When will the Sun die?How?

The Sun is a yellow, G2 V main sequence dwarf. Yellow dwarfs live about 10 billion years (from zero-age main sequence to white dwarf formation), and our Sun is already about 5 billion years old. Main sequence stars (like our Sun) are those that fuse hydrogen into helium, though the exact reactions vary depending on the mass of the star. The main sequence phase is by far the most stable and long- lived portion of a star's lifetime; the remainder of a star's evolution is almost an afterthought, even though the results of that evolution are what are most visible in the night sky. As the Sun ages, it will increase steadily in luminosity. In approximately 5 billion years, when the hydrogen in the Sun's core is mostly exhausted, the core will collapse -- and, consequently, its temperature will rise -- until the Sun begins fusion helium into carbon. Because the helium fuel source will release more energy than hydrogen, the Sun's outer layers will swell, as well as leaking away some of its outer atmosphere to space. When the conversion to the new fuel source is complete, the Sun will be slightly decreased in mass, as well as extending out to the current orbit of Earth or Mars (both of which will then be somewhat further out due to the Sun's slightly decreased mass). Since the Sun's fuel source will not have increased in proportion to its size, the blackbody power law indicates that the surface of the Sun will be cooler than it is now, and will become a cool, deep red. The Sun will have become a red giant. A few tens or hundreds of millions of years after the Sun enters its red giant phase (or "helium main sequence"; the traditional main sequence is occasionally referred to as the hydrogen main sequence to contrast the other main sequences that a massive star enters), the Sun will begin to exhaust its fuel supply of helium. As before, when the Sun left the (hydrogen) main sequence, the core will contract, which will correspondingly lead to an increase in temperature in the core.

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