The sun is gradually cooling down
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The genuine reason for the future putting out of the sun is the large number of the meteorites and celestial rocks which have been swallowed by the sun, so that its density will increase and its surface will cool leading to the formation of an earthy crust.
Most of the meteorites and celestial rocks are obtained when one of the terrestrial planets of this sun will come to a standstill; that is because meteorites, celestial rocks and moons are attracted to planets, and when the planet will be depleted of its central heat, it will stop its rotation; for the same reason, its gravitation will be lost; because the gravitation is the result of the central heat of the planet.
At that time the meteorites, celestial rocks and moons attracted by that planet will move towards the sun by its gravitational force, and the sun will swallow them.
The time required for putting out of the sun is not more than two thousand years (of our present years calculations), after which the sun will become an earth with a cold crust. The indication of that is what has been mentioned, in the past pages, about the formation of our Earth; for it also was a sun which became an earth after its life had finished, and its surface had become cold, and that had lasted two thousands of our years, then it burst and became nine planets.
The same will be the fate of our sun, which will be put out, and its surface will become cold in two thousand years time, after which it will burst and break up into nineteen pieces."
http://quran-ayat.com/universe/new_page_4.htm#Sunspots