Description: All objects emit electromagnetic radiation in a way directly related to their temperature. This radiation is called thermal radiation. For high enough temperature, some of the thermal radiation is visible to the naked eye, e.g.: a hot stove-top, a heating element, or a poker in the fire, glowing red; molten metal glowing yellow-white; the blueish-white glow during certain welding processes, so bright welders require eye protection. For cooler objects, we do not see the glow, but these objects emit thermal radiation in the infrared range or as radio waves.

The simplest form of thermal radiation is "blackbody radiation": Thermal radiation emitted by an ideal object that absorbs all incoming radiation, regardless of wavelength. The spectrum of that radiation (the information about how the radiation energy is distributed between the different wavelengths) is governed by a mathematical function called the Planck curve, which only depends on the object's temperature. The Planck curve also specifies that the total energy output of thermal radiation steeply increases with temperature.

The radiation we receive from stars is thermal radiation that closely follows the Planck curve. This, in turn, makes it possible to assign an "effective temperature" to each star. The Sun, with a temperature of 5500 degrees Celsius (5780 kelvins) shines in a range of colors that add up to what we define as white. Stars with a lower effective temperature appear reddish, such as red giants or red dwarfs. Hotter stars appear blueish and are typically very bright. At temperatures of tens of thousands or even millions of degrees, most thermal radiation is emitted in the ultraviolet, X-ray, or even gamma ray regions.

Thermal radiation is related to the concept of thermal equilibrium. Objects are in thermal equilibrium if they have the same temperature. Thermal radiation is a consequence of objects striving towards thermal equilibrium with the electromagnetic fields permeating all of space.

Related Terms:

• Blackbody Radiation
• Electromagnetic Radiation
• Infrared (IR)
• Magnetic Field

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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