For the moment, let's ignore the fact that the Sun is theorized to swallow Earth when it does decay into a red giant. We will pretend that it gets big enough to eat Venus, and stops there.

It will grow to massive size in the sky, but also become dimmer. Blue light will probably be mostly gone, so the sky will probably be red or pinkish, and daytime might look like a horror film with no blues or greens to speak of.*

My question is mainly this: would you be able to go about your life as we do now, minding not to look directly at the day star? Or would the greater surface area + proximity of burning plasma result in a truly unbearable luminosity?

I strongly prefer scientific analysis answers. Aspects to cover in an ideal answer:

  • How does the size of the Sun influence the amount of energy that hits Earth? If physics apply, please cite.
  • How about the proximity of the Sun?
  • How does "going red" militiate against these two factors, i.e. by what proportion does a star dim when it turns?

In the best-case scenario, I would like to leave this question knowing 3 formulae for the above questions, and I will be able to find a point in the space [size, brightness, proximity] that satisfies my quest for worldbuilding. Thank you for reading!

* I now know this isn't true, you would perceive colors as usual! What would a Rainbow look like on an earth-like Planet orbiting a Red Sun?

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    $\begingroup$ Side note: It is possible to look directly at our sun without immediate or even medium term consequences. When I was about ten I would star at the sun in fascination. I'm now 50 and still have my vision. Will something happen eventually because I used to stare at the sun? Maybe/probably. Is the advice to never look directly at the sun given with some extra caution? I believe so. $\endgroup$ Commented Jun 21 at 5:48
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    $\begingroup$ Shall we assume that a modern human traveled in time to when the Sun gets to that phase? Otherwise, if we are talking about our future descendants, they will evolve to tolerate those sorts of lighting conditions, as it happens slowly enough. $\endgroup$
    – vsz
    Commented Jun 21 at 8:07
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    $\begingroup$ Most of the answers are focusing on luminosity, which is the wrong quantity to focus on. The Andromeda galaxy has a luminosity billions of times higher than the sun, but it's completely safe to look at even if you're right in the middle of it. The relevant quantity for eye damage is luminous intensity measured in cd/m^2. $\endgroup$
    – David
    Commented Jun 21 at 9:28
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    $\begingroup$ @David Even that is not really relevant to the question whether it is save to look at it. Sure, too much intensity and it will cook you whole, not just your eyes. But for eye damage its much more relevant how bright it is visually per square degree. Thats why lasers are so dangerous to the eye, even when relatively low powered. $\endgroup$
    – LazyLizard
    Commented Jun 21 at 9:58
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    $\begingroup$ Can you look at something that merely is "red hot," without harming your eyes? Absolutely, yes. (You can do this at home if you have a toaster.) Can you stand, for any significant length of time, in the presence of something that is red hot and, is so huge or so close that it nearly fills your entire field of vision without being roasted? No. (You cannot do this at home if your home contains anything flammable. Maybe try the experiment while visiting a working steel mill. Or, you can get a hint by standing close to a really big bonfire at some celebration.) $\endgroup$ Commented Jun 21 at 16:20

7 Answers 7


Stars that leave the main sequence to become giants towards the end of their lives are generating more energy... you need that extra energy to hold the outer layers up against the pull of gravity, after all. This means they're gonna become a lot more luminous. The luminosity rises and falls based on various factors and how old the star is, but you get subgiants, red giants, asymptotic giant branch stars with various temperatures, colors and flavors.

An asymptotic giant branch star like Chi Pegasi has about the same mass as the Sun, but is 400-450 times more luminous. Its radius is 53 times larger than the Sun, but that's only ~0.14 AU which isn't even enough to eat Mercury. Arcturus is a regular solar-mass red giant but is still ~170 times more luminous than the Sun, with 25 times the radius.

At the "surface", the Sun has a radiant flux of ~6.3x107W/m2. Chi Pegasi has more like 9x106W/m2 (though it has a much less well defined "surface", being fuzzier around the edges than a regular main sequence star). That's less intense, but the 2500-fold increase in surface area makes up for it by blasting you with a lot more heat and light.

Because of how black body radiation curves work, A red star will be producing about 50% less output in the visible part of the EM spectrum, with much more infrared making up the balance. I mention this for completeness as it'll still be blindingly bright and incineratingly hot and nearby viewers won't be in any position to appreciate the difference.

