Stefana

Meet Stefana, who glows sixteen times brighter every time her temperature doubles.

Every surface above absolute zero radiates energy. Josef Stefan found from experiments in 1879, and Ludwig Boltzmann derived from thermodynamics in 1884, that the power radiated grows as the fourth power of absolute temperature: P = εσAT⁴. Here σ is the Stefan–Boltzmann constant, about 5.67×10⁻⁸ W·m⁻²·K⁻⁴, and ε is the emissivity, 1 for a perfect blackbody and less for real surfaces.

The fourth power makes temperature hugely important. Doubling T in kelvin multiplies the power by 2⁴ = 16. A perfect blackbody at room temperature, 300 K, radiates about 460 watts per square metre; at 600 K, about 7,300.

Astronomers combine this law with a star's temperature and luminosity to work out its radius. It also governs Earth's energy balance: the planet sheds heat to space as infrared.

The law gives the total power emitted. An object also absorbs radiation from its surroundings, so the net loss depends on both temperatures.