Re: How do I measure extremely high and low temperatures?

First, a couple thermal physics basic facts for anyone who reads this
question and response to set the stage:

1)  Absolute Zero (0 Kelvin) is unattainable by any finite number of steps
in a cooling process.  This is known as the Third Law of Thermodynamics.
1a)  Associated grammar fact:  Since Kelvin is the name of the unit in the
absolute temperature scale, it's just "Kelvin" and not "degrees Kelvin." 
Celsius and Fahrenheit are the names of temperature scales, and the units
of variation in these scales are degrees, hence "degrees Fahrenheit" and
"degrees Celsius."  Almost no one, even experienced physicists, gets this
right.

2)  There's really no upper limit on temperature.  10 million Kelvin is
hot, but there are stars that are hotter and explosions (supernova) that
make 10 million Kelvin look cool in comparison.

The basic process of measuring such extreme temperatures is the same, to
observe light either reflected from or emitted by such matter.  To observe
extremely cold matter (nanoKelvin), we observe how the matter disperses
after it's cooled.  The speed of the atoms is a direct measurement of the
temperature.  We can't touch the cooled matter with a thermometer, because
the thermometer itself would heat it...so we have to cool the matter in a
vacuum and just watch it.  As we go up in temperature, we can apply
physical probes to matter.  We know the relationship between electrical
conductivity and temperature for a variety of materials.  For very cold
materials (liquid helium, etc...), we use small probes at the end of very
long, thin wires (so as not to heat the material we want to measure the
temperature of).  This requirement is situation-dependent and mostly just a
matter of proper engineering of experiments.  Once matter is hot enough
that it emits a spectrum of light that we can easily observe, we can simply
look at the light that it gives off.  This can be from microwaves (cosmic
background radiation tells us the basic temperature of the universe...about
4 Kelvin) to x-rays and gamma-rays (for high-energy exploding matter in
supernova or for accretion discs surrounding black holes).  The basic
blackbody spectrum of radiation emitted tells us very accurately the
temperature of the matter we're observing.  There's enough overlap in these
methods to measure matter at practically any achievable temperature.  But
at very low and very high temperatures the "thermometers" we use aren't
directly in contact with the hot material, they're observations.  Any
precautions?  Yes, matter at low and high temperatures can create
hazards...but if you're simply observing matter from a distance you're
pretty safe.

The measurement of temperature is one of the newer and more transforming of
the fundamental measurements in physics (length and time came much sooner).
 "A Matter of Degrees" by Gino Segre is an excellent popular book on the
subject.  Technical information on temperature measurement is a vast field
of information, and depends largely on what you're trying to measure the
temperature of.  A Google search of "measuring the temperature of X," where
"X" is what you want to measure the temperature of, is a good start.