Blackbodies in Astrophysics
Thermal Electromagnetic Radiation from an Ideal Blackbody Radiator
The wavelengths of the electromagnetic radiation emitted by an ideal blackbody radiator depends only on the temperature.
Ideal Blackbody Radiator
An ideal blackbody radiator is defined as an object that absorbs all the electromagnetic radiation that strikes it. The electromagnetic radiation includes the entire electromagnetic spectrum: gamma rays, X rays, ultraviolet light, visible light, infrared radiation, and radio waves of all frequencies.
An ideal blackbody radiator is an idealized concept that probably does not actually exist. The idealization is however a useful concept in physics. There are many objects in nature that are very good approximations of ideal blackbody radiators. Describing the properties of an ideal blackbody, helps us understand the properties of any object that is a good approximation of an ideal blackbody.
The sun and other stars are an example from astrophysics of objects that are good approximations of ideal blackbodies.
Properties of Blackbodies
The most important property of blackbody radiators is the way in which they emit electromagnetic energy. Blackbodies in thermal equilibrium emit energy to balance the energy they absorb and remain at a constant temperature. This energy is in the form of electromagnetic radiation.
The wavelengths at which an ideal blackbody emits electromagnetic radiation depends on the temperature and absolutely nothing else. That is why the radiation emitted by an blackbody is often called thermal radiation. A radiating blackbody emits energy at all wavelengths and the hotter a blackbody is, the more total energy it emits.
Most importantly, A hotter blackbody will emit most of its electromagnetic energy at shorter wavelengths. Wien's law states that the wavelength in nanometers at which the most energy is emitted equals 3,000,000 divided by the temperature in degrees Kelvin.
Even though the wavelengths at which the energy is emitted depends only on the temperature, the total amount of energy emitted depends on the surface area of the blackbody. If two ideal blackbodies are at the same temperature, they will emit electromagnetic radiation at the same wavelengths, but the larger one will emit more total energy.
Applications to Astrophysics
Stars are very good approximations of ideal blackbodies. Blue light has a shorter wavelength than red light. Hot stars are therefore blue to blue-white in color. Only the very hottest stars emit large amounts of ultraviolet radiation.
Cool stars on the other hand are red in color and the coolest stars emit significant amounts of infrared radiation.
Planets with temperatures comparable to Earth and objects on the Earth at room temperature emit most of their energy at infrared wavelengths.
The big bang theory for the origin of the universe predicts that in the nearly 15 billion years since the unimaginably hot big bang the universe has cooled to about 3 degrees above absolute zero (3K). Microwaves are a specific wavelength of radio waves, which have a longer wavelength than visible or infrared light. A blackbody at 3K will emit most of its energy as microwaves. The universe therefore emits the 3K blackbody microwave cosmic background radiation. This background radiation was discovered by Penzias and Wilson in the 1960s.
Ideal blackbodies may not actually exist in nature, but many objects are good approximations. The properties of ideal blackbodies help us understand these objects that approximate ideal blackbodies.
