They can swell up to 100 times their original size and have temperatures which range from 2000 to 3500 K. Red giants radiate most intensely in the near-infrared region. Red dwarfs are the most common of all stars. They are much smaller than our Sun and are the coolest of the stars having a temperature of about 3000 K which means that these stars radiate most strongly in the near-infrared. Many of these stars are too faint in visible light to even be detected by optical telescopes, and have been discovered for the first time in the near-infrared. As we enter the mid-infrared region of the spectrum, the cool stars begin to fade out and cooler objects such as planets, comets and asteroids come into view.
Planets absorb light from the sun and heat up.
They then re-radiate this heat as infrared light. This is different from the visible light that we see from the planets which is reflected sunlight. The planets in our solar system have temperatures ranging from about 53 to 573 degrees Kelvin. Objects in this temperature range emit most of their light in the mid-infrared. For example, the Earth itself radiates most strongly at about 10 microns.