(1) The fusing of Helium into heavier elements will result in -
(a) fusing of Sun
(b) swelling of Sun
(c) swelling of Earth
Answers
The Sun will become a Red Giant.
Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant-branch (RGB) phase where the Sun will spend around a billion years and lose around a third of its mass.
After RGB the Sun now has only about 120 million years of active life left, but they are highly eventful. First the core ignites violently in the helium flash, and the Sun shrinks back to around 10 times its current size with 50 times the luminosity, with a temperature a little lower than today. It will then have reached the red clump or horizontal branch (HB), but a star of the Sun's mass does not evolve blueward along the HB. Instead it just becomes mildly larger and more luminous over about 100 million years as it continues to burn helium in the core.
When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-giant-branch (AGB) phase, and the Sun is alternately burning hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early AGB, the Sun becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level and the radius to over 1 AU. Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the RGB produce smaller, less luminous stars at the tip of the AGB, perhaps only 2,000 times the luminosity and less than 200 times the radius. For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a planetary nebula. By the end of that phase – lasting approximately 500,000 years – the Sun will only have about half of its current mass.
The post AGB evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a planetary nebula as the exposed core reaches 30,000 K. The final naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black.
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