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The sun is a main-sequence star; as I understand it the sun will expand into a red giant in a few billion years. I have heard conflicting reports of what this entails - some sources I found say the sun will expand until the current orbit of Earth is contained within it, others say it will stop expanding short of Earth's current orbit.

Every source has agreed that the planet will have long-since stopped being able to support life as we know it.

Assuming humanity takes no action to significantly alter the natural course of events, such as dramatic alterations of our atmosphere, what will the last few decades of Earth's life be like? Will the planet be heated to the point where its surface is like Mercury's sun-side? Will the sun's output drop low enough that it instead cools, like Mars? Will the sun expand and engulf the planet, or will the planet continue to orbit an expanded sun as a lifeless ball?

Will the sun's expansion be gradual or rapid? How will this expansion affect the rocky planets nearest to it?

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    $\begingroup$ I understand that there can't be a definitive answer (at least not for a few billion years) but I'm wondering what current models predict and if we've found any exoplanets which might shed light upon this question. $\endgroup$ – Jeff Jul 7 '15 at 20:00
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    $\begingroup$ I have read an article somewhere or watched a documentary, I forget which one but it stated that astronomers have found exoplanets that has been "engulfed" by it's star but still "survived". However these planets were Jovian sized so if this happened to earth, I think we will just slowly evaporate. $\endgroup$ – NuWin Jul 7 '15 at 20:20
  • $\begingroup$ @NuWin: I was pretty sure that if the sun engulfed Earth the Earth would be melted, shredded, sheared, or otherwise obliterated. I was more thinking about the time pre-engulf. Still, neat info about Jovian exoplanets within stellar bodies - I'd love to see more details. $\endgroup$ – Jeff Jul 7 '15 at 20:46
  • $\begingroup$ So I did some reading because your question got me intrigued. There is real no difinitive answer to this scenario from what I can tell but in the time of the pre-engulf, the intensive heat from the sun will evaporate our oceans and the solar radiation will blast away the hydrogen in our water. I guess you can use your imagination for how it would be like on earth. Essentially the earth will start a square one, a molten rock. $\endgroup$ – NuWin Jul 8 '15 at 4:16
  • $\begingroup$ Here's an article on 2 planets that survived a star's red-giant phase. space.com/14012-survivor-alien-planets-dying-star.html The article says that follow-up observations are needed. But, for example, when our sun goes into it's red giant phase, it's outer sphere will be very thin, (too lazy to do the math), but probably quite a bit thinner than our atmosphere, and red hot, so a couple thousand degrees not white hot. It's not crazy to think that a rocky and certainly an Iron core could survive in such an environment. $\endgroup$ – userLTK Jul 10 '15 at 23:08
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A lot of the answer depends on Orbital mechanics and calculating that isn't easy. If no action is taken, Earth's orbit is still likely to change, but how much and in which direction is hard to say.

Long before the sun goes red giant it will get too hot for the Earth. If the earth doesn't move, the sun's increased temperature will likely make the earth uncomfortable in a few hundred million years and perhaps scorching in as little as a billion years. Warmer air and more water vapor in the air will trap heat and at a certain point, you'd get a run-away green house effect, long before the red-giant phase. Short term, changes in CO2 in the atmosphere might slow that. We could see ice ages before we see run-away heating, but over time, with no human action, run-away heating is likely inevitable - on a very large timescale, like a billion years or so. Longer if the Earth drifts farther from the sun.

what will the last few decades of Earth's life be like? Will the planet be heated to the point where its surface is like Mercury's sun-side? Will the sun's output drop low enough that it instead cools, like Mars? Will the sun expand and engulf the planet, or will the planet continue to orbit an expanded sun as a lifeless ball?

Will the sun's expansion be gradual or rapid? How will this expansion affect the rocky planets nearest to it?

I'm going to answer the 2nd part first. The sun's expansion will first be gradual, but the red giant stage will be fairly rapid. Over the next 4 billion years or so, the sun will get some 60% more luminous, so significantly much more heat at the same orbit. A very rough rule of thumb is 10% more luminous every billion years. (that's not for every star, just for 1 solar mass) - and that's a pretty ballpark estimate.

But just 10% more heat from the sun would raise the Earth's temperature uncomfortably, and with the added water vapor in the air and greenhouse effect, the earth might be a permanent steam room, even at the poles, at just 10% more heat from the sun.

asking about the last few decades is the wrong time frame as changes would probably happen much more slowly, though atmospheric/feedback mechanisms in the climate might make changes happen more quickly - ice age speed for example, a few thousand years or so. Also, as Antarctica drifts away from the south pole, we might see an end to ice ages - there's lots of moving parts to long term climate predictions, but in general, you'd see gradual heating to the point where complex life might have a hard time adapting.

The suns output won't drop until after it goes red giant, it's on a steady but gradual increase, followed by a relatively fast increase at the red giant stage.

Now, if we terraform Titan, that might be a safe distance to watch the sun go red giant from and it would probably be quite a spectacular show, well, over enough time.

As to how fast it will go red-giant, fast for a star means, about 200 million years, so, from the perspective of a human lifetime, watching a star go red giant would be like watching grass grow.

A much more intelligent discussion on that than I can give, here: https://physics.stackexchange.com/questions/25622/how-fast-will-the-sun-become-a-red-giant

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  • $\begingroup$ The Sun will not get 60% hotter over the next 4 billion years. Do you mean 60% more luminous? $\endgroup$ – Rob Jeffries Jul 11 '15 at 7:49
  • $\begingroup$ You're right, I meant it will give off 60% more energy from the Earth's perspective, assuming a consistent distance, so 60% more luminous is probobly more correct. Answer edited. $\endgroup$ – userLTK Jul 11 '15 at 8:34
  • $\begingroup$ Well you could check that this is correct and possibly cite the evolutionary models you are using. However I do know that the Sun's temperature will get a little hotter before reaching the "terminal age main sequence" and then it cools quite significantly in the subgiant phase. $\endgroup$ – Rob Jeffries Jul 11 '15 at 8:40
  • $\begingroup$ I went from memory, but here's a link. 67% brighter over the next 4.8 billion years. 30% less bright 4.5 billion years ago. faculty.wcas.northwestern.edu/~infocom/The%20Website/… Since most of the energy the earth receives from the sun is in the form of light, 60% brighter means 60% more heat received on earth - everything else staying the same, of-course. $\endgroup$ – userLTK Jul 11 '15 at 9:52
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    $\begingroup$ This is an excellent link and a comprehensive answer to the OP. The models won't have changed that much - though the exact mass-loss in the giant phases is somewhat uncertain. Note that the Earth survives! However this link does not inlude any consideration of whether the planetary orbits remain stable - this is not by any means certain. $\endgroup$ – Rob Jeffries Jul 11 '15 at 10:36

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