Friday, 25 March 2011

The Sun as a red giant

The Sun is predicted to become a red giant in approximately five billion years.[7] It is calculated that the Sun will become sufficiently large to engulf the current orbits of the solar system's inner planets, up to Earth,[12][13][14] and its radius will expand to a minimum of 200 times its current value.[15] The Sun will lose a significant fraction of its mass in the process of becoming a red giant, and there is a chance that Mars and all the outer planets will escape as their resulting orbits will widen.[16] Mercury and most likely Venus will have been swallowed by sun's outer layer at this time. Earth's fate is less clear. Earth could technically achieve a widening of its orbit and could potentially maintain a sufficiently high angular velocity to keep it from becoming engulfed. In order to do so, its orbit needs to increase to between 1.3 AU (190,000,000 km) and 1.7 AU (250,000,000 km).[17] However the results of studies announced in 2008 show that due to tidal interaction between sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the sun before the sun reaches its largest size, despite the sun losing about 38% of its mass.[18] Before this happens, Earth's biosphere will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years.[19] Earth's atmosphere and lithosphere will become like that of Venus. Over another billion years, most of the atmosphere will get lost in space as well;[16] ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.

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