Future Sun. Artwork of the future Sun as seen from a molten Earth landscape. The Sun has become a swollen red giant, and has nearly filled the orbit of the Earth. This stage in stellar evolution occurs when the hydrogen fuel for nuclear fusion in the central core of the star has been exhausted. A period of gravitational collapse allows hydrogen burning to restart in a shell around the inert core. The energy from this stage causes the massive expansion of the outer layers of the star. It is estimated that the Sun will enter this stage of its life in around 4.5 billion years. The Moon (centre left) and several other planets are also seen, with comet-like tails.