The color of a star indicates its surface temperature. As stars age and evolve, their color changes as their surface temperature increases or decreases. The color specifically indicates the peak wavelength of light being emitted from the star’s photosphere. Here are some key things the color of a star can tell us:
Hot, Blue Stars
The hottest stars tend to appear blue or blue-white in color. These are stars with surface temperatures of over 25,000 Kelvin. The peak wavelength of light emitted is in the blue part of the visible spectrum. Examples of hot, blue stars include:
| Star Type | Surface Temperature |
| O-type main sequence star | 30,000 – 60,000 K |
| B-type main sequence star | 10,000 – 30,000 K |
| Blue giant star | 10,000 – 20,000 K |
| Blue supergiant star | 20,000 – 50,000 K |
The blue color indicates these are young stars still fusing hydrogen in their cores. They are very hot and luminous. Blue main sequence stars are typically massive stars, at least 5-10 times the mass of our Sun. Blue giants and supergiants are post-main sequence stars that have exhausted the hydrogen fuel in their cores and expanded outward.
White stars
Stars that appear white in color have surface temperatures around 5,000 – 10,000 K. Their peak light emission is in the yellow-white part of the spectrum. Examples of white-colored stars include:
| Star Type | Surface Temperature |
| A-type main sequence star | 7,500 – 10,000 K |
| White dwarf star | 5,000 – 20,000 K |
| DA spectral type white dwarf | 40,000 – 100,000 K |
White main sequence stars are hotter than our yellow Sun but cooler than blue main sequence stars. White dwarfs are very dense, compact stars that are the remnants of low-mass stars after they have exhausted their nuclear fuel. DA white dwarfs have only hydrogen absorption lines in their spectra.
Yellow stars
Stars that appear yellowish in color have surface temperatures of around 5,000 – 6,000 K. Their peak light emission falls in the yellow part of the spectrum. The most well-known example is our Sun, which is classified as a G-type main sequence star with a surface temperature of 5,778 K. Other yellow-colored stars include:
| Star Type | Surface Temperature |
| G-type main sequence star | 5,300 – 6,000 K |
| K-type main sequence star | 3,700 – 5,200 K |
These are main sequence stars fusing hydrogen into helium in their cores. They emit yellowish light with peak wavelengths around 550 nm. Our Sun will gradually get brighter and evolve into a red giant in about 5 billion years as it exhausts its core hydrogen fuel.
Orange and Red Stars
The coolest stars have surface temperatures below 5,000 K and appear orange or red. These include:
| Star Type | Surface Temperature |
| K-type giant star | 3,500 – 5,000 K |
| M-type main sequence star | 2,400 – 3,700 K |
| Red giant star | 3,000 – 4,000 K |
| Red supergiant star | 3,500 – 4,000 K |
The orange-red color is an indication of a cooler stellar atmosphere. Red giants and supergiants are evolved stars that have exhausted hydrogen fuel in their cores and dramatically expanded their outer layers. Red main sequence stars are the smallest, coolest, and least massive stars on the main sequence.
Extremely Cool Brown Dwarfs
Brown dwarfs are substellar objects that are too low in mass to sustain hydrogen fusion reactions. They have very cool surface temperatures below 2,700 K and appear magenta or brownish. The peak wavelength of light emitted is in infrared rather than visible light. Sometimes brown dwarfs are referred to as “failed stars.”
Advanced Stellar Evolution
As stars continue to evolve after main sequence, their colors can change dramatically depending on the nuclear fusion processes occurring:
| Star Type | Color | Process |
| Red giant | Orange-red | Shell fusion of hydrogen |
| Helium fusion star | Blue-white, blue | Helium fusion in core |
| Carbon star | Red | Buildup of carbon from helium fusion |
| Iron star | Pale/white | End stage, iron core |
Understanding what makes a star a certain color aids our models of stellar structure, evolution, and the lifecycle of stars. Color can also indicate the composition, age, mass, and evolutionary stage of a star.
Conclusion
The color of a star provides key insights into its surface temperature, which is related to composition, mass, age, and fusion processes occurring inside. Hot, young, massive stars appear blue. Yellow stars like our Sun fuse hydrogen on the main sequence. Red giants and supergiants are older stars that have exhausted their core fuel. Brown dwarfs are failed stars too small to ignite fusion. A star’s color evolves as the star ages and its internal processes change over billions of years. Observing a star’s color gives astronomers clues to its underlying physics and place on the stellar evolutionary path.