To sum up:

How does the size of the Sun influence the amount of energy that hits Earth?

To become larger, the sun must also become more luminous. The surface cools as it expands, but the radiant flux received at the Earth will be higher.

How about the proximity of the Sun?

That's just the inverse square law. As it gets larger, it gets closer.

How does "going red" militiate against these two factors, i.e. by what proportion does a star dim when it turns?

It doesn't dim, and that's what'll sizzle you up.

Would you be able to look directly at the Sun if it were a red giant?

From the distance of the Earth? Briefly.

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    $\begingroup$ Rough estimate, 400 times more luminous but 53 larger in diameter. Thats 2800 times larger in visual area. So per square degree it would be only about 14% as bright as the sun. So looking at it should be safer for your eyes than the sun. Less safe for your whole body though. $\endgroup$
    – LazyLizard
    Commented Jun 21 at 10:03
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    $\begingroup$ @LazyLizard Are you saying that its total perceived brightness is actually decreased due to the emissions being amortized across a greater surface area? I don't see that in any of the other answers; that's very interesting! $\endgroup$
    – order
    Commented Jun 21 at 17:19
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    $\begingroup$ @order I'm guessing I probably shouldn't have called it "radiant flux" when I said the same thing yesterday, huh. $\endgroup$ Commented Jun 21 at 19:22
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    $\begingroup$ @order Its perceived "overall brightness" (reflected, in the sense of the word, by how well it lights the landscape and at which time of evening you'll need a reading light in your living room) is vastly larger. It's just that looking at it may not damage your retina. (Even if you filter the infrared out it'll still heat everything up, violently, though, including your eyes.) $\endgroup$ Commented Jun 23 at 18:05

Luminosity will increase greatly

Although you are limiting your red giant about the orbit of Venus, this actually means the diameter of the red giant phase will be roughly 80 times its current diameter. This actually makes it on the small side for red giants.

Aldebaran is a well-known red giant that is even smaller, only about 45 times solar diameter, yet its measured luminosity is around 440 times solar luminosity. As a general trend, the larger the red giant, the greater its luminosity. So, you should expect the sun to eventually exceed current luminosity at least 440 times.

As the sun is already blindingly bright, this would be insanely bright when viewed at 1 AU. Also, you should expect earth temperatures to be around 1000 Centigrade, so that might be a problem too.


Hard no

First, you have to consider that the Sun will produce about 2200x as much light. This is the low end, because you're specifying a lower expansions size.

However, at the radius of Venus, the Sun would have 24,000x as much surface area. Thus, any individual spot on it would be less than 1/10th as bright.

If it were the same diameter, you could probably look at the Sun for a short period without burning out your retina. Unfortunately, it isn't. The Sun currently has a diameter of about .5 degrees. If it expanded to the radius of Venus, it would take up 90-ish degrees in the sky. Even the brightness of the landscape illuminated by the star would be bright enough to cause eye damage.

  • $\begingroup$ Eye damage from the landscape? How do you see that? Your eyes will cook along with the rest of you but it won't single out your eyes for extra damage. $\endgroup$ Commented Jun 23 at 2:19
  • $\begingroup$ @LorenPechtel: The back of the eye burns first because it's behind a lens. $\endgroup$
    – Joshua
    Commented Jun 23 at 15:12
  • $\begingroup$ @LorenPechtel, Eyes that look at bright light (like sunlight reflected off of waves for fishermen) suffer from photo-oxidative damage, even in cases where the individual's temperature is regulated. I do agree that this question is a bit like questions about black holes where they ask you to ignore heat and radiation. $\endgroup$ Commented Jun 23 at 21:04

As the sun becomes a red giant, the luminosity may go up by a factor of 300 or so according to the Wikipedia article. This, I believe is the astronomical 'luminosity' which refers to the total radiated energy, and not just the visible wavelengths. If the Earth is to re-radiate the energy, and the radiant power goes roughly as the fourth power of the temperature, then the Earth will get something like four times hotter in degrees Kelvin.

Let us turn the question on its head. Let us suppose we have a star with a surface temperature of 3000K instead of the 6000K of the Sun. This is half the temperature, so about 1/16th of the power emitted per unit area. If the star subtended two degrees, rather than the half-degree that the Sun subtends at the Earth, then would would have four times the angle, or sixteen times the area. If you were a black body, you would be at roughly the same temperature. This is not a perfect calculation: clouds reflect a lot of the visibleradiation on Earth, but a red giant would give a larger fraction of infra-red, which would be absorbed. But it is a good enough back of the envelope figure.


