What determines what type of light an object in space will emit? For most stars, planets and dust in the Universe the type of light they give off is determined by the temperature of the object. The Sun's surface is about 10,000 degrees Fahrenheit and emits most of its energy in the visible part of the spectrum. Hotter stars emit light at shorter wavelengths and cooler stars give off most of their light at longer wavelengths. The infrared light that SPIREX (South Pole Infrared Explorer) observes has a wavelength of four times that of yellow light. That means that SPIREX sees the light of objects that are about 1/4 as cool as the Sun or 2,500 degrees Fahrenheit. Other infrared telescopes can see things at far infrared wavelengths one hundred times longer. These objects are as cold as 25 degrees Fahrenheit.
Cool stars, planets and dust emit light at infrared wavelengths. In particular, SPIREX is tuned to be able to see objects that are a couple of thousand to a few thousand degrees. These temperatures are far cooler than typical stars. Only the coolest stars we know emit at these temperatures. Stars being born should glow at these wavelengths. Young galaxies are expected to have billions of such stars. There may also be missing link objects called brown dwarves that are somewhere between the categories of stars and giant planets. SPIREX even saw the explosions that took place on Jupiter when pieces of the Shoemaker-Levy 9 struck in July 1994.
Infrared astronomers face two difficult problem that optical astronomers don't have. Most wavelengths of infrared light have trouble penetrating Earth's atmosphere. This is primarily because molecules in the air block the infrared light from space. To make matters even worse, the atmosphere and even the telescope emit infrared light since they are in the temperature range of things that give off infrared light. To get around these problems, infrared astronomers fly their telescopes in aircraft and on spacecraft to get above warm, moist air. On the surface of the Earth, the South Pole is the coldest and driest site we astronomers can use and therefore the best location for infrared astronomy on the planet.
From "What is Infrared Astronomy?", by J.S. Sweitzer, dated 11/22/94.