Stars have colour, so why do they nearly all look white at night?
― The colour isn't lost from the star. It's lost inside your eye
In picture books, stars are drawn red or icy blue. But look up at the real night sky and almost every star is just a plain white dot. The stars haven't lost their colour. When the light is dim, your eyes simply switch off their "colour-reading" duty.
One autumn night, you look up from your balcony. Away from city lights, you can usually make out a few dozen stars.
You remember learning in science class that "some stars are red, some are blue." But every star in front of you is just glowing a pale white.
Look closer, and among the brightest stars you spot a faint orange one. Oddly, colour only shows up in the bright ones.
There are only two reasons stars look white
Red stars and blue stars both give off a little of every colour in the rainbow. The difference is only in which colour is slightly stronger — nowhere near as sharp as paint colours.
Your eye has cells that can tell colour apart, and cells that are good in the dark but can't tell colour. Light from a faint star only triggers the latter.
The first reason is about the star; the second is about your eye. Put the two together, and the night sky becomes an almost black-and-white world. Let's look at each in turn.
Red stars and blue stars both give off every colour of the rainbow
A star's colour depends on its surface temperature. Cooler stars glow orange, a sun-like star glows pale yellow-white, and very hot stars shine blue-white. It's the same mechanism as the red glow of charcoal or a heater: the hotter something is, the more blue light it adds.
But hot objects never give off just one colour. Look at Figure 1. The mountain-shaped curve shows how strong the star's light is at each colour (that is, at each wavelength). The curve is very wide, wide enough to cover the whole band of colours we can see (the rainbow strip).
Turn the dial to change the temperature, and the peak of the mountain shifts. But within the visible band, the curve is always just a gentle slope at any temperature. Red, green and blue are all present, so to the eye they blend into something close to white.
Even Betelgeuse, which looks orange, is thought to have its peak out in the invisible infrared. Blue-white Sirius has its peak out in the ultraviolet. For both stars, what we actually see is only a slice of the mountain's "foothills."
In dim light, your eye's "colour sensors" are off duty
At the back of your eye sit two kinds of light-receiving cells. One kind distinguishes colour: there are three types, each strongly tuned to red, green, or blue. The other kind is good in the dark, responding even to very faint light — but since there's only one type, it can't tell colours apart.
The colour-sensing cells need a certain minimum brightness to work at all. Most stars in the night sky never reach it. So only the dark-adapted cells respond, and the star shows up as nothing more than a white dot, bright or faint.
Figure 2 compares the colour a star actually emits (top row) with the colour you see with the naked eye (bottom row). Only the bright stars on the left are strong enough to trigger the colour sensors, so some colour survives in the bottom row too. The faint stars on the right turn pale grey regardless of their true colour.
Binoculars gather light with lenses far larger than your pupil. That makes the starlight brighter, switching on your colour sensors — so a star that looked white to the naked eye shows orange or blue through binoculars. Albireo, at the beak of Cygnus the Swan, is famous for appearing as a pair of stars, one orange and one blue.
Some observers deliberately defocus a telescope slightly when they want to see a star's colour. The light spreads from a point into a small disc, reaching more colour-sensing cells at once, which is said to make the colour easier to see.
Summary
Stars give off light coloured according to their temperature. But that light is a mix of every colour of the rainbow, so the differences are modest to begin with. On top of that, faint starlight is rarely strong enough to switch on your eye's colour sensors. That's why most stars in the night sky look white.
The stars haven't lost their colour.
On a dark night, it's your eyes that are off duty.
How your eyes adapt in the dark is covered in "Why can't you see anything for a while after stepping from a bright room into a dark one?", and the link between a hot object's colour and temperature is explained in "How do blacksmiths judge iron's temperature without a thermometer?". For why stars twinkle, see this article.
- Find a spot away from streetlights and let your eyes adjust to the dark for about 10 minutes. Avoid looking at your phone screen.
- Pick out a few of the brightest stars and compare their colours. In autumn, orange Aldebaran rising in the east late at night is a good landmark.
- Next, try a faint star and see if you can tell its colour. If you have binoculars, look at the same star through them and compare how the colour changes.
If brighter stars show more colour, and binoculars make the colour richer, you've confirmed the article's second reason with your own eyes. Watch your footing when observing at night, and go with an adult.
For readers who want more ― terms, formulas, and textbook linksWe flag which level each part belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics, earth science, or biology
- High school+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist content (astrophysics, vision science)
- ResearchNot yet settled as "established fact" even at university — an active research question
Middle schoolTerms: this phenomenon has names
- Cones: colour-sensing cells in the retina. There are three types, strongly tuned to red, green, and blue, and they work in bright conditions.
- Rods: dark-sensitive cells in the retina. They respond to very faint light, but since there's only one type, they can't distinguish colour.
