You Live in a Heated Universe

Zeta Ophiuchi is plowing through space.
All objects in the universe emit thermal radiation. For example, this infrared image shows stellar winds flowing out from a fast-moving star called Zeta Ophiuchi. Those winds create a "bow shock" in surrounding clouds of gas and dust. The interaction causes the clouds to glow brightly in infrared light. This is similar to the motion of stars in the center of the Milky Way Galaxy. NASA/JPL-Caltech

Thermal radiation sounds like one a geeky term you'd see on a physics test. Actually, it's a process that everyone experiences when an object gives off heat. It is also called "heat transfer" in engineering and "black-body radiation" in physics.

Everything in the universe radiates heat. Some things radiate much MORE heat than others. If an object or process is above absolute zero, it's giving off heat. Given that space itself can be only 2 or 3 degrees Kelvin (which is pretty darned cold!), calling it "heat radiation" seems odd, but it's an actual physical process. 

Measuring Heat

Thermal radiation can be measured by very sensitive instruments — essentially high-tech thermometers. The specific wavelength of radiation will entirely depend on the exact temperature of the object. In most cases ,the emitted radiation isn't something you can see (what we call "optical light"). For example, a very hot and energetic object might radiate very strongly in x-ray or ultraviolet, but perhaps not look so bright in visible (optical) light. An extremely energetic object might emit gamma rays, which we definitely can't see, followed by visible or x-ray light.  

The most common example of heat transfer in the field of astronomy what stars do, particularly our Sun. They shine and give off prodigious amounts of heat. The surface temperature of our central star (roughly 6,000 degrees Celsius) is responsible for the production of the white "visible" light that reaches Earth. (The Sun appears yellow due to atmospheric effects.) Other objects also emit light and radiation, including solar system objects (mostly infrared), galaxies, the regions around black holes, and nebulae (interstellar clouds of gas and dust). 

Other common examples of thermal radiation in our everyday lives include the coils on a stove top when they are heated, the heated surface of an iron, the motor of a car, and even the infrared emission from the human body.

How it Works

As matter is heated, kinetic energy is imparted to the charged particles that make up the structure of that matter. The average kinetic energy of the particles is known as the thermal energy of the system. This imparted thermal energy will cause the particles to oscillate and accelerate, which creates electromagnetic radiation (which is sometimes referred to as light).

In some fields, the term "heat transfer" is used when describing the production of electromagnetic energy (i.e. radiation/light) by the process of heating. But this is simply looking at the concept of thermal radiation from a slightly different perspective and the terms really interchangeable.

Thermal Radiation and Black-body Systems

Black body objects are those that exhibit the specific properties of perfectly absorbing every wavelength of electromagnetic radiation (meaning that they would not reflect light of any wavelength, hence the term black body) and they also will perfectly emit light when they are heated.

The specific peak wavelength of light that is emitted is determined from Wien's Law which states that the wavelength of light emitted is inversely proportional to the temperature of the object.

In the specific cases of black body objects, the thermal radiation is the sole "source" of light from the object.

Objects like our Sun, while not perfect blackbody emitters, do exhibit such characteristics. The hot plasma near the surface of the Sun generates the thermal radiation that eventually makes it to Earth as heat and light. 

In astronomy, black-body radiation helps astronomers understand an object's internal processes, as well as its interaction with the local environment. One of the most interesting examples is that given off by the cosmic microwave background.  This is a remnant glow from the energies expended during the Big Bang, which occurred some 13.7 billion years ago. It marks the point when the young universe had cooled enough for protons and electrons in the early "primordial soup" to combine to form neutral atoms of hydrogen. That radiation from that early material is visible to us as a "glow" in the microwave region of the spectrum.

Edited and expanded by Carolyn Collins Petersen