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How do we know that nuclear fusion is still going on in the Sun?

The process of nuclear fusion is very slow, and when photons reach the conduction zone, the Sun eventually disperses tons upon tons of energy. I've heard that since the photons disperse sporadically and slowly, it could take a million years or so for that energy to reach the Earth. If it takes a million years for the Sun's energy to reach the Earth, why would it matter if the Sun would stop working for one hour or even for one day? Whatever energy the Sun is producing right now wouldn't be seen for a million years anyway according to this.

We know that the Sun is still doing nuclear fusion right now because it hasn't exploded, right? Is there any other reason besides the exploding Sun as to why we know the Sun is still doing nuclear fusion right now, as opposed to it stopping a million years ago? Just because we can "see" it (the Sun, the light) right now doesn't necessarily prove the Sun is still doing nuclear fusion if indeed what we "see" is from a million years ago, right? The hard evidence of it exploding if nuclear fusion stopped a million years ago would prove that the Sun is still doing nuclear fusion right this very minute and not a million years ago, right? I hope I understand the process correctly, but that is why I am asking the question, so please help.

Indeed, it is true that light which is generated by nuclear fusion reactions in the center of the Sun takes a very long time to escape and reach Earth. A million years is towards the high end of the estimates I have seen, but it certainly takes many thousands of years. (For a discussion of how this is calculated, see this question from another Ask an Astronomer site.)

The reason it takes so long is simply that the atmosphere in the center of the Sun is very dense and opaque (so opaque that if you were there, you wouldn't be able to see the nose in front of your face!). Therefore, light in the Sun's interior can only travel a tiny distance before it is absorbed and reemitted, or scattered, in a random direction by the surrounding plasma. This means that the light will bounce around randomly for many thousands of years before it's lucky enough to get far enough out to escape the Sun and travel the additional eight minutes across space to reach Earth.

(In fact, the light we see from the Sun is not really even the "same" light that was produced in the core; the light generated by nuclear fusion in the core is primarily X-rays and gamma rays, but it gets absorbed and reemitted by cooler material so many times along the way out that most of what is actually emitted from the Sun's surface is regular old visible light.)

Nonetheless, despite the fact that there is such a long delay between the production of the light and it reaching Earth, we can rest assured that nuclear fusion is still going on in the core of the Sun today. Our main evidence for this probably comes from neutrinos, elementary particles which are produced in nuclear reactions in the center of the Sun and which have been observed using detectors on Earth. Unlike light, neutrinos can pass through the Sun untouched (in fact, they can pass through pretty much anything untouched, as memorialized in a famous poem by John Updike). Because of this, they reach the Earth about eight minutes after they are produced, with the delay being due only to the travel time between the Sun and the Earth. Therefore, observations of neutrinos from the direction of the Sun show that the Sun is undergoing nuclear fusion reactions right up until the present day.

If the fusion reactions in the Sun's core were to suddenly "turn off" for some reason, then there are plenty of ways we'd know about it! The fact that we'd stop receiving neutrinos from the Sun is by far the least drastic. The only reason that the Sun is able to sit stably, without collapsing under its own weight, is that the nuclear reactions in the core are constantly providing the heat and pressure needed to hold it up. Without the nuclear reactions, the Sun would start to collapse. However, it would actually be a long time before we noticed the Sun shrinking; the collapse would not proceed in a "free fall" manner because there would still be a lot of latent heat left in the Sun whose pressure would be able to hold it up for a while, until it cooled off. So the collapse would proceed slowly.

However, even within an hour or a day, my guess is that there would still be some noticeable effects. We'd probably see some strange pulsations at the Sun's surface, due to all the turmoil that would be going on in the Sun's core pushing up on the layers above it (very roughly, the Sun would be undergoing "earthquakes" as its interior structure sloshed around). So while the Sun's fusion turning off would not necessarily cause any kind of explosion or major collapse on a one-hour timescale, I think you are right in assuming that there are plenty of ways we'd see it without having to wait for light from the core to diffuse outwards thousands or even millions of years later.

January 2005, Dave Rothstein (more by Dave Rothstein) (Like this Answer)

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