Since this weekend marks the 20th anniversary of the launch of the Hubble telescope, I thought it would be worthwhile to explain the false-colour images taken by the Hubble’s cameras — why they’re in false colour, what the colours represent, and how it’s done. False-colour astrophotography is not unique to Hubble; it’s used at observatories on the ground around the world, and by professional and amateur astronomers alike. I could do it myself, down the road. So here’s my best stab at explaining it.
Visible light represents only a small piece of the total electromagnetic spectrum, which ranges from from high-frequency, high-energy and short-wavelength gamma rays to low-frequency, long-wavelength and low-energy radio waves. Tucked between the ultraviolet and the infrared, visible light is, by definition, what part of the spectrum detectable by the human eye: wavelengths between about 380 nanometres (violet) and 760 nm (red) — or, to put it another way, frequencies between 400 terahertz (red) and 790 THz (violet). In a nutshell, each colour we perceive is simply light at a specific wavelength.
(For reasons that will become clear later on, I’m going to talk about wavelengths in nanometres, rather than frequencies in terahertz.)