Measuring Extreme Temperatures
It is easy to measure temperatures that occur naturally on the surface of the earth. From the temperature of the air on a warm day, to the temperature of snow collected on the ground, a simple mercury thermometer can give a reading within a degree of accuracy. But what about temperatures near 0 Kelvin, and as high as the surface of the Sun? Obviously, 0 Kelvin cannot be found anywhere on the earth (the lowest temperature recorded on Earth is around 183 K), and even if it could, a mercury thermometer would not be able to reach such a low temperature because the mercury cannot read measurements anywhere near 0 K, and a thermometer cannot be taken to the surface of the Sun, no matter the type or brand.
When attempting to determine the temperature of the Sun, an understanding of what the body is is required. Our sun is a giant nuclear furnace, fusing Hydrogen to Helium (converting Hydrogen into Helium) atoms together at its core. As this furnace burns, it is a huge source of light. As this light passes from the core outward, through mostly Hydrogen, it bounces in every direction randomly, eventually reaching the surface. What we consider to be the surface is the distance from the core that we see the outline of the Sun; the location of the final bounce within the hydrogen. The surface of our Sun is not like the surface of the Earth; it is more like a translucent globe with the core being a light bulb at the center. This is also the reason that the Sun appears larger than the core that is producing its light; as in our example, the light bulb is the source of light rather than the globe itself. Knowing this, the temperature can be estimated mathematically by considering that the fusion produced within the core yields light at every wavelength at intensities proportional to the surface's temperature. This not only gives us a way to measure the surface temperature at any distance, it verifies that our Sun does in fact perform fusion in its core. Using this method, the surface of our Sun is estimated to be near 5880 Kelvin.
In order to measure temperatures near 0 Kelvin, one technique used is to measure the Kinetic Energy that is present by measuring the attributes of the magnetic and superconducting materials in material being examined. This technique is very complex and is based on attempting to reach a temperature that does not naturally exist in nature. The Physics of calculating such a number require highly unnatural circumstances, which in themselves require a very high level of knowledge in Physics. For instance, Hydrogen does not freeze in nature; though it is possible to freeze every element of the periodic table (hence, absolute 0), to do so requires much more than what is required to freeze water (H2O).
In laboratory experiments, it is possible to calculate a freezing point of a periodic element; furthermore, trial and error is not the only technique when attempting the find the freezing point of any element. As an example, water (H2O) freezes at 0 degrees Celsius, (273.16 Kelvin). This can be determined by taking the molar mass of a specific element - what the element is made up of - and determining a what temperature the element will freeze by plotting the two temperature points on a graph and calculating the intersection point. Though this method does not require the physical testing of such a low temperature, it uses realistic equipment, which can be used to plot numerous calculations to maximize the precision.
Whether measuring the extremely low temperatures, or measuring extremely low temperatures, it is possible to use our understanding of the known universe to expand our understanding of what would be extremely difficult, if not impossible to test directly. Distances in space are much the same way. In order to estimate the distance to Alpha Centauri, attempting to build an "AU-Ruler" would be prone to failure; but the use of what we already know, the revolving of the Earth around the Sun, allows for further knowledge to be gained.
