What Does a Star's Colour Mean? Temperature Written in Light

Any hot object emits light across a range of wavelengths, with the distribution depending on temperature, and the wavelength at which the emission peaks shifts towards the blue as the temperature rises. A cool object emits mostly at long wavelengths and appears red, and a hotter one shifts its peak through yellow and white towards blue. That relationship is a fundamental property of thermal radiation rather than anything specific to stars, and it applies equally to a heated iron bar, which glows red, then orange, then white as it is heated further. Stars approximate this behaviour closely enough that measuring the colour gives the surface temperature directly, and the range runs from around three thousand degrees for the coolest visible stars to tens of thousands for the hottest.

Turning an impression into a number requires a defined procedure:

• •Brightness is measured through standard filters passing defined bands of wavelength
• •The difference between the magnitudes in two bands is the colour index, which is a single number
• •A negative index means bluer and hotter, and a positive one means redder and cooler
• •The index must be corrected for interstellar dust, which reddens light and mimics a cooler star
• •Spectroscopy gives a better temperature by measuring which absorption lines are present and how strong
• •The spectral classification sequence, running through letters that are not alphabetical for historical reasons, orders stars by exactly this

Stars are conspicuously less colourful to the eye than photographs suggest, and the reason is in the observer rather than the stars. Colour vision depends on cone cells which require reasonably bright light, and at the light levels stars provide the eye relies largely on rod cells which do not distinguish colour, so most stars appear white or nearly so. Only the brightest show colour readily, with a few red giants and blue stars distinguishable without equipment, and binoculars help by gathering more light. Long-exposure photography accumulates light the eye cannot, which is why astronomical images show colours no observer sees. That gap is worth knowing, since a beginner comparing the view through a telescope with a published photograph is comparing two different things and is generally disappointed.

Colour is the crudest thing a star's light reveals and spreading it into a spectrum gives much more. Dark lines at specific wavelengths identify which elements are present in the outer layers, since each element absorbs at a characteristic set of wavelengths, which is how the composition of objects nobody can visit became known. The widths of those lines indicate pressure and therefore whether the star is a giant or a dwarf, which separates objects of the same temperature. A shift of every line towards longer or shorter wavelengths gives the speed towards or away from us, which is how binary companions are detected and how the expansion of the universe was found. Regular variation in that shift betrays orbiting planets. All of it comes from the same light that gives the colour, read more carefully.

Plotting colour against brightness for a population of stars produces the central organising diagram of stellar astronomy. Most stars fall along a diagonal band called the main sequence, running from hot bright stars at one end to cool faint ones at the other, and a star's position on that band is determined almost entirely by its mass. Above the band sit giants and supergiants, which are cool and bright and therefore must be large. Below sit white dwarfs, which are hot and faint and therefore small. A star moves across the diagram as it evolves, which means the distribution of a cluster's stars records its age, since the most massive leave the main sequence first. The diagram was constructed independently by two astronomers around 1910 and carries both their names.

The wavelength of peak emission shifts towards blue as temperature rises, which is a general property of hot objects, so colour measures surface temperature directly. The difference in brightness through two filters gives a single number, corrected for dust which reddens and mimics a cooler star. Stars look washed out because colour vision needs more light than they supply.