Infrared light is invisible to the human eye, but it's extremely valuable for studying the universe because:

* It can penetrate dust clouds: Unlike visible light, infrared can pass through the dust and gas clouds that often obscure our view of stars and galaxies.

* It reveals cooler objects: Infrared light is emitted by cooler objects, like planets, brown dwarfs, and even cool stars that don't shine brightly in visible light.

* It provides information about temperature and composition: By analyzing the wavelengths of infrared light, astronomers can learn about the temperature and chemical composition of celestial objects.

Spitzer was particularly adept at studying the far-infrared part of the spectrum, which allowed it to see even colder objects and penetrate even thicker dust clouds.

The Spitzer Space Telescope was decommissioned in January 2020, but its legacy continues to shape our understanding of the universe.