what is the basis for scientific predictions about the future of the sun?

Inspired by this thread, in which posters cite predictions of the sun’s future properties, including changes in size and power output. Such changes are expected to take place billions of years from now, but we haven’t been observing stars very closely for more than a couple of centuries. No doubt we have observed distant stars with very different properties, but what changes have we seen stars undergo within that period that lead us to believe we can reliably extrapolate our observations to a period that is millions of times longer?

How much confidence do scientists have about our predictions for the future of the sun?

Folks will be along shortly with the details, but there’s effectively three things that matter here:

First, stars are relatively simple things, given enough handwaving. Gravity pulls in, fusion pushes out, and their chemical composition can be read in their spectrum. So it’s possible to just apply math and some known physics, and get reliable answers.

Second, our star is a main sequence star, typical for the stars in the galaxy. It doesn’t have any odd properties that would make us believe it’s radically different from any other main sequence star, so we can extrapolate anything we know about the others to our own sun.

And finally, we have the “space is mind-bogglingly big” part: there are an awful lot of observable stars from Earth (tens of thousands), so combined with the first two, a couple centuries of observation (and a few decades of really detailed observation) is enough to find examples of almost any stage of stellar evolution and observe it.

In general, we gain confidence in theories as we use them to make predictions and those predictions are verified.

One of the misconceptions that some people have is that if an event took place in the past, or takes a very long time relative to the human lifespan, we can’t make predictions. Not so.

Because whenever we collect data, we can make predictions about what the data will be.
Haven’t viewed the UV spectrum of that red giant? Well, here’s what the model tells us you’ll record.
And stellar evolution is quite nice for this, as we have millions of snapshots of stars at various stages of their life. We can constantly explicltly and implicitly test our understanding.

There are a few things to take into account.

  1. The process stars use to fuse materials is sufficiently nailed down to allow us to make extrapolations as to the impact of the various concentrations of metals in a star on its temperature profile

  2. We can then take our theoretical models and look out at hundred of millions of candidate stars to test them.

So while we can’t examine our G2 star 2 billion years ago we can observe a G2 star which is 2 billion years younger than ours or one that appears to be 2 billion years older. This is ultimately what the Hertzsprung–Russell diagram is all about.

In unscientific terms, you don’t have to observe a human for 100 years to know that a person is to learn to walk, learn to talk, go through puberty, mature, age, and die wrinkly. You can walk down the street in an afternoon and see hundreds or even thousands of people just like you at all different stages in their lives, and put together what that life cycle looks like.

Stars are kind of the same way.

Grey is correct, we’ve basically nailed down all the physics that occurs within a G2 star … so we know at some point the helium collecting in the core of our sun will itself start the fusion process … and that’ll be a bad thing for life on Earth.

A G2 star isn’t really that big, so the energies don’t really get too frickin’ high. There’s no supernova in our future.

By itself, the H-R diagram is just a survey of the relative abundances of various star types in the universe. The fact that there is a densly-populated main sequence could imply that all stars spend most of their lives in the main sequence and then leave it, or it could imply that main sequence stars are just more-commonly created for some reason.

What really cements the connection between the main sequence and stellar evolution is studies of globular clusters, which contain stars formed at about the same time, but with a range of masses. More massive stars age more quickly than less massive stars, so if you create the H-R diagram for the stars in a single cluster, you see the less massive stars still on the main sequence, but the more massive stars drifting off, which really makes much more sense in terms of stellar aging.

Exactly, and then you compare the observed HR diagram to a computer-generated HR diagram based on your stellar evolution models. This builds confidence that your model is accurate. It is said that we understand the inside of stars much better than we understand the inside of the Earth.

The OP should be safe in buying unripe bananas for the time being, but stay tuned …

It is really a combination of many kinds of observations that reinforce each other, in combination with our understanding of basic physics. The HR diagrams, especially in globular clusters, our knowledge of nuclear fusion and the influence of “metals” (to an astronomer, every element except hydrogen and helium is a “metal”) on reactions, and so on. There is also good geological evidence that the sun has been been getting hotter through the ages. It all hangs together and gives great confidence that their understanding is correct.

A couple years ago, I watched a great course on astronomy from the Learning Company. The lecturer was named Alex Philipenko. It was 48 hours long but at the end you will feel you know the answer to this question.