<blockquote>
<p>If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?</p>
</blockquote>
<p>I'd say no for a couple of reasons.</p>
<ol>
<li><p>@PM2Ring's comment:</p>
<blockquote>
<p>FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the <a href="https://en.wikipedia.org/wiki/Main_sequence" rel="noreferrer">main sequence</a> and becoming a red giant. See <a href="https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Future_of_the_Earth,_the_Solar_System_and_the_universe" rel="noreferrer">Timeline of the far future</a>.</p>
</blockquote>
</li>
<li><p>Our eyes and vision processes accommodate (adjust for) changes in brightness, we walk indoors from a sunny day to a well lit room and the brightness of things has dropped by 99% or <a href="https://chemistry.stackexchange.com/a/111065/16035">nearly a factor of 100</a> within a few seconds and yet though we are somewhat aware of the change of tens of percent per second we hardly notice it.</p>
</li>
<li><p>Our vision system is constantly adjusting its <a href="https://en.wikipedia.org/wiki/White_point" rel="noreferrer">white point</a> to accommodate changes in the color of available light.<sup>†</sup> A white sheet of paper looks white to us even as the color of ambient light changes because our vision is always &quot;color balancing&quot; (not exactly the right term but it's something like that).</p>
</li>
</ol>
<p><sup>†</sup>From <a href="https://astronomy.stackexchange.com/questions/35388/our-sun-becoming-a-red-giant/35391?noredirect=1#comment68568_35391">@Luaan's comment</a></p>
<blockquote>
<p>It's called &quot;white balancing&quot; - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved)</p>
</blockquote>

CC BY-SA 4.0
2020-03-03T07:17:20.273Z
It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved).
Luaan
CC BY-SA 4.0
2020-03-03T09:01:54.643Z
@Luaan aaaaaahhhh!!! I've stumbled into meme galaxy! :-) Thanks, I'll make an a̶c̶c̶o̶m̶m̶o̶d̶a̶t̶i̶o̶n̶  edit.
uhoh
CC BY-SA 4.0
2020-03-02T22:48:29.317Z
2020-03-03T09:04:50.720Z
2020-06-17T09:47:40.653Z
uhoh
<p>If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence? </p>

CC BY-SA 4.0
2020-03-02T20:35:40.777Z
What would you consider the actual "start" of this process?
Carl Witthoft
CC BY-SA 4.0
2020-03-02T22:12:07.260Z
FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the [main sequence](https://en.wikipedia.org/wiki/Main_sequence) and becoming a red giant. See [Timeline of the far future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Future_of_the_Earth,_the_Solar_System_and_the_universe).
PM 2Ring
CC BY-SA 4.0
2020-03-02T23:18:52.777Z
@PM2Ring I quoted you below
uhoh
CC BY-SA 4.0
2020-03-03T11:19:35.847Z
The Sun (or any other star) increases its radiative output almost constantly when at main sequence stage. See "faint young sun paradox". The process is rather slow (say, 20 or 30% for all its past history of, say, 4.5 bn years). If we interpolate over the humanity history (say, 100ky) we get some 0.0001% increase. Not a big deal, since 11-year sun cycle gives about 1% difference.
fraxinus
CC BY-SA 4.0
2020-03-06T21:05:54.203Z
While the expansion of the Sun is enormously slow and not visible on a human time-frame, Earth's eccentricity varies a bit faster, and about 200,000 years ago Earth had an eccentricity of about .05, similar to the Moon's eccentricity.   That means, at Perihelion, it was like having a "super sun", similar to the super moons we get today.   .05% eccentricity mean a 10% variation in degrees of arc and a 21% variation in brightness that the Sun would swing back and forth every 6 months.    I think the idea of super-suns and 21% variation in the visible size of the Sun is kinda cool.
userLTK
CC BY-SA 4.0
2020-03-15T19:13:59.160Z
I did a rough calculation based on a model (Figure 1) in this paper https://arxiv.org/pdf/0801.4031.pdf that I found referenced in a Physics Stack Exchange Question.
Jack R. Woods
CC BY-SA 4.0
2020-03-15T19:24:53.033Z
I did a rough calculation based on a model (Figure 1) in this paper https://arxiv.org/pdf/0801.4031.pdf that I found referenced in a Physics Stack Exchange Question. The sun changes the most right before its peak of expansion (past the Earth's current orbit). According to the model, in the last 100,000 years or so, before the peak, the sun will lose 20% of it's mass and increase in size about 60%. With an estimated temperature T=2600K, this means that the Luminosity (L) will increase from about 1040 L(sun) to 2730 L(sun). This is still only about .003 increase in 100 years.
Jack R. Woods
<the-sun>
Our sun becoming a red giant
charly
# Our sun becoming a red giant

