Double the temperature
The peak shifts to a wavelength half as long. The total flux emitted per square metre increases by a factor of 16.
Available when twice the temperature remains within the experiment’s range.
The portal / Laboratory / Stellar colours
One temperature.
A whole spectrum of light.
Change the temperature and discover how the spectrum shifts, how the visible colour changes and how much power is radiated.
This experiment uses an ideal blackbody to approximate a star’s continuous radiation. Spectral lines from its atmosphere are not simulated.
A continuous spectrum.
Selected model 5,800 K reference, if enabled
Night mode changes the displayed colours. Switch back to normal mode to compare colours.
The blackbody colour is integrated using the CIE 1931 colour-matching functions. Ultraviolet and infrared radiation are not converted to visible colours.
—Spectral peak per nm
—Total flux per unit area
A continuumAll wavelengths contribute
The peak is not a single colour emitted by the blackbody.
Where does the energy go?
Distribution of all emitted radiation, before absorption by an atmosphere or dust.
Ultraviolet—Below 380 nm
Visible—380–780 nm
Infrared—Above 780 nm
Illustrative boundaries and rounded percentages. These fractions integrate the complete spectrum, beyond the plotted window of 50 to 3,000 nm.
View spectrum values
| Wavelength (nm) | Selected model | 5,800 K reference |
|---|
Three experiments with light.
A peak in the green?
At 5,500 K, the model’s peak falls in the green. Yet other wavelengths are also emitted: their mixture produces a colour close to white.
Find the Sun
The rounded reference of 5,800 K approximates the Sun’s effective temperature. This continuum model does not reproduce every feature of the solar spectrum.
Temperature, spectrum and energy.
λmax = b / TWien’s law gives the position of the peak of Bλ. Here, b ≈ 2.897772 × 10⁶ nm·K. Doubling the temperature halves this wavelength.
F = σ T⁴The Stefan–Boltzmann law gives the total flux emitted per unit area of a blackbody. Calculating a star’s total luminosity would also require its surface area.
Bλ(T) = (2hc²/λ⁵)
/ [exp(hc/λkT) − 1]Planck’s law builds the continuum. In this formula, λ is in metres; the tool then converts the radiance to express it per nanometre.
Colour ≠ peak wavelengthThe responses of human vision to different wavelengths are combined. A thermal continuum does not select a single colour from the spectrum.
Model, colours, data and sources
The model describes an ideal thermal emitter, not an individual star. A real stellar atmosphere adds features such as absorption lines. Dust, reddening, Earth’s atmosphere, instruments and adaptation of the eye are not simulated. Temperature alone does not give a star’s age, radius or total luminosity.
Colour is calculated by integrating Planck radiation with the CIE 1931 2° standard observer functions, from 360 to 830 nm in steps of 1 nm, then converting XYZ to sRGB with a D65 reference white. Linear channels are normalised to their maximum, then clipped to the gamut before applying the sRGB transfer function. The disc’s size and brightness are normalised: only its colour is compared. This is a screen preview, not a prediction of night vision or a photograph.
The chart shows Bλ per nanometre from 50 to 3,000 nm on a logarithmic λ axis. In shape mode, each curve is divided by its own maximum. In power mode, all values are divided by the same maximum of the 5,800 K blackbody. The vertical axis adjusts; the area on screen is not an energy integral over wavelength.
The ultraviolet, visible and infrared fractions use the illustrative thresholds of 380 and 780 nm; visual sensitivity changes gradually. They are integrated across all radiation by changing variables to x = hc/(λkT), using the function x³/(exp(x)−1). They are not derived from the widths of the coloured bands in the drawing.
SI constants: h = 6.62607015 × 10⁻³⁴ J·s; c = 299,792,458 m/s; k = 1.380649 × 10⁻²³ J/K. σ is calculated from these constants; the peak in wavelength uses the root 4.9651142317… of Wien’s equation. The CSV exports physical values and the 5,800 K reference, regardless of display mode. Calculations run locally, without an account or saved settings.
Colour data: CIE 2019, Colour-matching functions of CIE 1931 standard colorimetric observer, International Commission on Illumination (CIE), Vienna. DOI 10.25039/CIE.DS.xvudnb9b. Data licensed under CC BY-SA 4.0. Adaptation: CSV values packaged as a JavaScript array, with the licence retained. Original CSV · Metadata and licence · Adapted data.
References accessed on 19 October 2026: NASA/ESA/CSA · continuous stellar spectra ; NASA · radiation laws ; NIST/CODATA 2022 · constants ; sRGB / ICC specification, hosted by the W3C ; NASA · the Sun’s temperature ; ESO · stellar colours and perception.
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