An artist's conception of a red dwarf star. Red dwarfs constitute the majority of all stars

According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0. The Sun (Sol is the Star at the center of the Solar System. 075 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic

Description and characteristics

An artist's impression of a planet in orbit around a red dwarf

Red dwarfs are very low mass stars with no more than 40% of the mass of the Sun. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. ^[1] Consequently they have relatively low temperatures in their cores and energy is generated at a slow rate through nuclear fusion of hydrogen into helium via the proton-proton (PP) chain mechanism. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The proton-proton chain reaction is one of several fusion reactions by which Stars convert Hydrogen to Helium, the primary alternative being the Thus these stars emit little light, sometimes as little as 1/10,000th that of the Sun. But even the largest red dwarf has only about 10% of the Sun's luminosity. The solar luminosity, L_\odot is a unit of Luminosity ( power emitted in the form of Photons conventionally used by Astronomers to ^[2]

In general red dwarfs transport energy from the core to the surface by convection. Convection in the most general terms refers to the movement of molecules within Fluids (i Convection occurs because of opacity of the interior, which has a relatively high density compared to the temperature. Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation especially visible Light. As a result, it is more difficult for photons to travel toward the surface by radiative processes. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Convection takes over energy transport because it is a more efficient process. ^[3]

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. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Thus red dwarfs have an enormous estimated lifespan; from tens of billions up to trillions of years depending upon mass. These lifespans are longer than the estimated age of the universe. The lower the mass of a red dwarf, the longer the lifespan. ^[1] 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. ^[4]

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 clusters to be estimated by finding the mass at which the stars turn off the main sequence. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A subdwarf star, sometimes denoted by "sd" is Luminosity class VI under the Yerkes spectral classification system The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Subgiant star is a class of Stars that are slightly brighter than normal Main sequence (dwarf stars of the same spectral class but not as bright as A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. The luminosity class II in the Yerkes spectral classification is given to bright giants. Supergiants are among the most massive Stars In the Hertzsprung-Russell diagram they occupy the top region of the diagram A hypergiant ( luminosity class 0) is a Star with a tremendous Mass and Luminosity, showing signs of a very high rate of mass loss In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard Star clusters are groups of Stars which are gravitationally bound This provides a lower, stellar, age limit to the Universe and also allows formation timescales to be placed upon the structures within the Milky Way galaxy, namely the Galactic halo and Galactic disk. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the The galactic coordinate system is a Celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy

One mystery which has not been solved as of 2007 is the absence of red dwarf stars with no metals. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen (In astronomy, a metal is any element heavier than hydrogen or helium). The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and trace amounts of lithium. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. 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. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen Alternative explanations, such as 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, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11. Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 05), as are twenty of the next thirty nearest. This list of stars nearest to the Earth is ordered by increasing distance out to a maximum of 5 Parsecs (16 However, due to their low luminosity, individual red dwarfs cannot easily be observed over the vast interstellar distances that luminous stars can; in fact, none are visible to the naked eye. ^[5]

Planets

Extrasolar planets were discovered orbiting the red dwarf Gliese 581 in 2005, about the mass of Neptune, or seventeen earth masses. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 Gliese 581 (ˈgliːzə is a Red dwarf Star with spectral type M3V located 20 Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. It orbits just 6 million kilometers (0. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand 04 AU) from its star, and so is estimated to have a surface temperature of 150 °C, despite the dimness of the star. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The Celsius Temperature scale was previously known as the centigrade scale. 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; it lies 390 million km (2. 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, 6 AU) from the star and its surface temperature is −220 °C (56 K). The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic

In 2007, a new, potentially habitable extrasolar planet, Gliese 581 c, was found, orbiting Gliese 581. Planetary habitability is the measure of a Planet 's or a Natural satellite 's potential to develop and sustain Life. Gliese 581 c (ˈgliːzə is a " Super-earth " Gliese 581 (ˈgliːzə is a Red dwarf Star with spectral type M3V located 20 If the mass estimated by its discoverers (a team led by Stephane Udry), namely 5. Stéphane Udry (born 1961 in Sion Switzerland) is an astronomer at the University of Geneva in Switzerland, whose current work is primarily 03 times that of the Earth, is correct, it is the smallest extrasolar planet revolving around a normal star discovered to date. (There are smaller planets known around a neutron star, named PSR B1257+12. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a Pulsar located 980 Light-years from the Sun. ) The discoverers estimate its radius to be 1. 5 times that of the Earth. This planet is within the habitable zone of Gliese 581, and is the most likely candidate for habitability of any extrasolar planet discovered so far. In Astronomy a habitable zone ( HZ) is a region of space where conditions are favorable for Life as it may be found on Earth. ^[6]

Habitability

Main article: Habitability of red dwarf systems‎

Planetary habitability of red dwarf star systems is subject to some debate. Determining the habitability of red dwarf systems could help reveal how likely Extraterrestrial life is to exist as Red dwarfs make up a majority of all the stars Planetary habitability is the measure of a Planet 's or a Natural satellite 's potential to develop and sustain Life. 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. A separate article treats the phenomenon of Tidal resonance in Oceanography. 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 life (as we know it) to evolve. On the other hand, recent theories propose that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet.

Another potential problem is that red dwarfs emit most of their radiation as infrared light, while on Earth plants use energy mostly in the visible spectrum. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of But, perhaps the most serious problem may be stellar variability. Red dwarfs are often covered in starspots, reducing stellar output by as much as 40% for months at a time. A sunspot is a region on the Sun 's surface ( Photosphere) that is marked by intense magnetic activity which inhibits Convection, forming At other times, some red dwarfs, called flare stars, can emit gigantic flares, doubling their brightness in minutes. A flare star is a Variable star which can undergo unpredictable dramatic increases in brightness for a few minutes This variability may also make it difficult for life as we know it to survive near a red dwarf star. Gibor Basri of the University of California, Berkeley claims a planet orbiting close to a red dwarf star could keep its atmosphere even if the star flares. The University of California ( UC) is a Public university system in the state of California. [1]

See also

• Aurelia and Blue Moon
• Brown dwarf
• Cataclysmic variable star
• Hertzsprung-Russell diagram
• Flare star
• Nemesis (star)
• Red giant
• Stellar evolution
• White dwarf
• Yerkes luminosity classification

References

• A. Burrows, W. B. Hubbard, D. Saumon, J. I. Lunine (1993). "An expanded set of brown dwarf and very low mass star models". Astrophysical Journal 406 (1): 158-171. doi:10.1086/172427. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• "VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars", European Southern Observatory, November 19, 2002. Retrieved on 2007-01-12. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 475 - Basiliscus becomes Byzantine Emperor, with a coronation ceremony in the Hebdomon palace in Constantinople  
• Neptune-Size Planet Orbiting Common Star Hints at Many More

External links

• Variable stars
• Stellar Flares - D. Montes, UCM.
• Red Dwarfs
• Red Star Rising : Small, cool stars may be hot spots for life - Scientific American (November 2005)