The Color of Heat: Why Hot Things Glow - Incandescence, Blackbody Radiation, and the Color of Stars
Heat a horseshoe in a forge and it tells you its temperature by color alone: dull red, then cherry, then orange, yellow, and finally a dazzling white. Every warm object in the universe does this - a stove coil, a light-bulb filament, the Sun, a distant blue giant - and they all follow one elegant curve discovered by Max Planck. That single law explains why cooling embers fade to red, why the Sun is white and not yellow, and why the hottest stars are blue. This is the physics of glowing things.
Light straight from heat
Most of the color around you is reflected - objects take white light and subtract some of it. But there is a second, more primal way to make light: get something hot enough and it emits light of its own. This is incandescence, and it is why a filament, an ember, molten metal, and the Sun all shine without any outside light at all.
What makes incandescence so orderly is that a hot object's color depends on almost nothing except its temperature. A perfect emitter - physicists call it a blackbody - radiates a spectrum fixed entirely by how hot it is, described by Planck's law. So color becomes a thermometer: the blacksmith reading the forge, the astronomer reading a star, and the lighting engineer specifying a bulb are all reading the same curve. Let's build it up piece by piece.
The Planck curve
In 1900, Max Planck wrote down the exact shape of a blackbody's spectrum - a curve of how much light it emits at each wavelength. Two things happen as you raise the temperature: the whole curve rises (more light everywhere), and its peak slides toward the blue. The color you see is the eye's blend of that entire curve across the visible band. Heat it up and watch.
One curve, set by temperature
The Planck radiation curve at your chosen temperature, drawn across the near-ultraviolet, the visible band (shaded with its true spectral colors), and into the infrared. Each curve is scaled to its own peak so the shape is always visible. The swatch shows the resulting color; the dotted line marks the emission peak.
Wien's law: the peak moves
Where exactly is that peak? Wien's displacement law gives it in one line:
λ_max ≈ 2,898,000 / T nanometers, with T in kelvin. The consequence is
striking: a body at room temperature, or even a 3000 K bulb, peaks deep in the
infrared - you feel its heat but see only its faint visible tail. Only around
5000-7000 K does the peak actually land inside the colors we can see. Trace the peak against the
visible window.
Why most hot things glow invisibly
The peak emission wavelength plotted against temperature. The shaded band is the visible spectrum (about 380-700 nm). Below roughly 4000 K the peak sits off to the right in the infrared - the object radiates heat you can't see; only at high temperatures does the peak cross into visible light.
The glow progression
Put it together and you get the blacksmith's color scale. Below about 798 K - the Draper point - an object emits so little visible light that it looks black in a dark room, even while radiating plenty of invisible infrared. Cross that threshold and it glows dull red, then brightens and warms up the scale to white-hot. Heat the bar and read its temperature by color.
Reading temperature by color
A metal bar at your chosen temperature, drawn in its true incandescent color and brightness. Below the Draper point it stays dark; above it, it climbs the classic forge scale - dull red, cherry, orange, yellow, white. The brightness rises steeply with temperature, just as it does in reality.
The color of stars
Stars are nearly perfect blackbodies, so their color is a direct read-out of their surface temperature - the basis of the O B A F G K M spectral sequence. Cool red dwarfs smolder near 3000 K; our Sun sits at 5772 K and shines white; blue supergiants blaze past 10,000 K. Counterintuitively, blue means hottest and red means coolest - the opposite of the kitchen tap. Meet a few by temperature.
Blue is hot, red is cool
Familiar stars placed on the temperature scale, each drawn in its true blackbody color. Our Sun lands in the white middle; red giants sit at the cool end, blue supergiants at the hot end. Select a star to see its temperature, spectral class, and why it looks the way it does.
The terms, defined
The vocabulary of glowing things.
λ_max ≈ 2.898×10⁶ / T nm. The hotter the body, the shorter
(bluer) its peak emission wavelength.What it means for color
Test your understanding
Six questions on incandescence, Planck's law, Wien's law, the Draper point, and star color. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.
Quick check
Continue your journey
The glow curve connects to color temperature, the spectra of light sources, chromaticity, the sky, and the body clock it drives - here's where to go next.
Color Temperature and White Balance
The Kelvin scale this glow curve defines.
Physics · 70Emission: Neon, LEDs, and Gas-Discharge Light
The non-thermal way to make colored light.
Physics · 4Spectral Power Distributions and Why RGB Is Not Enough
Reading any light source by its spectrum.
Colorimetry · 10Chromaticity Diagrams Without the Intimidation
Where the Planckian locus lives.
Physics · 40Color in Nature
The Sun's light after the sky gets to it.
Vision · 111Light and the Body Clock
How a light's color temperature reaches your biology.