The color of a star seen in the night sky depends primarily on its surface temperature. The hotter a star is, the bluer its color. Cooler stars appear more red or orange. Most stars seen without a telescope appear white or bluish-white to the human eye because they are hotter than our sun. However, the true colors of stars span the visible spectrum and beyond, from deep reds to brilliant blues.
Surface Temperature Determines a Star’s Color
A star’s color is a direct result of its surface temperature. Stars are essentially massive spheres of superheated gas. The temperature at the surface determines the wavelength of light emitted. Hotter stars emit more blue and ultraviolet light from their photosphere (visible surface). Cooler stars radiate more red and infrared light.
Surface temperature is related to stellar classification. The major categories are O, B, A, F, G, K and M stars. O stars are the hottest, with surface temperatures above 30,000 K. M stars are the coolest, with temperatures of 2400K and below. Each successive category is cooler than the last. So B stars are hotter than A stars, which are hotter than F stars, and so on.
This table shows the relationship between stellar class, surface temperature, and color:
| Class | Temperature (K) | Color |
|---|---|---|
| O | Above 30,000 | Bluish white |
| B | 10,000 – 30,000 | Blue white |
| A | 7500 – 10,000 | White |
| F | 6000 – 7500 | Yellowish white |
| G | 5000 – 6000 | Yellow |
| K | 3500 – 5000 | Orange |
| M | 2400 and below | Red |
As this shows, O and B stars appear blue or blue-white, while K and M stars appear orange and red. Our sun is a G type star with a temperature around 5800K and a yellowish hue.
Apparent Colors Depend on Brightness
The actual color of a star depends on its surface temperature. However, the color it appears to the human eye also depends significantly on how intrinsically bright it is.
Brighter stars appear whiter or bluer. Dimmer stars look more red or orange. That’s because the rods and cones in our eyes respond differently to different wavelengths at different brightness levels.
Stars with high absolute magnitude look blue-white or just white. Lower absolute magnitude stars appear distinctly colored – yellow, orange, or red. Absolute magnitude is related to luminosity and indicates the intrinsic brightness of a star.
So Sirius, the brightest star in the night sky, appears brilliant white. It is an A type star with a surface temperature around 9940K. But if Sirius were dimmer, it would look yellowish or orange despite its high temperature. That’s because our eyes perceive brighter sources of monochromatic light as more white.
Betelgeuse, on the other hand, is a red supergiant star. Its surface temperature is around 3600K, giving it a strong orange/red color. But because it is so intrinsically luminous, if Betelgeuse were even brighter it would appear more white or pinkish.
Interstellar Reddening Alters Color
Interstellar dust and gas that lies between us and the stars can also affect their apparent color. This interstellar reddening preferentially scatters shorter wavelength blue light, allowing longer wavelength red light to pass through.
This can make some distant stars appear redder than their true color based on surface temperature. Reddening is most pronounced for stars over 1000 light years away, beyond local dust clouds.
For example, the star Rigel in the constellation Orion is a blue-white supergiant, classified B8. But it appears as the blue-white color we expect only because it is nearby at about 860 light years distance. If Rigel were farther away, interstellar reddening would give it a more whitish appearance.
Stellar Spectral Lines Determine Color
A star’s spectrum reveals much more about its true color than just surface temperature. A spectrum spreads out the component wavelengths of light, revealing patterns of bright and dark spectral lines.
Different chemical elements in the star’s outer layers absorb and emit specific wavelengths of light. This creates a unique fingerprint that reveals the star’s chemical makeup. Various spectral lines can enhance different color contributions to subtly influence a star’s net color.
For example, strong hydrogen lines in A type stars contribute blue tint. Titanium oxide in M stars boosts redness. Sodium and calcium lines add a subtle yellowishness to some stars.
Astronomers use spectrometry to discern the detailed colors of stars from their chemical compositions. But these spectral nuances are imperceptible to the naked eye.
Bright Star Colors Range from Red to Blue
The brightest stars visible in the night sky without a telescope include very hot and cool types. Despite differences in surface temperature and true color, most appear white or bluish-white.
Sirius, Canopus, Rigel, Vega, and Capella are all hot type A or B stars. So they look brilliant white or blue-white, the colors corresponding to their high temperatures around 10,000K.
