<p>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!</p>

CC BY-SA 4.0
2019-01-22T11:46:49.997Z
Actually, it will likely never swallow Earth at all.
Ken G
CC BY-SA 4.0
2019-01-22T11:53:05.823Z
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.
Ken G
CC BY-SA 4.0
2019-01-22T16:05:23.793Z
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.
ProfRob
CC BY-SA 4.0
2019-01-22T21:08:27.153Z
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, http://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!
Ken G
CC BY-SA 4.0
2019-01-23T16:36:19.227Z
[veering off into SciFi]   Never, since our highly technologically advanced descendants will have transported Earth (with its moon) to orbit around a Sol-like star but much younger and thus not doing all those bad things.  :-)
Carl Witthoft
CC BY-SA 4.0
2019-01-22T03:30:19.697Z
2019-01-22T03:40:06.307Z
2019-01-22T03:40:06.307Z
Ken G
<p>According to a <a href="https://phys.org/news/2016-05-earth-survive-sun-red-giant.html" rel="nofollow noreferrer">Physics.org</a> article: </p>

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

<p>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.</p>

CC BY-SA 4.0
2019-01-21T19:32:14.347Z
2019-01-22T19:09:26.713Z
2019-01-22T19:09:26.713Z
BillDOe
<p>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?</p>

CC BY-SA 4.0
2019-01-22T05:00:55.927Z
FWIW, the Earth will be unliveable long before then because the Sun is gradually heating up. See  https://en.wikipedia.org/wiki/Timeline_of_the_far_future
PM 2Ring
CC BY-SA 4.0
2019-01-22T08:50:56.023Z
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.
ProfRob
<the-sun><red-giant>
How fast will the sun grow when it becomes a red giant?
klutt
# How fast will the sun grow when it becomes a red giant?

(Asked by: username_0 on 2019-01-21)

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?

**Tags:** the-sun, red-giant
> **Comments:**
> **username_1**: FWIW, the Earth will be unliveable long before then because the Sun is gradually heating up. See  https://en.wikipedia.org/wiki/Timeline_of_the_far_future  
> **username_2**: 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.  

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## 2 answers

### Answer by username_3 on 2019-01-22. Score: 6

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!

> **Comments:**
> **username_3**: Actually, it will likely never swallow Earth at all.  
> **username_3**: 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.  
> **username_2**: 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.  
> **username_3**: 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, http://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!  
> **username_4**: [veering off into SciFi]   Never, since our highly technologically advanced descendants will have transported Earth (with its moon) to orbit around a Sol-like star but much younger and thus not doing all those bad things.  :-)  

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### Answer by username_5 on 2019-01-21. Score: 1

According to a [Physics.org](https://phys.org/news/2016-05-earth-survive-sun-red-giant.html) 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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