A stars apparent colour is determined primarily by the temperature of its photosphere, because temperature controls the distribution of electromagnetic radiation emitted from the stellar surface. Hotter stars emit proportionally more radiation at shorter wavelengths, causing them to appear blue or blue white, while cooler stars emit proportionally more at longer wavelengths, causing them to appear orange or red. Intermediate temperature stars can appear white or yellowish white.
This relationship follows the approximate behaviour of blackbody radiation and is described by Wiens displacement law. The peak wavelength shifts towards shorter wavelengths as temperature increases. Stars are not perfect blackbodies because their atmospheres contain atoms and ions that selectively absorb radiation, but the overall continuum colour is still strongly governed by effective temperature. Consequently, stellar colour is an important observational indicator of surface temperature and can be used to classify stars and investigate their evolutionary states.
The effective temperature of a star is the temperature a perfect blackbody would need to have to emit the same total energy per unit surface area as the actual stellar surface. It is not necessarily identical to a simple physical thermometer reading from a sharply defined surface because stars have extended atmospheres and their radiation emerges from different depths at different wavelengths. Nevertheless, effective temperature provides a powerful standardised quantity.
Hot blue stars can have effective temperatures exceeding 20000 kelvin and in some cases substantially higher, whereas cool red stars can have temperatures below 4000 kelvin. The Sun has an effective temperature of roughly 5800 kelvin and appears broadly white when viewed from space, although atmospheric scattering can make sunlight appear yellowish from the surface of Earth. Stellar colour therefore depends on both intrinsic spectral energy distribution and the observing environment.
Astronomers quantify stellar colour using photometric measurements through filters that isolate particular wavelength ranges. A colour index is the difference between magnitudes measured in two filters, such as blue and visual or visual and red bands. Because hotter stars emit relatively more short wavelength radiation, their colour indices differ systematically from those of cooler stars. Photometric colour can therefore be used to estimate temperature, classify stars and identify unusual objects.
Interstellar dust complicates this interpretation because dust grains preferentially scatter and absorb shorter wavelength light, making distant stars appear redder than their intrinsic colour. This effect is called interstellar reddening or extinction. Astronomers can estimate and correct for reddening using observations across multiple wavelengths and knowledge of the intervening dust. Consequently, the colour recorded by a telescope is not always the same as the intrinsic colour produced at the stellar surface.
Stellar colour is related to spectral classification, which organises stars according to their spectra and temperature. The familiar sequence O, B, A, F, G, K, M runs from the hottest to the coolest common stellar spectral classes, with additional classes such as L, T and Y used for cooler brown dwarfs and related objects.
O type stars are typically blue and extremely hot, B type stars blue white, A type stars white, F type stars yellow white, G type stars broadly yellow white, K type stars orange and M type stars red. The spectral sequence is not simply a colour chart because spectral classes are determined from detailed absorption line patterns and physical properties as well as continuum temperature. Subclasses numbered from 0 to 9 refine the classification within each major class, with lower numbers representing hotter stars within a class. Luminosity classes further distinguish stars according to surface gravity and evolutionary state.
Surface temperature is not the only factor affecting observed stellar colour. Chemical composition alters the absorption lines and therefore modifies the detailed spectrum. Surface gravity affects atmospheric pressure and line formation, while stellar rotation can broaden spectral features and affect temperature distributions. Magnetic activity can produce emission at particular wavelengths and alter the spectrum. Starspots are cooler regions on magnetically active stars and can change integrated colours as the star rotates.
Binary stars can also produce combined colours that do not correspond neatly to a single stellar temperature because the observed light is the sum of radiation from two or more stars. Dust and gas between the star and observer can further modify the spectrum. Therefore, colour is a useful first indicator of temperature but is not by itself a complete description of a stars physical condition.
Human perception also affects how stellar colours are described. Bright stars can stimulate colour vision more effectively than faint stars, while atmospheric scattering and the sensitivity of the human eye influence visual impressions. Under dark adapted conditions, human vision becomes less sensitive to colour because rod cells dominate, making many faint stars appear white. Photographic and electronic detectors can record spectral differences that are subtle or invisible to the unaided eye.
The colour assigned to a star in an astronomical image may also depend on the filters, detector response and image processing used. Astronomical images are therefore often constructed using multiple wavelength bands rather than representing exactly what the human eye would see. Scientific colour measurements use calibrated photometry rather than subjective visual descriptions.
Stellar colour can provide information about stellar populations and evolution. A colour magnitude diagram plots a measure of brightness against colour and reveals structures such as the main sequence, red giant branch, white dwarf sequence and other evolutionary populations. Hot young massive stars tend to occupy blue regions, while cool evolved giants occupy red regions. White dwarfs begin very hot and can appear blue or white before cooling and becoming progressively redder and fainter.
Star clusters are particularly valuable because many member stars formed at approximately the same time and distance, allowing differences in colour and luminosity to reveal mass dependent evolutionary stages. Colour therefore becomes a tool not only for identifying temperature but also for studying stellar ages, populations, distances, dust and evolution.
A common misconception is that a red star is necessarily less energetic overall than a blue star. A red star has a cooler surface temperature and emits less energy per unit surface area than a blue star at the same radius, but a sufficiently large red giant can have a much greater total luminosity than a smaller blue or white star. Another misconception is that the Sun is intrinsically yellow.
Its approximate visible spectrum corresponds to a surface temperature near 5800 kelvin and its integrated light is broadly white, although Earths atmosphere selectively scatters shorter wavelengths and can make the Sun appear yellow, orange or red under particular conditions. Stellar colour is therefore best interpreted quantitatively through temperature, spectral energy distribution and photometric measurements rather than through simple everyday colour categories.