I think you underestimate how bright the Sun is.

When we see the yellow ball that is the Sun in all those infographics and NASA images, that isn't how it actually looks. That's clearing away a vast amount of excess light to see some parts of the spectrum, and that's not even all of it (the sun actually emits more green light than any other color, but because of the balance the spectrum looks yellow). Whether you're in space or on Earth or anywhere, the Sun looks so blindingly bright that it damages your eyes just looking at it.

A red giant isn't going to be any more bearable. It's still equivalent to watching trillions of nuclear bombs going off every second, and it's still going to be looking directly at a star. Even if it stops at Venus's orbit, looking at the Sun will still be looking at the Sun, and the surface and corona actually just got closer to you by a lot, so even if the luminosity decreases substantially (it will actually increase dramatically, not decrease) you're still gonna be looking at a star and it will still damage your eyes.

Of course, if this actually happens and the Sun decides to expand into a red giant out to Venus's orbit over the next few decades, we will not survive. We will be much closer to the Sun (obviously), and the resulting temperature rise will bake everyone on the planet and boil the oceans. Worse yet, the proximity to an active star means that the solar wind will carry enough kinetic energy to blow away the atmosphere, because while our magnetosphere is strong it's not nearly strong enough to withstand the magnetic and kinetic forces that are present that close to a star.

Besides, once the Sun gets that big, the increased solar wind density will make Earth's orbit essentially pass through the Sun's "atmosphere" of more-dense solar wind, which will over time cause its orbit to decay, eventually causing it to spiral into the Sun even though the Sun didn't just straight up consume the Earth.

Don't mess with the Sun.

  • $\begingroup$ Is "the resulting temperature rise", solar wind increase, etc. due to the surface getting closer, and energy/mass emissions becoming more dense? Are there formulae for calculating these things? $\endgroup$
    – order
    Commented Jun 20 at 18:35
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    $\begingroup$ The resulting temperature rise is just because the total amount of light you receive from the Sun is inversely proportional to the square of the distance from it, and also because for some reason the area immediately surrounding the star (the corona) is way hotter than the surface itself and we don't know why. Unfortunately formulae regarding temperatures around stars are very hard to come by, but I can tell you for sure it'd be too hot for humans (or any other life as we know it). $\endgroup$ Commented Jun 20 at 18:37

The nearest everyday example for the brightness of a 3000K object is a tungsten-halogen lightbulb.

A car headlight variety has a few square mm heated surface and is quite unpleasant if looked at from a meter or two distance. And it still will cover smaller visual angle than even the present-day Sun.

In short, don't do this at home.

Well, the late evening Sun near the horizon is sometimes safe to look at. This will most probably fail at 100x luminosity either, unless one finds a better-scattering atmosphere.


I see several answers here that I don't think give a properly direct to your first two questions

It seems like you're thinking here that the sun is bigger in the sky (both because it is larger and the surface is closer) so we must get hit with more radiation from it. That isn't the case. The answer to your first two questions is "they have no effect". As long as you stay outside of the sun, the size of the sun doesn't matter.

The way to intuit this is to imagine a spherical surface centered on the Sun at an arbitrary distance. Assume the sun radiates energy equally in all directions so the energy hitting that sphere is evenly distributed. The portion of that energy something absorbs is directly related to the portion of the surface of that sphere it is blocking. Block all of it and you absorb all the energy; Block half of it and you absorb half the energy; block 1% of it and you absorb 1% of the energy.

The amount of that sphere blocked is just the ratio of the cross sectional area of the blocker to the surface area of the sphere. Area of the blocker only depends on the size of the Earth. Area of the sphere only depends on the distance from the center of the Sun to the Earth. Neither of these are changing.
The only thing changing is the total amount of energy emitted and it is going up. You won't get the red giant otherwise. The size and color are a consequence of the increased energy production.

To your worldbuilding questions: There aren't two competing effects here as you'd hoped. The sun produces more energy and the Earth gets hotter. To change that you need to somehow change the portion of the Sun's energy absorbed by the Earth. To do that Something you could consider looking at is the albedo of the Earth. Unfortunately, most things on earth absorb redder light better (water in particular), but not everything.


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