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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