Topic 5.3: Light, Color and the Blackbody Curve
Look carefully at the stars on a clear night. Some, like Betelgeuse in Orion, are a deep orange-red. Some, like Rigel on the opposite side of Orion, are blue-white. Capella is yellow, like the Sun. The color is not decoration: it tells us the star's temperature.
Light Is a Wave
Light is an electromagnetic wave. The distance from one crest to the next is its wavelength, and the number of crests passing each second is its frequency. They are tied together by the speed of light:
c = wavelength x frequency
The full range of wavelengths is the electromagnetic spectrum:
| Kind | Wavelength | A source |
|---|---|---|
| Radio | meters to kilometers | radio stations, pulsars |
| Microwave | millimeters | microwave ovens, the cosmic microwave background |
| Infrared | 1,000 nm to a millimeter | heat from your skin, cool stars |
| Visible | 400 to 700 nm | the Sun, a light bulb |
| Ultraviolet | 10 to 400 nm | hot stars, sunburn |
| X-rays | 0.01 to 10 nm | hot gas around black holes |
| Gamma rays | less than 0.01 nm | exploding stars |
(1 nanometer, nm, is one billionth of a meter.) Our eyes see only a thin slice, from red (about 700 nm) to violet (about 400 nm). That is why space telescopes that see the other kinds of light show so much that we could never see ourselves.
Hot Things Glow
Anything hot gives off light: an electric stove, a fire poker, a star. As the temperature rises the object glows first dull red, then orange, yellow, white, and blue-white. The pattern of brightness at each wavelength is called a blackbody curve (a blackbody is an ideal object that absorbs and gives off light perfectly; stars are very close to it).
Two facts about the curve make astronomy work:
-
Wien's law: the hotter the object, the shorter the wavelength of its peak.
peak wavelength (nm) = 2,898,000 / T (kelvins)
- The Sun: T = 5,800 K gives a peak at 500 nm (green-yellow). Spread across all the colors, the Sun looks white.
- A 3,000 K red dwarf peaks at 966 nm, in the infrared; the light we see from it is mostly orange-red.
- A 10,000 K star peaks at 290 nm, in the ultraviolet; its visible light is blue-white.
-
Stefan-Boltzmann law: hotter things are far brighter. A star's power output grows as the fourth power of its temperature and as the square of its radius:
L = 4 x pi x R squared x sigma x T to the 4th
Double a star's temperature and it gives out 16 times as much light from every square meter. This is why the very hot blue stars can outshine cooler stars of the same size so easily.
Reading a Star's Color
| Color | Temperature | Example |
|---|---|---|
| Red | 2,400 to 3,700 K | Betelgeuse, Antares |
| Orange | 3,700 to 5,200 K | Arcturus |
| Yellow | 5,200 to 6,000 K | The Sun, Capella |
| White | 6,000 to 10,000 K | Sirius, Vega |
| Blue-white | above 10,000 K | Rigel, Spica |
You will never see a green star, because a star's light is a broad mix of colors, and mixing the whole rainbow looks white. A star whose peak is green still looks white.
Worth a Pause
The psalmist praises the Lord as one "who coverest thyself with light as with a garment: who stretchest out the heavens like a curtain" (Psalm 104:2). Light was the first thing spoken into being in Genesis, and it is still the way creation reaches us. Everything we know about a star a thousand trillion miles away arrives as a stream of light, and every color of it carries a message about the star's temperature. We are reading letters that were mailed across the universe. One of the videos says our eyes evolved to see the colors the Sun gives off best. Whether you hold that theory or not, notice the fit: eyes made for exactly the light the Sun sends. Genesis says, "Let there be light" (Genesis 1:3). Ask which story, evolution or creation, explains the fit better, and be willing to look at both.
Try It: Spectrum Detective. Tell rainbows from bright and dark lines, match an element's fingerprint, and read a star's color. Play Spectrum Detective »
Practice Problems
- What is the relationship between wavelength, frequency and the speed of light?
- List the electromagnetic spectrum from longest to shortest wavelength.
- Use Wien's law to find the peak wavelength of a 6,000 K star.
- Which is hotter, a red star or a blue star? How do you know?
- By what factor does a star's power per unit area change if its temperature doubles?
- Why does the Sun look white even though its peak is in the green-yellow part of the spectrum?
- A star peaks at 500 nm. Estimate its temperature.