|MadSci Network: Astronomy|
Most temperatures in an astronomical setting are measured by the peak wavelength of light that the object emits. This does give us a feel for the surface temperature only, of course, since it is the surface of the object which we see emitting the light. The radiation is called "blackbody radiation". Asteroids are very cool objects, so the peak of their radiation would be in the infrared. If we were to take a spectrum, and then fit it with a blackbody model we could see what temperature the asteroid is at. [Moderator's note: asteroids are often used as "calibration sources" for infrared telescopes!] Comets in general are also cool; however they do heat up as they approach the sun. Still, they also emit the majority of their radiation in the infrared. In contrast, the sun's surface is at 5800K and the peak of its blackbody curve is in the optical - greenish yellow, to be more precise, and that is probably why our eyes have evolved to see best at those wavelengths. The core temperatures of such objects are not directly measured but calculated by complicated radiative transfer codes. A website which has a java applet that shows you blackbody spectra and discusses them briefly is at: http://javalab.uoregon.edu/dcaley/prf/PRF_plugin.html (There are other java applications at: http://javalab.uoregon.edu/dcaley/java.html) Another java applet which has some of the parameters that would go into a model for the calculation of the interior temperature is the build-an-asteroid webpage at: http://home.att.net/~eepalmer/Build/build.html
Try the links in the MadSci Library for more information on Astronomy.