Red dwarf
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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. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K.
Description and characteristics
Red dwarfs are very low mass stars with no more than 40% of the mass of the Sun. 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. 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.
In general red dwarfs transport energy from the core to the surface by convection. Convection occurs because of opacity of the interior, which has a relatively high density compared to the temperature. As a result, it is more difficult for photons to travel toward the surface by radiative processes. Convection takes over energy transport because it is a more efficient process.
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. 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. 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.
("dwarfs")
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. 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.
One mystery which has not been solved as of 2007 is the absence of red dwarf stars with no metals. (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. If such stars included red dwarfs, they should still be observable today, but none have yet been identified. The perferred 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 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 neighbourhood of the Sun. Proxima Centauri, 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 over the vast interstellar distances that luminous stars can.
Planets
Extrasolar planets were discovered orbiting the red dwarf Gliese 581 in 2005, about the mass of Neptune, or seventeen earth masses. It orbits just 6 million kilometers (0.04 AU) from its star, and so is estimated to have a surface temperature of 150 °C, 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; it lies 390 million km (2.6 AU) from the star and its surface temperature is −220 °C (56 K).
In 2007, a new, potentially habitable extrasolar planet, Gliese 581 c, was found, orbiting Gliese 581. If the mass estimated by its discoverers (a team led by Stephane Udry), namely 5.03 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.) 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.
Habitability
Planetary habitability 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. 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. 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. At other times, some red dwarfs, called flare stars, can emit gigantic flares, doubling their brightness in 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.
