A good question! Here's a similar one - how do we know what the Earth is like inside/ I mean all the way inside, not just a couple hundred feet or so? The reason I ask this one back is because the method we use on the Sun is essentially the same as the method we use on the Earth to find out what the insides look like without digging all the way to the core. The method is sound waves! Let me talk first about the Earth, because it is maybe more familiar. A sound wave is just an oscillation in pressure in any kind of material - gas, liquid or solid (this is why we can hear, because of small pressure changes that our earsdrums pick up). Something like an earthquake on the Earth, or a seismic blast set off deliberately, sends these pressure/sound waves running through the inside of the Earth. The strength of the waves is not very great, but they can be large enough to detect on the other side of the planet, in many different places. Depending on what kind of material those waves traveled through, and how far they had to go, they can arrive a bit sooner or a bit later, a bit weaker or a bit stronger. By combining the information on an earthquake wave from all different parts of the planet, you can work out (with some fancy math) not only where the Earthquake happened, but what kind of substance the earthquake wave traveled through - in other words what kind of substance is inside the planet. This is called seismology. Now in the case of the Sun it is a bit more complicated. There are such things as sunquakes, when a large solar flare happens, but that is not very often. However, the whole sun is vibrating all the time because of its own random gas motions. Again the size of these vibrations is teeny-tiny, but with sensitive enough instruments they can be measured across the whole sun. Then the vibrations from one part of the sun are compared with vibrations on every other part, to see whether they are related and whether it is the same waves giving rise to both. Then if you find two spots on the sun which are vibrating in sympathy then you can use some even more fancy math to work out how the wave is traveling inside the Sun between these two spots, which leads on to what the Sun is like inside. This is called helioseismology. hope this helps! Lyndsay