- Magnitude: the scale of star brightness. Smaller numbers mean brighter; a 1st-magnitude star is defined as about 100 times brighter than a 6th-magnitude star.
Middle schoolHigh schoolChecking with a formula: where is the "peak" of a star's light?
The wavelength at which a hot object's light is strongest depends only on its temperature. Let's use this to calculate where the "peak" falls for an orange star and a blue-white star.
| In symbols | λmax = b ÷ T |
| In words | Wavelength of peak brightness = a fixed constant ÷ surface temperature |
| Where it comes from | This relation (Wien's displacement law) comes from finding the peak of the "Planck formula," which describes a hot object's light intensity at each colour |
| λmax | The wavelength of peak brightness, in nanometres (nm) |
| b | Wien's displacement constant, in nm·K |
| T | The star's surface temperature, in kelvin (K) |
| Wien's displacement constant | about 2898000 nm·K |
| Betelgeuse's surface temperature | thought to be about 3500 K |
| Sirius's surface temperature | thought to be about 9900 K |
| The Sun's surface temperature | about 5800 K |
| Range of visible light | about 380–780 nm |
| Betelgeuse's peak | 2898000 ÷ 3500 ≒ 828 nm |
| Overshoot past the red edge of visible range | 828 − 780 = 48 nm |
| Sirius's peak | 2898000 ÷ 9900 ≒ 293 nm |
| Overshoot past the violet edge of visible range | 380 − 293 = 87 nm |
| The Sun's peak | 2898000 ÷ 5800 ≒ 500 nm |
The orange star's peak spills over into infrared, and the blue-white star's peak spills over into ultraviolet. Either way, only the slope of the mountain falls within the visible range. So the colour difference isn't "red" versus "blue" — it's more like "white with a touch of orange" versus "white with a touch of blue."
High schoolHigh school+Star colour, temperature, and the brightness scale
High school earth science covers the link between a star's colour (spectral type) and surface temperature. In the order O, B, A, F, G, K, M, temperature drops and colour shifts from blue-white through white, yellow, orange, to red. Biology covers the different roles of cones and rods in the retina.
High school+ Magnitude is a scale expressing brightness ratios on a logarithmic scale. It's defined so that a difference of 5 magnitudes equals exactly 100 times brightness, making a difference of 1 magnitude about 2.512 times. This scheme is called "Pogson's formula," after the person who proposed it. In astronomy, the difference in brightness measured through blue and yellow-green filters (the colour index) gives a numeric value for a star's colour.
UniversityBlack-body radiation, and mesopic/scotopic vision
Starlight can largely be treated as "black-body radiation." Its intensity distribution is described by Planck's radiation law, and finding the peak of that law gives Wien's displacement law. On the vision side, the state where only cones are active is called photopic vision, where only rods are active is scotopic vision, and the in-between state where both work is mesopic vision. Whether you can see a star's colour with the naked eye depends on whether its light reaches a brightness that triggers the cones. What's more, the eye's central "fovea" is packed with cones and has almost no rods. So looking at a faint star and looking for colour actually use different parts of the retina.
📖 For the derivation of the formula and further reading: Wien's displacement law (Japanese Wikipedia) ／ Magnitude (astronomy) (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How colour is perceived in mesopic vision At brightness levels where rods and cones work together, exactly how the two signals combine to produce colour perception is still being studied in detail.
- Differences between individuals Looking at the same star, some people see orange and others see white. It isn't clear how much this comes down to age-related yellowing of the lens or individual differences in cone count.
- Old records of star colour Some old records describe a star as "red" that now looks white. Whether the star itself changed, or the record or perception was off, remains debated.
In other words, even this article describes things "as currently understood." How we see star colour depends on both the physics of the star and the workings of the eye.
Links to the curriculum (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light, structure of the eye | Starlight as a mix of every colour; the eye's two cell types |
| High school | Earth science: star colour and temperature / Biology: vision | Temperature and colour in Figure 1; cones and rods |
| High school+ | Advanced earth science: magnitude and colour index | Brightness ratio of 1st- vs 6th-magnitude stars |
| University | Astrophysics / vision science | Planck's radiation law; photopic, scotopic, mesopic vision |
| Research | Visual psychophysics | Colour perception in mesopic vision; individual differences |
| ― | Everyday connections | The same mechanism makes flower and clothing colours hard to tell apart at night |
- Wien's displacement law - Wikipedia (Japanese)
- Magnitude (astronomy) - Wikipedia (Japanese)
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (Chronological Scientific Tables), Maruzen Publishing (stellar surface temperatures and spectral types)
- Vision Society of Japan (日本視覚学会), ed., Handbook of Visual Information Processing, Asakura Publishing (cones, rods, photopic and scotopic vision)
※This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the mechanism. Stellar surface temperatures vary between sources.