False-colour astrophotography explained
The Hubble Space Telescope captured this image of the Carina Nebula on February 1 and 2, 2010. The colours follow the so-called Hubble palette, which I describe later in this entry: red represents sulfur, green hydrogen and nitrogen, and blue oxygen. Image credit: NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team (STScI).
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.
But before we can explore what false colour is, and why astronomers use it, we need to explain what true colour is — and that requires us to talk a little bit about light.
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.)