Tell me about the lifetime of the Sun
Or more specifically, how long is the Sun likely to take to burn out and how is this calculated? What alternative sources of energy will be available when the Sun dies?
This answer is courtesy of Henrik Lundstedt, Lund University, Solar-Terrestrial Physics Division, Sweden.
What we know about the evolution of the Sun we have learned from experiments in laboratories and from studies of stars similar to the Sun, but of different ages.
The Sun was born from a contracting gas cloud. The formation of the Sun took no more than 10 million years. When the temperature was high enough in the core (15 million degrees), hydrogen started to burn into helium (nuclear fusion). This is the most stable state of the Sun ("main sequence"), and we live during this period of the Sun.
The Sun is 4.6 billion years old and is estimated to live in this stable phase another 4.6 billion years. However, there is probably an uncertainty in this estimate due to doubts about the precise value of the Sun's initial helium content. The higher is the helium content, the less hydrogen there is to be converted into helium before the Sun leaves the stable phase.
After the Sun leaves the stable phase, the Sun is predicted to become so luminous and large (a red giant star) that the Earth will be engulfed. The Sun will eventually put an end to life on Earth. So I guess alternative sources of energy are in low demand.......
The central temperature of the Sun is now so high that nuclear reaction can convert helium into carbon, and subsequently oxygen. This process before its final dying state is believed to take no longer than about 10% of the total stable lifetime. Finally the Sun dies as a compact star: a so called "white dwarf".
More information about the structure and evolution of stars.
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Special Thanks to H. Lundstedt