1893

Star Color = Star Temperature

Gustav Kirchhoff (1824–1887), Max Planck (1858–1947), Wilhelm Wien (1864–1928)

Physicists made major advances in the second half of the nineteenth century in understanding light and energy. For example, the German physicist Gustav Kirchhoff developed fundamental equations describing the way a hypothetically perfect light-absorbing object called a blackbody should emit energy as electromagnetic radiation at a particular temperature. He found that blackbodies at the temperatures of typical real-world objects radiate a continuous spectrum of energy, from the long-wavelength part of the radio and infrared spectrum to the higher-energy, shorter-wavelength visible and ultraviolet wavelengths.

The German physicist Wilhelm Wien expanded upon those ideas, and in 1893 derived a simple relationship, now called Wien’s law, that showed that the peak wavelength of the energy being emitted by an object is inversely proportional to its temperature. That is, hotter objects emit most of their energy at shorter UV and visible wavelengths; cooler objects emit mostly in the infrared. Max Planck, another German physicist, would further expand on these ideas about blackbodies and light and help to create the field of Quantum Mechanics.

Astronomers took advantage of this new understanding of light and energy to begin to understand objects that they could observe visually. Specifically, Wien’s law helped astronomers deduce the relative temperatures of the stars: Hotter stars should be emitting more of their energy at shorter wavelengths and thus should appear bluer, and cooler stars should be emitting most of their energy at longer wavelengths, with their spectra peaking toward the yellow, orange, and red end of the spectrum. Our Sun, for example, is a yellowish star, putting it near the average to slightly cooler-than-average end of the stellar color scale.

The colors of the stars thus became a key observational parameter that could be used to begin to classify them according to temperature and, thus, in the twentieth century, to develop a systematic understanding of their origin, evolution, inner workings, and ultimate fate.

Hubble Space Telescope photo of part of the globular star cluster Omega Centauri (NGC 5139), a cluster of more than 10 million stars all bound together gravitationally. The wide range of star colors indicates a wide range of temperatures, from blue/white (hottest) to orange/red (coolest).