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We all know that our sun will become a red giant in a couple of billion years? But how fast will the process be when it starts? Would it be theoretically possible (disregarding the deadly effects of this event) for a human to experience the sun as its present size until it swallows earth?

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    $\begingroup$ FWIW, the Earth will be unliveable long before then because the Sun is gradually heating up. See en.wikipedia.org/wiki/Timeline_of_the_far_future $\endgroup$ – PM 2Ring Jan 22 at 5:00
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    $\begingroup$ The Sun will not become a giant for another 6 billion years and will then go through the horizontal branch phase before becoming an asymptotic red giant branch star that might engulf the Earth. $\endgroup$ – Rob Jeffries Jan 22 at 8:50
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Yes, the time is very long, but exactly how long depends on when you "start the clock" for the expansion to a red giant. If you take the end of the "main sequence" (when the core hydrogen runs out) as the start of the expansion phase, it's more like a billion years to complete the process of becoming a maximally expanded red giant. But the answer that gets 5 million years focuses only on the phase of most rapid expansion (which begins after the star is already quite a bit larger than the Sun). Either way, our lives a the smallest blink of an eye to a star!

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  • $\begingroup$ A red giant Sun (after the main sequence) will only reach about $20 R_{\odot}$ and will not engulf the Earth. That comes later. $\endgroup$ – Rob Jeffries Jan 22 at 8:51
  • $\begingroup$ Actually, it will likely never swallow Earth at all. $\endgroup$ – Ken G Jan 22 at 11:46
  • $\begingroup$ Also, the Sun will be noticeably larger than it is now even by the end of the main sequence lifetime, but none of those seemed like the key aspect of the question. $\endgroup$ – Ken G Jan 22 at 11:53
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    $\begingroup$ Whether it will immeditately engulf the Earth is still moot and depends on how much mass is lost. However, tidal dissipation will then shrink the Earth's orbit and it probably will be engulfed. $\endgroup$ – Rob Jeffries Jan 22 at 16:05
  • $\begingroup$ I agree the issue is how much mass is lost, and it appears Earth is right on the hairy edge where it is really difficult to tell what our fate will be. For example, adsabs.harvard.edu/abs/2016Ap%26SS.361..122G finds evidence we will not be engulfed, but it's clear we really don't know and likely never will unless the species survives long enough to see it! $\endgroup$ – Ken G Jan 22 at 21:08
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According to a Physics.org article:

When the sun does begin to expand, it will do so quickly, sweeping through the inner solar system in just 5 million years.

So, no. No human alive could possibly see the sun change from its more-or-less present size to a red giant in their life time, unless extreme advances in human longevity approaching science fiction were to occur before then.

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