What is the surface of the Sun made of?

The sun itself is made of gas, mostly hydrogen and helium, and so does not have a solid surface like you would find on Earth or Mars.

However, if you look at a photo of the Sun, you can see it has a definite edge to it. This is not because the gas stops at this point, it actually carries on diffusely for thousands of kilometres.

The Sun is hotter in the centre and cooler towards the outside, and there is a point where the hot gas becomes cool enough to become transparent; this point is what we see as the surface of the Sun.

The temperature at the surface of the Sun is about 5000 degrees Celsius; inside it's millions of degrees. This is sufficiently hot for nuclear fusion reactions to occur inside the Sun; these are much like nuclear bombs going off all the time, releasing huge amounts of energy.

There is also material streaming off the surface of the Sun all the time, which forms the "Solar Wind". This charges past the Earth at about 1.5 million kilometres per hour, deflected past us by our magnetic field. This interaction also gives rise to the Aurora borealis and the Aurora australis, the "Northern Lights" and "Southern Lights" respectively.