|A selection of Kelvin temperatures|
|Absolute zero||0.00 K|
|Cosmic Background Radiation||2.73 K|
|Freezing point of hydrogen||13.97 K|
|Boiling point of hydrogen||20.41 K|
|Surface of Pluto (mean)||53 K|
|Freezing point of water||273.16 K|
|Surface of Earth (mean)||288 K|
|Boiling point of water||373.16 K|
|Surface of Mercury (mean)||445 K|
|Freezing point of iron||1,811 K|
|Boiling point of iron||3,110 K|
|Surface of a red (M-type) star||< 3,500 K|
|Surface of the Sun||c. 5,700 K|
|Surface of a W-type star||> 50,000 K|
A scale of temperature measurement based on the absolute zero point, which lies at -273.16° on the more familiar Celsius scale.
The Celsius scale uses the freezing and boiling points of water as its baselines, and divides the scale between these two temperatures into exactly one hundred units. The Kelvin system retains these units, so that there are one hundred degrees between ice and steam on both systems. The Kelvin system, though, uses a different, absolute, zero point.
The temperature of a substance is the result of the relative motion of the molecules that make it up; the more active the molecules, the higher its temperature. Above 100°C water molecules are very active, their motion is random and rapid, and so water takes on a gaseous form: steam. Below 100°C, the same molecules show much less motion. In fact, they pack themselves into a regular structure to give the water a solid form: ice. The water molecules in a block of ice, though, are by no means motionless. As the molecules show less and less motion, the temperature becomes colder and colder. We need to use negative numbers to represent this on the Celsius scale.
If temperature arises from molecular motion, then it follows that there must be a minimum possible temperature: the
point at which that motion stops altogether. This point is absolute zero, the coldest temperature possible,
273.16°C below the freezing point of water, and the baseline of the Kelvin scale.