the Sun is going to run out of fuel in about 7-8 billion years. What will happen to the planets then, especially to Earth? Research suggests, if tidal forces are taken into account, then the Earth is likely to evaporate when the Sun treads towards its death ( Icarus , Vol 151, p130).
The Sun belongs to a class of stars called the main sequence stars. Thermonuclear fusion reactions in their core convert hydrogen atoms into helium atoms and in the process releases a lot of energy. This energy holds up the Sun against its own gravitational force. In the absence of any energy generation, the material in the Sun would collapse into the core. This very energy also provides us with our life force on Earth.
And though the sun is massive, it holds a limited volume of hydrogen. At some point of time all the hydrogen is bound to be converted into helium. At this point, the Sun will enter the 'red giant phase' when the helium in the core will undergo thermonuclear reaction to produce energy and other heavier elements. The Sun will become huge and probably engulf planets as distant as the Venus' orbital radius. Towards the end of the red giant phase, the helium in the core will ignite and emit a huge flash. After a couple of hundred million years, the Sun would run out of nuclear fuel to ignite and then begin its compression. It will, depending on its mass and size, settle down to a dense cold object that could be a white dwarf or a neutron star. The fate of our solar system will then hinge on how the Sun will move from a main sequence star to a red giant.
Some calculations suggest that the red giant would be big enough to cover the orbit of Earth while others feel that it will only engulf Venus. The calculations are horrendously difficult to perform because of all the various variables that need to be taken into account.
K Rybicki, researcher at the Polish Academy of Sciences, Warsaw and C Denis, researcher at the University of Lige, Belgium have calculated the future of our own planet taking into account tidal forces. As the Sun becomes huge and its envelope of hot gases reaches out to the Earth's orbit, the planets will experience tidal forces. These forces will change the orbits significantly because of the huge mass and distances involved. For instance, if there were no tidal forces, calculations suggest that Venus (and all the other planets beyond that) would move away from the red giant. If the tidal forces are taken into consideration, then Venus will spiral into the Sun.
As for the Earth, it will probably stay out of the Sun's envelope but may get trapped in one of the heat pulses that come after a star has finished its red giant phase. If it gets trapped, then the earth will be sucked into the Sun and eventually evaporate. On the other hand, if these pulses did not last for too long, Earth may, like the outer planets, continue in its orbit around the much cooler and denser Sun. In any case, life as we know it would have long become extinct, either because of too much heat or ultimately because of too little heat and energy!
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