The common perception that stars are yellow is a visual trick caused by our atmosphere. In reality, stars exhibit a striking range of colors, including deep red, vibrant blue, and pure white. A star’s true color is not random but a direct indicator of its surface temperature, determined by fundamental physics. Understanding stellar color requires analyzing the light stars emit before it reaches Earth.
Stellar Color is Determined by Temperature
A star’s color is directly linked to the physics of blackbody radiation, which describes the light emitted by any object based on its heat. Stars are massive, nearly perfect radiators of energy, meaning their surface temperature dictates the spectrum of light they produce. The hotter an object is, the shorter the wavelength of light at which it emits the maximum amount of energy, a principle known as Wien’s Law.
This physical relationship creates a clear color-to-temperature scale. The coolest stars, with surface temperatures around 3,000 Kelvin, primarily emit longer, lower-energy wavelengths perceived as red light. Conversely, the hottest stars, exceeding 30,000 Kelvin, peak at shorter, higher-energy wavelengths, making them appear brilliant blue or blue-white.
Astronomers use the spectral classification system (O, B, A, F, G, K, and M) to categorize stars by temperature and color. O-type stars are the hottest and bluest, while M-type stars are the coolest and reddest. Color differences are determined solely by the heat of the star’s outer layers, not by chemical composition.
Stars with intermediate temperatures (6,000 to 7,500 Kelvin) emit light that peaks in the green-yellow part of the spectrum. However, they also emit substantial light across all other visible wavelengths. The combination of these colors averages out to appear white to the human eye, meaning truly green or yellow stars are extremely rare when viewed without atmospheric effects.
The Sun’s Classification and Actual Color
Our own star, the Sun, illustrates how a star’s actual color differs from its common name. The Sun is classified as a G2V star, informally known as a “Yellow Dwarf,” despite its light not being yellow. Its effective surface temperature is approximately 5,772 Kelvin.
Applying Wien’s Law reveals that the Sun’s peak emission wavelength is around 500 nanometers, which falls directly in the green-blue portion of the visible spectrum. If the Sun were a perfect single-color emitter, it would look green.
The Sun appears white, not green, because it emits a continuous spectrum of light across all visible colors, from violet through red. Our eyes perceive a combination of all wavelengths of light together, in approximately equal amounts, as white light. When viewed from space, the Sun is a white star.
The “Yellow Dwarf” moniker is largely a historical classification and a result of visual filtering from Earth. Its G-type classification places it in the middle temperature range, often associated with yellow in astronomical charts, even though the star is white. This common name contributes to the incorrect belief that the Sun and similar stars are truly yellow.
Why Stars Look Yellow from Earth
The yellow or reddish appearance of stars, including the Sun, is an illusion caused by the Earth’s atmosphere acting as a selective filter. This phenomenon is known as Rayleigh scattering, which describes how light interacts with small particles in the air, such as nitrogen and oxygen molecules.
Rayleigh scattering is much more effective at scattering shorter wavelengths of light (blue and violet) than it is at scattering longer wavelengths (red and yellow). When starlight enters our atmosphere, the blue light is scattered away in all directions.
Because blue light is preferentially removed from the direct line of sight, the light reaching our eyes is depleted of its blue components. The transmitted light is dominated by the longer, less-scattered yellow and red wavelengths. This filtering process is why the Sun, a white star, appears yellow when high in the sky and more orange or red at sunrise and sunset.
The same atmospheric effect applies to distant stars, often leading to the misidentification of white or blue-white stars as yellow. When observing stars near the horizon, the light passes through a much greater thickness of atmosphere, intensifying the scattering effect and making them appear redder than their true color.