(Asked by: username_0 on 2020-03-02)

If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?

**Tags:** the-sun
> **Comments:**
> **username_1**: What would you consider the actual "start" of this process?  
> **username_2**: FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the [main sequence](https://en.wikipedia.org/wiki/Main_sequence) and becoming a red giant. See [Timeline of the far future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Future_of_the_Earth,_the_Solar_System_and_the_universe).  
> **username_3**: @PM2Ring I quoted you below  
> **username_4**: The Sun (or any other star) increases its radiative output almost constantly when at main sequence stage. See "faint young sun paradox". The process is rather slow (say, 20 or 30% for all its past history of, say, 4.5 bn years). If we interpolate over the humanity history (say, 100ky) we get some 0.0001% increase. Not a big deal, since 11-year sun cycle gives about 1% difference.  
> **username_5**: While the expansion of the Sun is enormously slow and not visible on a human time-frame, Earth's eccentricity varies a bit faster, and about 200,000 years ago Earth had an eccentricity of about .05, similar to the Moon's eccentricity.   That means, at Perihelion, it was like having a "super sun", similar to the super moons we get today.   .05% eccentricity mean a 10% variation in degrees of arc and a 21% variation in brightness that the Sun would swing back and forth every 6 months.    I think the idea of super-suns and 21% variation in the visible size of the Sun is kinda cool.  
> **username_6**: I did a rough calculation based on a model (Figure 1) in this paper https://arxiv.org/pdf/0801.4031.pdf that I found referenced in a Physics Stack Exchange Question.  
> **username_6**: I did a rough calculation based on a model (Figure 1) in this paper https://arxiv.org/pdf/0801.4031.pdf that I found referenced in a Physics Stack Exchange Question. The sun changes the most right before its peak of expansion (past the Earth's current orbit). According to the model, in the last 100,000 years or so, before the peak, the sun will lose 20% of it's mass and increase in size about 60%. With an estimated temperature T=2600K, this means that the Luminosity (L) will increase from about 1040 L(sun) to 2730 L(sun). This is still only about .003 increase in 100 years.  

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## 1 answers

### Answer by username_3 on 2020-03-02. Score: 12

> If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?

I'd say no for a couple of reasons.

1. @PM2Ring's comment:

   > FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the [main sequence](https://en.wikipedia.org/wiki/Main_sequence) and becoming a red giant. See [Timeline of the far future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Future_of_the_Earth,_the_Solar_System_and_the_universe).
2. Our eyes and vision processes accommodate (adjust for) changes in brightness, we walk indoors from a sunny day to a well lit room and the brightness of things has dropped by 99% or [nearly a factor of 100](https://chemistry.stackexchange.com/a/111065/16035) within a few seconds and yet though we are somewhat aware of the change of tens of percent per second we hardly notice it.
3. Our vision system is constantly adjusting its [white point](https://en.wikipedia.org/wiki/White_point) to accommodate changes in the color of available light.† A white sheet of paper looks white to us even as the color of ambient light changes because our vision is always "color balancing" (not exactly the right term but it's something like that).

†From [@Luaan's comment](https://astronomy.stackexchange.com/questions/35388/our-sun-becoming-a-red-giant/35391?noredirect=1#comment68568_35391)

> It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved)

> **Comments:**
> **username_7**: It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved).  
> **username_3**: @Luaan aaaaaahhhh!!! I've stumbled into meme galaxy! :-) Thanks, I'll make an a̶c̶c̶o̶m̶m̶o̶d̶a̶t̶i̶o̶n̶  edit.  

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