Temperature is a measure of how much energy there is in a substance. Even when we think something is very cold, such as a piece of metal left outside on a very cold winter day, the atoms of that piece of metal continue to vibrate within their molecules. They still have a lot of energy. Examine the scale of temperatures on the right. It's measuring the range of temperatures that things can have. The units are primarily in degrees Kelvin, which is a scale used for convenience, based on where 'absolute zero' is. Absolute zero is the temperature at which all motion within a substance stops. Nothing we know of can get that cold on its own, not even in outer space. Scientists, however, are able to achieve temperatures of a fraction of a degree above absolute zero using artificial cooling methods. Absolute zero turns out to be -273 degrees Celsius. For simplicity, the Kelvin scale defines this point as zero degrees Kelvin; any Kelvin temperature is then equivalent to a Celsius temperature of 273 degrees less. Room temperature, for example, is 293 deg K, or 20 deg C. The next thing to notice about the scale is that it is logarithmic. It goes up by powers of 10 (from 1 degree upwards). Each level up is ten times hotter than the previous level. This is necessary on a diagram like this where the range is a huge number. It would be difficult to label the interesting points on a graph you could see, when some of the values are less than 1000 degrees, and others are tens of millions. Steps of 'times ten' make everything fit. Now lets look at some of the interesting points on the scale above. Most of the natural temperatures that occur on the surface of the earth are in the green band. Nowhere does it get cold enough to drop into the blue. (The coldest-ever recorded temperature was -70 deg C, or 203 deg K). Lava from volcanoes may be into the yellow band slightly. When temperatures reach tens of thousands of degrees C, atoms begin to be ripped apart; the electrons and nuclei flow together as 'plasma'. Eventually, when the temperature reaches many hundreds of millions of degrees, the nuclei themselves come apart. This only happens in the hottest stars where the pressure is also very high. Resources Content, HTML, graphics & design by Bill Willis 2023