The natural color of stars is determined by their surface temperature. Stars emit light across the electromagnetic spectrum, but the wavelength distribution of this light depends on how hot the star’s surface is. Hotter stars emit more blue light, while cooler stars emit more red light. To our eyes, hotter stars then tend to appear bluer in color, while cooler stars appear redder. A star’s color can therefore reveal key information about its properties and stage of evolution.
Blackbody Radiation
Stars can be approximated as blackbodies that emit electromagnetic radiation according to their temperature. A blackbody is an idealized physical body that absorbs all radiation that impinges on it and re-emits this energy at wavelengths dependent on its temperature. As the temperature increases, the peak of the blackbody radiation distribution shifts to shorter wavelengths. The relationship between temperature and peak wavelength is described by Wien’s displacement law.
For stars, hotter surface temperatures thus lead to bluer blackbody spectra, while cooler surface temperatures lead to redder blackbody spectra. Our sun, with an effective surface temperature of about 5800 K, emits most intensely in the green part of the spectrum. A hotter, blue star could have a temperature of 10,000 K or more and emit most strongly at blue wavelengths. Meanwhile, a relatively cool red star might have a temperature of 3000 K and peak in the red.
Color Index
| Spectral Type | Temperature (K) | Color |
| O | 30,000 – 60,000 | Blue |
| B | 10,000 – 30,000 | Blue-white |
| A | 7,500 – 10,000 | White |
| F | 6,000 – 7,500 | Yellow-white |
| G | 5,000 – 6,000 | Yellow |
| K | 3,500 – 5,000 | Orange |
| M | 2,000 – 3,500 | Red |
Astronomers characterize a star’s color using a spectral classification scheme. Each spectral type corresponds to a range of surface temperatures and characteristic color:
– O and B stars are the hottest and bluest stars with temperatures exceeding 10,000 K.
– A stars are white in color with temperatures around 10,000 K.
– F stars are yellowish-white with temperatures of 6000-7500 K.
– G stars like our sun are yellow with temperatures of around 5000-6000 K.
– K stars have an orange hue and temperatures of 3500-5000 K.
– M stars are the coolest stars with temperatures 2000-3500 K and a red color.
So in general, the sequence goes from the hottest blue O stars to the coolest red M stars.
Effects of Composition
While surface temperature is the primary factor determining a star’s emitted wavelengths of light, surface composition can also have some effect. Stars with a higher abundance of heavy elements in their outer layers emit more strongly at specific wavelengths related to those elements.
For example, very hot O and B stars with significant carbon, nitrogen, and oxygen absorption lines are categorized as peculiar stars. Due to gas absorption at specific wavelengths, these O and B stars may take on a yellowish or reddish tint.
Metallic A and F stars enriched in metals like iron also emit more strongly in the blue and ultraviolet compared to non-metallic stars. This pushes the peak of their blackbody-like spectra toward bluer wavelengths. The overall effect is that metal-rich A and F stars tend to appear whitish or slightly bluish rather than yellowish.
Role of Stellar Evolution
The color of most stars changes over their lifetimes as they evolve. Stars are born hot and blue before cooling as they age. The most massive O and B stars have extremely short lifetimes of just millions of years. Over this time their initial hydrogen fuel is rapidly consumed, and they transition to successively cooler temperature and redder color.
Our sun provides an example of more moderately sized star evolution. Currently on the main sequence, our G2 yellow sun is gradually brightening and warming over billions of years. In several billion years, models predict the sun’s increasing luminosity will cause warming oceans to make Earth uninhabitable.
Eventually as the hydrogen fuel runs low, the sun will cool into a red giant phase. Its outer layers will expand and cool, reddening the sun’s apparent color. Later stage evolution includes expulsion of the outer layers as a planetary nebula, leaving behind a hot blue remnant white dwarf core.
Role of Extinction
Intervening dust along the line of sight to a star can also redden its apparent color. Dust grains preferentially scatter shorter wavelength blue light compared to longer wavelength red light. This extinction effect shifts the stellar spectrum toward the red end.
Astronomers quantify the amount of extinction using color excess measurements. The stellar classification is determined from the intrinsic color after correcting for reddening effects of dust. Extinction maps of our Milky Way galaxy show dust is concentrated in the galactic plane. Stars observed through greater distances of dust appear redder than their intrinsic color.
Apparent Color Changes in Binary Stars
Another interesting color effect occurs in binary star systems with a bright primary and dimmer companion. As the dim companion star orbits the primary, its spectral contribution changes depending on whether it is behind or in front of the primary.
When the dimmer star is located behind, its light is overwhelmed by the bright primary. The system has the color of the primary star. But when the dimmer star is in front, its spectral contribution reddens the total observed color. This apparent color change recurs over each orbital period and is known as beta Persei variability.
Conclusion
In summary, the natural color of stars is fundamentally linked to their surface temperature, with hotter stars appearing blue and cooler stars red. But stellar composition, evolution, extinction, and binarity all affect the observed color. Careful measurement of a star’s spectrum reveals its surface temperature and intrinsic properties. Tracking color changes over time can provide clues to stellar evolution and system dynamics. A star’s color is thus a key diagnostic that provides deep insights into fundamental stellar properties.
