Life cycle
<dl><dd>
Main articles: Formation and evolution of the solar system and Stellar evolution
</dd></dl> The Sun's current age, determined using
computer models of
stellar evolution and
nucleocosmochronology, is thought to be about 4.57 billion years.<sup id="_ref-Bonanno_0" class="reference">
[5]</sup>
The Sun is about halfway through its
main-sequence evolution, during which
nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million
tonnes of matter are converted into energy within the Sun's core, producing
neutrinos and
solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10
billion years as a main sequence star.
The Sun does not have enough mass to explode as a
supernova. Instead, in 4-5 billion years, it will enter a
red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 MK, and will produce carbon and oxygen. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. However, Earth's water will be boiled away and most of its atmosphere will escape into space.
Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a
planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a
white dwarf over many billions of years. This
stellar evolution scenario is typical of low- to medium-mass stars.<sup id="_ref-future-sun_0" class="reference">
[6]</sup><sup id="_ref-Sackmann_0" class="reference">
[7]</sup>
