Kelvin Scale:

Obviously, it is possible to freeze water and stay cooling it down or boil it all away into vapor and then continue heating it up. Temperatures can plunge far below 0°C and can increase far above 100°C. Are there limits to how low or how high the temperature can get?

Fascinatingly, there is an absolute limit to how low the temperature in degrees Celsius can become, though there is no limit on the upper end of the scale. We may take unexpected efforts to cool a chunk of ice down to see how cold we can make it, though we can never chill it down to a temperature any lower than around 273 degrees Celsius below zero (-273°C). This is termed as absolute zero. Object at absolute zero cannot transfer energy to anything else since it possesses no energy to transfer. There is supposed to be no such object in our universe, however some atoms in the vast reaches of intergalactic space come nearer.

Absolute zero is the basis for the Kelvin temperature scale (K). A temperature of -273.15°C is equivalent to 0 K. The size of the Kelvin degree is similar as the size of the Celsius degree, therefore 0°C = 273.15 K, and + 100°C = 373.15 K. Note that the degree symbol is not used with K.

On high end, it is possible to keep heating matter up indefinitely. Temperature in the core of star rises into the millions of degrees Kelvin. No matter what the real temperature, the difference between the Kelvin temperature and the Celsius temperature is forever 273.15 degrees.

At times, Celsius and Kelvin figures can be consider equivalent. Whenever you hear someone say that a specific star's core has a temperature of 30 million K, it means the same thing as 30 million °C for the purposes of many discussions since 273.15 is a negligible difference value relative to 30 million.