[pfdietz]

Since the Sun settled onto the Main Sequence after its formation, it has gradually brightened. It is now 30% brighter than it was then.

The brightening is caused by accumulation of helium ash in the Sun's core. This causes the core's equilibrium state to be denser and hotter, with more energy being produced.

In another billion years or so, this will drive the Earth into a runaway greenhouse effect. The oceans will evaporate and the hydrogen will be lost to space. Not long after, the Earth will be a dead planet devoid of any life, even microorganisms. You sometimes see statements about how when the Sun expands into a red giant it will destroy life on Earth, but the Earth will have long been lifeless by that time.

[bloopernova]

In case others were curious about the term Helium ash:

Ash is the name given to the energetic alpha-particles or helium nuclei produced by fusion reactions in a deuterium-tritium plasma, even though helium bears no physical resemblance to ash from a fire.

[pfdietz]

It should be noted that deuterium-tritium fusion is not what is happening in the Sun. The primary reaction chain involves proton-proton fusion, which yields deuterium. The deuterium immediately (within seconds) reacts with another proton to make helium-3. Helium-3 eventually fuses with another He-3 to make He-4 and two protons. There are some side chains, but the ultimate overall reaction is four protons + 2 electrons --> 4He + 2 neutrinos.

[galangalalgol]

The way I read it, it isn't a co2 greenhouse, in fact the death stroke is the increased co2 absorption at those temperatures due to weathering of rock. Driving co2 below what plants can survive. Building a parasol at l1 that can be modulated to vary what is passed and what is harvested will be necessary at some point. It could get a couple extra billion years potentially.

[pfdietz]

That destroys higher life, but tough microorganisms survive until the runaway steam greenhouse sterilizes the planet.

[naveen99]

Anyone else question why astronomers are so sure about things that happened 5+ billion years ago ? I guess otherwise there’d be a probably or a maybe in every sentence ? But then how do you tell when they really are sure ?

[wongarsu]

There is always an implied "as far as we know". But astronomy has a couple reasons to be fairly certain about many things. We can see a lot of stars, and because light takes time to travel the further away they are the older the state we are observing. We only see each star as it is right now, but from looking at a lot of them you get a good idea of how they can develop. Like how you can get a good idea of how humans age by just looking at a lot of people of different ages; you don't need to follow each of them for 90 years. The other advantage is that most of it is well understood physics that can be reasoned about and simulated. And then we can compare those simulations to what we are observing across the universe to see if our simulations make sense.

[naveen99]

Feels more social than science.

[wongarsu]

Science is forming a testable hypothesis and then testing it. So you hypothesize "as stars accumulate fusion products their cores get heavier and the rate of fusion increases, making them brighter" and test this both with modeling and looking if this matches the stars we can see. How is that now science?

[naveen99]

Models are not stars and very incomplete. They are different stars. Feels like you could drive a supernova through the black holes in the sloppiness.

[wolfendin]

Feels like?

What evidence do you have for the sloppiness and how does the current model fail to explain the deviation in observations?

[beej71]

So do it! :) Then we'll all have some new science to learn.

[naveen99]

Ok. Good explanation. Thanks.

[wizzwizz4]

They build a model of the universe's laws, as simple as they can get it while matching the evidence. They then look at other evidence they hadn't looked at before. It turns out, that quite simple models can predict a lot of different phenomena, which makes us reasonably confident in our assumptions that (1) the universe is governed by simple, fundamental laws; and (2) those laws are similar to our model.

If we make the assumption that our model applies everywhere and at every time (colloquially, that the laws of physics don't change), we can ask our models what happened in the past. Under our assumption, that's probably quite close to what actually happened. This assumption is called the "principle of induction", the "uniformitarian principle", the "cosmological principle", the "Copernican principle", and many other things besides.

[pfdietz]

The models are so good that when solar neutrinos were not as predicted, it turned out to be because of new physics (neutrino oscillation) not flaws in the solar model.

[tomrod]

Science is a powerful tool.

[baxtr]

Someone else in the thread is saying it’s 8% brighter.

So who is right?

[pfdietz]

Both can be right. The 8% is per billion years (and I believe the rate has been accelerating slightly over time.)

[baxtr]

Ah ok, got it. Thanks

[baxtr]

So what are we gonna do in a billion years from now??!

[krapp]

We'll be extinct long, long before that don't worry.