<P> The Sun today is roughly halfway through the most stable part of its life . It has not changed dramatically for over four billion years, and will remain fairly stable for more than five billion more . However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes, both internally and externally . </P> <P> The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars . This age is estimated using computer models of stellar evolution and through nucleocosmochronology . The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago . Studies of ancient meteorites reveal traces of stable daughter nuclei of short - lived isotopes, such as iron - 60, that form only in exploding, short - lived stars . This indicates that one or more supernovae must have occurred near the location where the Sun formed . A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity . As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure . Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies . Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion . Thus, the Sun was born . </P> <P> The Sun is about halfway through its main - sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium . Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation . At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun . The Sun will spend a total of approximately 10 billion years as a main - sequence star . The Sun is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the hydrogen atoms that were fused . The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the centre and experience a stronger gravitational force, according to the inverse - square law . This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs . This process speeds up as the core gradually becomes denser . It is estimated that the Sun has become 30% brighter in the last 4.5 billion years . At present, it is increasing in brightness by about 1% every 100 million years . </P> <P> The Sun does not have enough mass to explode as a supernova . Instead it will exit the main sequence in approximately 5 billion years and start to turn into a red giant . As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth . </P>

Our sun is in which stage of its life cycle