That's a great question that took surprisingly long into the history of physics to figure out! I'll skip the historical details (although you should read about it here) and cut to the chase: It turns out that everything with a non-zero temperature emits light all the time. This isn't necessarily the type of light that your eyes can detect, but it's there all the same. It was discovered experimentally long before it was explained theoretically (it took until the year 1900), but now we have a pretty good understanding of what's happening inside things when they glow. In fact, it's the same reason that a hot object in a fire will begin to glow red, and why fire itself is orange at the top.
This radiation is called Blackbody Radiation, named so because the form of the radiation is derived by assuming that the body is completely black, i.e., it absorbs all incoming light and doesn't scatter or reflect any. The only light that is emitted is actually generated inside the object, not just reflected from a nearby flashlight. The useful thing about blackbody radiation is that the spectrum of light that's emitted is determined by the temperature and nothing else. That means that we can measure the spectrum of emitted blackbody radiation from an object and immediately know its temperature.
There are some caveats though; some materials don't play by the rules, instead choosing to emit light that's not well-described by the blackbody radiation spectrum. Luckily for us, the sun is one of the most ideal blackbody radiators that we've discovered: that means our estimate of the temperature (on the surface anyway) is really quite accurate.
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