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Red dwarf

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

          This article is about the type of star known as a red dwarf. For
          the British television series, see Red Dwarf.

   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
   diameter and mass of less than one-third that of the Sun (down to 0.08
   solar masses, which are brown dwarfs) and a surface temperature of less
   than 3,500 K. Red dwarfs fuse hydrogen to helium via the proton-proton
   (PP) chain. Due to the low temperatures in the core, fusion proceeds
   slowly. Thus red dwarfs have an enormous estimated lifespan; from tens
   of billions up to trillions of years depending upon mass. Consequently
   they emit little light, sometimes as little as 1/10,000th that of the
   sun. In general red dwarfs transport energy from the core to the
   surface via convection. As red dwarfs are fully convective, all the
   hydrogen in the star is available for fusion, which further increases
   their lifespan. Red dwarfs never initiate helium fusion via the triple
   alpha process and so cannot evolve beyond the red giant phase. In any
   event, there has not been sufficient time since the Big Bang for red
   dwarfs to evolve off the main sequence.

   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 one to date star clusters 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 2006 is the lack of red
   dwarf stars with no metals (in astronomy a metal is any element other
   than hydrogen and helium). The Big Bang model predicts the first
   generation of stars should have only hydrogen, helium, and lithium. If
   such stars included red dwarfs, they should still be observable today,
   but as yet none have been identified. One explanation is that without
   heavy elements, low mass stars cannot form. Alternatively as they are
   dim and could be few in number, we simply may not have observed them
   yet.

   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, magnitude 11.0), as are twenty of the
   next thirty nearest. However, due to their low luminosity, individual
   red dwarfs cannot easily be observed over the vast intergalactic
   distances that luminous stars can.

   Exoplanets have been discovered orbiting red dwarfs in 2005, one as
   small as the size 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 how dim the star is. .
   In 2006 a planet similar in size to Earth was found orbiting a red
   dwarf; it lies 390 million km (2.6 AU) from the star and its surface
   temperature is -220 °C (56 K).

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