Sun Bengins It's Red Giant Bloat - How Long Before We Notice?

I know that when stars go Nova, it happens (they say) in a matter of minutes or hours at most. Prior to that, how long would it take to notice the Sun has begun it’s Red Giant phase? How long before the Earth is consumed?

The reason I ask is that I grew up as a big fan of Car Sagan’s “Cosmos”, and his “last perfect day on Earth” line has stuck with me all these years. Always wondered, would it really be that quick? (I assume he was just being dramatic, but you never know).

What will you do with this information? If you have intentions of constructing a bunker I believe it will be for naught.

Stars don’t just suddenly go red giant; the process takes millions of years, thus is completely inperceptiable on human timescales.

Also, even before it goes red giant, the Sun will (and already is, since it formed; the Sun used to be about 70% of its current brightness) slowly increase in size and output to the point where Earth will become too hot for most life (save for perhaps some bacteria) in around a billion years - about 4 billion years before it becomes a red giant, and it will take several billion more years before the Sun actually absorbs the Earth:

Of particular note regarding the evolution of the Sun during the red giant phase:

So it will take around a billion years to go from the end of the main sequence to its maximum red giant size.

ETA: Even if the Sun could go supernova, there would still be many changes occurring millions of years before it actually blew up as it went from hydrogen burning to helium burning and so on.

Looks like you can book that extended vacation. Your home will still be there when you return.

Probably as soon as it takes off its t-shirt and we see the pink beer gut hanging over the solar belt buckle.

Eight minutes, give or take?


That always stuck with me, too. But no, it won’t really be that quick. The sun is gradually getting brighter, and has been since it was born. Long before the red giant phase starts, it will get bright enough to boil Earth’s oceans. The brightening of the sun is a gradual process, so it will probably gradually get hotter on Earth until it gets too hot for life.

Phil Plait’s Death From the Skies is a good book that talks about this, as well as other astronomical threats to life on Earth.

Any guesses as to whether evolution will be able to keep up with climate change during the 4 billion year period when the sun’s luminosity is increasing? I’m thinking in terms of non-humans here.

Since the luminosity is going to increase to the point of boiling away the oceans, I’m going to say no.

Given what we know about extremophiles, I’m not convinced that this alones necessitates global extinction. I think it really depends on the speed of evolution vs the speed of environmental change.

Even extremophile bacteria need liquid water to reproduce, or even to live in a non-sporulated form.

I’m going to go out on a limb here and hazard that some future intelligent life, whether human or some successor species, will contrive to move the Earth a bit further away from the Sun and avoid the ocean-boiling thing.

Given the several billion years remaining, I suspect there will be time for intelligence to reappear and develop sufficient technology to prevent the suns expansion. Unfortunately, such technology will likely develop AFTER a respect for the natural order of life and the universe and so would not be used.

Wait, what’s that reference? I didn’t find it easily on Wikipedia. How does the sun’s greater heat output in 1.1 billion years stop Earth’s plate tectonics? Or is Earth’s core likely to cool sufficiently, in that time period, to reduce plate tectonics to the point that the magnetic field fails enough so that greater solar wind strips Earth’s atmosphere?

A good reason for global warning, right?

Now, how much hotter has it become over the last, say, 53 years?

I ask because that’s the way I roll.

If humanity is still around by then and we haven’t developed interstellar starships or found habitable planets around other stars, it’s possible that a move to Mars or even beyond could buy us some more time. As Mars heats up it should become easier to terraform than it is right now, and hopefully we’ll have more advanced technology to do that anyway. Anyone know of any research as to how long a move to Mars would buy us? How about Europa or some other further world?

“Does this Kuiper Belt make me look fat?”

Talk about sprawl! The commute will be hell.

I don’t think Mars would be much of a solution. There’s not a lot of atmosphere there, even if you melt the frozen CO2. And transporting billions of people to Mars is a stretch even if we allow science-fiction technology. If we have that kind of tech, living permanently on space ships - or even going to other stars - should be an option. I think we’d buy more time on Earth just by digging below the surface or living under domes.

If anything, the Sun has gotten cooler over the past 53 years or so, just when global warming has really shown itself; the current solar cycle in particular has been rather anemic (but the last one was weaker than the previous one as well, and much weaker than the one in the 1950s, the strongest in the modern record, although new research casts doubt on the magnitude of solar variation recorded in the past, due to inconsistent sunspot measurements, but the 1950s cycle was clearly much stronger than later ones, the current one is similar to those in the early 1900s).

Interestingly, the Earth was as warm or even warmer than today when the Sun was only 70% as bright - the so-called “faint young sun problem”, the discrepancy being due to much higher levels of greenhouse gasses (a major Snowball Earth episode followed the rise of oxygen-producing organisms as they depleted GHG levels).

Oh, and if the gradual brightening of the Sun was enough to cause noticeable global warming in 50 years, we’d have been cooked long ago; even over 50 million years it didn’t offset a cooling trend due to geological changes (mountain building, particularly the Himalayas, reduced CO2 levels, along with ocean circulation changes in Antarctica and the closing of the Isthmus of Panama).

I had no idea that Earth was so far over the hill. Only 1.1 billion years left before the Sun cooks away our oceans?! We’d better make the most of them!