Every Object Emits Light Based on Its Temperature

The warmth you feel from the sun or a campfire is a direct experience of a universal phenomenon. All matter with a temperature above absolute zero is composed of atoms in constant motion, and this jiggling of charged particles results in the emission of electromagnetic energy, known as thermal radiation. For objects at everyday temperatures, this energy is primarily released as infrared radiation, which is invisible to our eyes but can be detected by thermal imaging cameras. This is why even objects that are not visibly glowing, including the human body, are constantly radiating energy.

The journey to understanding this process began in 1800 when astronomer William Herschel discovered an invisible form of light beyond the red end of the spectrum, which he called "radiant heat" and we now know as infrared. This discovery laid the groundwork for later scientists. In the late 19th and early 20th centuries, physicists like Max Planck studied the idealized concept of a "black body"—a perfect absorber and emitter of radiation. Planck's revolutionary work to mathematically describe the observed spectrum of black-body radiation was a pivotal moment in physics, as it could not be explained by classical theories and helped launch the development of quantum mechanics.

As an object's temperature increases, the radiation it emits shifts to higher frequencies and shorter wavelengths. When the temperature reaches about 525 degrees Celsius (977°F), often called the Draper point, it begins to emit enough energy in the visible spectrum for us to perceive a dim red glow. If the temperature continues to rise, the color will shift from red to orange, yellow, and eventually white or even blue for extremely hot objects like some stars. This predictable shift in the color of emitted light with temperature is a fundamental principle that allows astronomers to determine the surface temperature of distant stars.