The color of a star depends primarily on its surface temperature. Stars come in a range of colors from bright blue to deep red. This range of colors is directly linked to the temperature on the star’s surface. The hotter a star is, the bluer its color. The cooler it is, the redder it appears.
This pattern is counterintuitive because we normally associate red with hot and blue with cool. However, when it comes to stars, bluer means a hotter star, while redder means a cooler star. So why do hotter stars appear blue while cooler stars are red? To understand this, we need to look at how temperature affects the light emitted by stars.
Blackbody Radiation
Stars produce thermal radiation known as blackbody radiation. The amount and wavelength of light emitted depends directly on the star’s surface temperature. Hotter objects emit more total energy per unit area than cooler objects. Stars act like what are called “blackbodies” – idealized physical objects that absorb and reemit all radiation.
As a blackbody gets hotter, it emits light across a wider range of wavelengths. However, the peak of intensity shifts to shorter wavelengths. Shorter wavelengths of visible light appear blue to our eyes. Longer wavelengths appear red. So hotter blackbody stars put out more blue light, making them appear blue. Cooler blackbody stars emit less blue light and more red, making them look redder.
This principle is called Wien’s displacement law. It states that there is an inverse relationship between the peak wavelength of blackbody radiation and its temperature. As temperature rises, peak wavelength decreases toward the blue end of the spectrum. As temperature falls, peak wavelength increases toward the red end.
| Star Temperature | Peak Wavelength | Star Color |
|---|---|---|
| 30,000 K | Blue | Blue white |
| 10,000 K | Ultraviolet | Blue white |
| 7,500 K | Near Ultraviolet | White |
| 5,000 K | Green | Yellow white |
| 3,500 K | Orange | Orange |
| 3,000 K | Infrared | Red |
This table demonstrates how peak wavelength decreases toward the blue end of the spectrum as temperature rises from red to blue white.
Stellar Classification
Astronomers group stars into a spectral class system from hottest to coolest based on these colors:
– O – Blue
– B – Blue to white
– A – White
– F – Yellow white
– G – Yellow
– K – Orange
– M – Red
This OBAFGKM sequence was originally ordered spectroscopically according to absorption lines in stellar atmospheres. But it was later determined that they fell in order of decreasing surface temperature. They provided a handy sequence to categorize stellar colors based on temperature.
There are also numerical subdivisions from 0 to 9 within each class to further categorize surface temperature. A star classified as B2 would be hotter than B5 and cooler than B0. Our sun is classified as a G2 star, indicating a yellow surface typical of its approximately 6,000 K surface temperature.
Star Composition
The composition of a star also affects its color. When astronomers first classified stars by color, they did not know about stellar composition. Later spectral analysis revealed that bluer stars are hotter than red ones partially due to differences in chemical makeup.
Blue O-type stars are extremely hot, bright, and massive. They tend to be much younger and contain more heavy elements than cooler, smaller, older stars. Extremely hot O-stars emit strong ultraviolet radiation that ionizes surrounding gas, making it fluoresce.
Red M-type stars are the coolest, least massive, and most abundant population of stars. They contain fewer heavy elements and less opaque atmospheres. Red dwarfs live much longer lifespans than hotter stars, slowly fusing fuel over trillions of years due to their lower mass.
Besides the trend toward redder colors in smaller stars, composition can turn stars redder through lower opacity. Red supergiants are huge, older stars that have moved off the main sequence. As heavy element fusion products build up in their cores, their outer layers expand and cool. This reveals deeper, redder layers compared to hotter blue main sequence stars.
Interstellar Reddening
Finally, the color of stars can be reddened by interstellar dust. The space between stars contains minute particles of gas and dust. Blue light is preferentially scattered by these particles more than red light is. Stars viewed through dense clouds can have their blue light scattered away, leaving them appearing redder – an effect called interstellar reddening.
Astronomers must correct for reddening effects to interpret the true color and atmospheric composition of stars. This is done by comparing a star’s spectral type to the amount of color shift detected. Calibrations have been developed to model the average degree of interstellar reddening along different galactic sight lines.
Conclusion
A star’s color is primarily linked to its surface temperature, following blackbody radiation principles. Hotter stars emit bluer colors with peak intensity at shorter wavelengths according to Wien’s law. The OBAFGKM spectral sequence categorizes stars from hottest to coolest based on color. Stellar composition and interstellar reddening also affect a star’s color but are secondary effects. Understanding what makes stars shine in different shades provides insights into their underlying physics and place in the cosmos.