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Thermal noise (or Johnson-Nyquist noise) is randomness added to a signal passing through any electronic component due to the component's heat. The heat (i.e., movement in the atoms) affects the flow of electricity by agitating the electrons that are passing along the flow of the charge. Thermal noise varies by component design, but at least a portion is always directly proportional to the temperature of the component during operation. The most sensitive radio telescopes depend upon cooling key circuits to cyrogenic temperatures to keep the thermal noise low, so as to allow extremely small signals to stand out sufficiently from the noise.
In electronics (thus, radio astronomy), random modifications to the received signal such as thermal noise are often quantified using temperature units (i.e., citing an equivalent noise temperature for each source of noise, such as an antenna temperature). The variations that constitute signal itself are also quantified in this manner, yielding one means of characterizing the signal-to-noise ratio.