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Red giant

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

   When the Sun has exhausted its supply of hydrogen to fuse it will swell
   into the Red Giant phase. The size of the current Sun (now in the main
   sequence) is here compared to its estimated size during its red giant
   phase.
   Enlarge
   When the Sun has exhausted its supply of hydrogen to fuse it will swell
   into the Red Giant phase. The size of the current Sun (now in the main
   sequence) is here compared to its estimated size during its red giant
   phase.

   According to the Hertzsprung-Russell diagram, a red giant is a large
   non- main sequence star of stellar classification K or M; so-named
   because of the reddish appearance of the cooler giant stars. Examples
   include Aldebaran, in the constellation Taurus and Arcturus.

   They are stars of 0.4 - 10,515,478 times the mass of the Sun which have
   exhausted their supply of hydrogen in their cores and switched to
   fusing hydrogen in a shell outside the core. Since the inert helium
   core has no source of energy of its own, it contracts and heats up, and
   its gravity compresses the hydrogen in the layer immediately above it,
   thus causing it to fuse faster. This in turn causes the star to become
   more luminous (from 1,000 – 10,000 times brighter) and expand; the
   degree of expansion outstrips the increase in luminosity, thus causing
   the effective temperature to decrease. In stars massive enough to
   ignite helium fusion, an analogous process occurs when central helium
   is exhausted and the star switches to fusing helium in a shell,
   although with the additional complication that in many cases hydrogen
   fusion will continue in a shell at lesser depth — this puts stars onto
   the asymptotic giant branch. , , The decrease in surface temperature
   shifts the star's visible light output to the red — hence red giant.
   Stars of spectral types O through K are believed to become red giants
   (or supergiants in the case of O and B stars).
   Artist's conception of the remains of artificial structures on the
   Earth after the Sun enters its red giant phase and swells to roughly
   100 times its current size.
   Enlarge
   Artist's conception of the remains of artificial structures on the
   Earth after the Sun enters its red giant phase and swells to roughly
   100 times its current size.

   Very low mass stars are thought to be fully convective , and thus may
   not accumulate an inert core of helium, and thus may exhaust all of
   their fuel without ever becoming red giants. Such stars are commonly
   referred to as red dwarfs.

   If the star is less than 2.57 solar masses, the addition of helium to
   the core by shell hydrogen fusing will cause a helium flash—a rapid
   burst of helium fusing in the core, after which the star will commence
   a brief period of helium fusing before beginning another ascent of the
   red giant branch. Stars more massive than 2.5 solar masses enter the
   helium fusing phase of their lives much more smoothly. The core helium
   fusing phase of a star's life is called the horizontal branch in
   metal-poor stars, so named because these stars lie on a nearly
   horizontal line in the Hertzsprung-Russell diagram of many star
   clusters. Metal-rich helium-fusing stars do not lie on a horizontal
   branch, but instead lie in a clump (the red clump) in the
   Hertzsprung-Russell diagram.
   Comparison between red giants and the Sun
   Enlarge
   Comparison between red giants and the Sun

Earth's Sun

   As Earth's Sun is of one solar mass, it is expected to become a red
   giant in about five billion years. It will become sufficiently large to
   engulf the current orbits of some of the solar system's inner planets,
   possibly including Earth's . The gravitational pull of the Sun will
   have weakened by then due to its loss of mass, and it is possible that
   Earth may escape to a wider orbit . The fate of the Earth with regard
   to the size of the expanding Sun is still hotly debated in the
   scientific community. Mercury and Venus will almost certainly be
   swallowed up by the Sun when it turns into a red giant. If the Earth
   does escape, it will be due to tidal acceleration.

Red giants in fiction

     * In the Superman movie series, the doomed planet Krypton orbited a
       red giant called Rao. It explodes in a supernova explosion, causing
       shockwaves that in turn destroys Krypton itself (the comic book
       series has a red dwarf star and Krypton exploding due to an
       unstable core). Current continuity attributes the source of
       Superman's powers to Earth's yellow sun.

     * According to The Magician's Nephew, part of C. S. Lewis' The
       Chronicles of Narnia, the White Witch Jadis comes from an accursed
       world called Charn. The star in Charn is described as being red,
       cold, and much larger than the Sun, just like the red giants in our
       own universe.

     * Within the Dune Universe, The Sun is in its Red Giant phase during
       the events that take place in The Butlerian Jihad. The book being
       the first of three prequel Novels. Written by Brian Herbert and
       Kevin J. Anderson. Current estimates require 4.5 billion years for
       the sun to reach its red giant phase.

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