Arcturus and Aldebaran are cooler orange K giants. But at magnitude -0.1 and 0.85, respectively, they appear orange-white. Their luminosity dominates over their orange hue.
Betelgeuse is one of the few naked eye stars to appear distinctly colored. As a bright M supergiant, it takes on a strong reddish-orange color despite its +0.5 magnitude.
This table of the brightest stars shows their stellar classification, surface temperature, true color, and visual color:
| Star | Class | Temperature | True Color | Visual Color |
|---|---|---|---|---|
| Sirius | A1V | 9940K | White | White |
| Canopus | F0Ib | 7350K | Yellow-white | White |
| Rigel | B8Iab | 12100K | Blue-white | Blue-white |
| Arcturus | K1.5III | 4290K | Orange | Orange-white |
| Vega | A0V | 9400K | Bluish-white | White |
| Capella | G1III + G0III | 4970K | Yellow-white | Yellow-white |
| Altair | A7V | 7550K | White | White |
| Aldebaran | K5III | 3910K | Orange | Pale orange |
| Betelgeuse | M1-2Ia-ab | 3600K | Orange-red | Orange-red |
Colors Depend on Star Mass and Age
A star’s mass and age also influence its color:
- More massive stars are hotter, bluer, and burn through their fuel faster
- Less massive stars are cooler, redder, and can burn for billions of years
- Newer stars tend to be hotter and bluer
- Older stars have cooler, redder colors
This happens because more massive stars need higher core pressures and temperatures to counteract their stronger gravity. This makes them hotter and bluer on the main sequence.
Lower mass stars can burn at a lower temperature and emit redder light, sometimes for trillions of years. As stars age, they eventually cool and expand into red giants and supergiants.
Cool Red Stars Are Most Common
Though hot blue stars are intrinsically brighter, cool red M dwarf stars are actually the most common in the universe. That’s because lower mass stars are easier to form and live longer.
About 76% of stars are red dwarfs. And the vast majority of the closest stars to the Sun are reddish M stars, like Proxima Centauri and Barnard’s Star.
The coolest red dwarfs have surface temperatures of 2400-2800K. But they appear very faint and dull even through telescopes. Red dwarf stars below about M5 are essentially invisible to the naked eye because they are so dim.
Despite being the most numerous, we only see red dwarfs if they are relatively close by. More distant red stars are too faint and simply appear black against the night sky backdrop.
Beyond Visible Colors in Star Spectra
While stars emit a rainbow of visible light, much of their energy is at invisible wavelengths like ultraviolet, infrared, radio and X-rays. Each part of the spectrum reveals clues to the star’s properties.
Hot blue O stars blast out most of their radiation in ultraviolet. Cool red M dwarf stars peak in infrared. And very active stars produce strong X-ray and radio emissions.
Astronomers examine a star’s full multiwavelength spectrum to decipher its composition, behavior, evolution and hidden companions. But the visual light our eyes see encodes the star’s surface temperature and intrinsic luminosity.
True Star Colors Need Photography
To see their true subtle colors, stars must be photographed with telescopes and specialized astronomical cameras. Long exposure astrophotography can reveal colors imperceptible to human vision.
Adjusting image saturation and contrast also brings out faint coloring differences between stars. Comparative colors photos of stars can highlight their variety of hues based on mass, temperature, age and composition.
This truer color differentiation is lost to the naked eye. Still, visible light tells us a lot about a star’s basic properties and place in the stellar lifecycle.
Conclusion
A star’s color provides key insights into its surface temperature, luminosity, mass, age, and composition. Hot young stars shine blue-white or white. Older cooler stars glow red and orange. But interstellar dust can redden any star’s color.
While telescopes reveal more nuanced hues, the human eye sees stars primarily as white or slightly bluish. A few exceptions like Betelgeuse and Antares stand out with their distinct orange/red colors.
A star’s spectrum encodes the details of its color. Different spectral lines boost specific wavelengths to fine-tune the overall visible color. But the majority of a star’s emissions are often at invisible ultraviolet and infrared wavelengths.
So a star’s true colors are far more diverse, subtle, and scientifically revealing than what we can see with just our eyes alone on a dark night gazing up at the heavens.
