phyllis Basri, a professor of dance and ballet, and Saul Basri, professor of physics at the University of Colorado, Basri grew up in Fort Collins, Colorado, together with his younger brother David...

 of the University of California, Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...

 claims a planet orbiting close to a red dwarf star could keep its atmosphere even if the star flares.

See also



  • Aurelia and Blue Moon
    Aurelia and Blue Moon
    Aurelia and Blue Moon are hypothetical examples of a planet and a moon on which extraterrestrial life could evolve. They are the outcome of a collaboration between television company Blue Wave Productions Ltd. and a group of American and British scientists who were collectively commissioned by...

  • Blue dwarf (red-dwarf stage)
  • Cataclysmic variable star
    Cataclysmic variable star
    Cataclysmic variable stars are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state...

  • Stellar classification, Class M


  • Hertzsprung-Russell diagram
  • Flare star
    Flare star
    A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection in the atmospheres of the stars. The brightness increase is...

  • Nemesis (star)
    Nemesis (star)
    Nemesis is a hypothetical hard-to-detect red dwarf star, white dwarf star or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU , somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to...

  • Red giant
    Red giant
    A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

  • Star count
    Star count
    Star Counts are bookkeeping surveys of stars and the statistical and geometrical methods used to correct the survey data for bias. The surveys are most often made of nearby stars in the Milky Way Galaxy....

    , star survey

  • Stellar evolution
    Stellar evolution
    Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

  • White dwarf
    White dwarf
    A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

  • Yerkes luminosity classification


External links