How the primary colours of light combine.
Credit: SharkD

Colour tells us about the light that an object reflects. When light hits an object, some is absorbed and some is reflected. Objects get their colour from the wavelengths of light they reflect. For example, a red object absorbs all colours other than red. It reflects the red light, so it looks red.

There are 3 primary colours of light: red, green and blue. We also often talk about the spectrum of visible light. This includes red, orange, yellow, green, blue, indigo, and violet. White light is made up of all the colours of the spectrum. So objects that look white reflect all the wavelengths of visible light. Objects that look black absorb all the wavelengths of visible light.

Visible light is the part of the electromagnetic spectrum that our eyes can see. It is wavelengths of light from 400 to 700 nanometres (nm). 1 nm is 1 millionth of a millimetre!

Ask an astronomer what colour the star Antares is, and they might give an odd reply. Rather than saying 'it's red' they might tell you its colour is 1.83. This is because astronomers define colour as the difference in brightness between 2 distinct 'bands'. A band is a range of wavelengths which represent a colour. For example, the B-band lets through wavelengths of light from about 400 nm to 500 nm. Light that our eyes would see as blue. The most common bands are U, B, V, R and I. 

Many telescopes have filters in front of the camera which let through distinct bands of light. The filters can be changed to suit the observation. So to measure the colour of Antares, we could observe it using a B filter and then a V filter. The difference in brightness between the two observations would be 1.83. This tells us its colour is towards the red end of the visible light spectrum.

Astronomers can create amazing colour pictures of objects in space. Objects like star-forming nebulae or supernovae relics. They do so by combining 3 images of the object taken using different filters. We call these kinds of pictures 3-colour images or representative colour images.

Astronomers can also add false colours to images. These pictures do not show the true colours of an object, but they can help our eyes see more details.