The Sun was born about 4,500 million years ago from a nebula. It's a gigantic ball composed mostly of hydrogen and helium. In this nebula, the gas condensed in its internal parts, cooled and there came a time when the pressure, density and temperature of this ball that was collapsing was so great that it began to perform nuclear reactions. In these fusion reactions, hydrogen is combined with helium and hence a new star results.
How does a star work?
In the Sun's core the pressures are a million times more powerful than on Earth, having a temperature of 15,000,000,000 degrees Celsius. What happens in its central part is that the atomic nucleus fuse (composed of neutrons and protons) and that is what makes hydrogen turn into helium. Thanks to these nuclear reactions is why the Sun shines and so it remains stable. The Sun has a very large mass and in this way it can shine for a great amount of time.
How much time does the Sun have left?
Our star still has fuel left for about 4,000 / 4,500 million years. It's important to explain that its end will not occur in a big explosion, when it runs out of hydrogen, which is the fuel with which it produces helium, the star itself will be compacted breaking the balance that existed until then between gravity that tends inward and the nuclear force that tends towards the outside.
When this imbalance occurs, the outermost layers of the Sun will swell and that will cause a red giant to form. At this point it will continue to swell and swallow Mercury, Venus and most likely swallow the Earth. The helium will then try to merge to form other elements (carbon, hydrogen and oxygen above all) although it will form a very small amount and finally the outer layers will end up expanding into space forming what is known as a planetary nebula.
At this point, the Sun's core will have become a white dwarf and will have a fuzzy gas halo with the material that has been created inside the Sun. Life as we know it would cease to exist, leaving this white dwarf that's a very dense and hot object. This process towards a white dwarf is estimated to take about 1,000 million years.
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