Stars come in a variety of colors that can provide insights into their surface temperature, size, age, and composition. The color of a star depends primarily on its surface temperature. Hotter stars tend to appear bluish or white, while cooler stars tend to appear reddish or orange. However, other factors like size and age also impact a star’s color. While most stars appear white or yellowish to the naked eye, when observed with telescopes and spectrographs, their true colors become apparent.
Surface Temperature
A star’s surface temperature determines the wavelength of light it emits. Stars emit light across the electromagnetic spectrum, but they emit the bulk of their light in the visible range that our eyes can detect. Hot stars emit more blue/violet light, while cool stars emit more red/orange light. This relationship between temperature and color is why cooler stars appear more reddish, while hotter stars appear more blueish.
The surface temperature of main sequence stars correlates strongly with their spectral type classification. O-type stars have surface temperatures over 30,000 K and appear bluish-white. B-type stars range from 10,000 to 30,000 K with a blue-white hue. A-type stars span 7,500 to 10,000 K and appear white. F-type stars from 6,000 to 7,500 K look yellowish-white. Our Sun is a G-type main sequence star with a temperature around 5,800 K, giving it a yellowish color. K-type stars from 3,500 to 5,000 K are orange. And M-type stars under 3,500 K are reddish.
Size
The size of a star impacts its color. Larger stars tend to be hotter and bluer, while smaller stars tend to be cooler and redder. This relationship exists because a star’s mass determines the gravitational pressure in its core, which controls the rate of nuclear fusion. More massive stars have higher core pressures and faster fusion rates, making them hotter.
Size can override the color tendencies of temperature for some stars. Supergiant stars have enormous diameters and low surface gravities. Though they have high core temperatures, their outer layers are inflated and cooler by comparison. A red supergiant may have a high core temperature over 20,000 K but a much cooler surface temperature under 4,000 K because its outer layers have expanded so much. Conversely, white dwarfs are very hot for their small size because they lack an outer envelope and their hot core is exposed.
Composition
A star’s chemical composition also influences its color. Stars are made of mainly hydrogen and helium, but have varying trace amounts of heavier elements. Stars with more metals (astronomy term for any element heavier than helium) in their outer layers appear whiter or bluer. Metal-rich stars have more opacity which drives faster outer convection, increasing the star’s surface temperature and blue hue.
Very old stars formed early in the universe’s history have fewer heavy elements since there were fewer previous generations of stars to produce them. These ancient Population II stars tend to be redder than younger Population I stars like our Sun with higher metal content. Extremely metal-poor halo Population II stars can have a pronounced orange or red tint.
Age
As stars age, their color tends to shift. When stars first form they are larger, hotter, and bluer. But as they consume their core hydrogen fuel over billions of years, they gradually expand into red giant or red supergiant stars which are much cooler and redder.
Our Sun has warmed by about 30% over its 4.5 billion year lifetime as its composition changed. When it was just forming, the Sun would have appeared slightly bluer and cooler than today. In another 5 billion years as it runs out of hydrogen fuel, the Sun will expand into a red giant phase growing over 100x larger and much redder.
Duplicity
Stars that are part of multiple star systems can also exhibit different colors than they otherwise would as single stars. A close binary pair where two stars orbit each other will usually share gas back and forth. This can lead to enhanced surface metal content and hotter bluer colors than average.
Some binary pairs are eclipsing binaries where they pass in front of each other from our perspective. When the dimmer, cooler star eclipses its brighter, hotter companion it can make the system temporarily appear redder as more of the cooler star’s light is visible. The light curves of eclipsing binaries vary periodically over each orbit between bluer and redder hues.
Spectral Types
Here is a table summarizing the typical color and temperature range for each main sequence spectral type:
| Spectral Type | Color | Temperature (K) |
|---|---|---|
| O | Bluish-white | Over 30,000 |
| B | Blue-white | 10,000 – 30,000 |
| A | White | 7,500 – 10,000 |
| F | Yellowish-white | 6,000 – 7,500 |
| G | Yellow | 5,300 – 6,000 |
| K | Orange | 3,500 – 5,000 |
| M | Reddish | Under 3,500 |
As this table shows, O-type stars are the hottest and bluest main sequence stars, while M-type stars are the coolest and reddest. But there are non-main sequence stars that extend beyond this sequence in both directions.
Non-Main Sequence Stars
While the typical color sequence above covers main sequence hydrogen-burning stars, some very young or very old stars can exhibit more extreme colors:
- Blue hypergiant stars – Young supermassive stars over 100x the Sun’s mass that are extremely hot and blue, but will soon explode as supernovae.
- Blue stragglers – Rejuvenated stars appearing anomalously blue and hot for their age.
- White dwarfs – Very old, faded stellar remnants of moderate temperature but enormous surface gravity and density.
- Red supergiants – Swollen, evolved massive stars like Betelgeuse that are cooler than 1,000 K.
- Brown dwarfs – Failed stars without enough mass for fusion that keep slowly cooling and dimming.
- Black dwarfs – Theoretical endpoint for white dwarfs that have cooled for trillions of years and emit no visible light.
These diverse objects demonstrate stars can appear in a vast range of visible colors from searing blue to the invisible glow of black dwarfs. Even single low-mass main sequence stars will gradually shift from hot, blue newly formed stars to bloated, red giants in their elder years as their core hydrogen is exhausted.
Unusual Star Colors
Under special circumstances, some stars can exhibit peculiar colors that deviate from the normal sequence of their spectral class:
- Pink stars – Extremely hot O-type stars with powerful stellar winds that strip hydrogen to produce bright emission lines of ionized helium and give a pinkish hue.
- Yellow hypergiants – Stars with extended atmospheres like Rho Cassiopeiae can appear yellow despite having higher temperatures.
- Purple/violet stars – Fast spinning stars seen pole-on can have their colors doppler shifted toward the violet end of the spectrum.
- Garnet stars – Cool red stars with emission lines from trace metals can take on a deep red garnet color.
- Green stars – Rare green colors can come from unusual chemical compositions or optical illusions with bright blue companions.
So while stars usually progress from hot/blue to cool/red over their lifetimes, stellar winds, rapid rotation, binarity, and unusual chemistry can all contribute exceptions to a star’s expected color.
Multi-Colored Binary Systems
Binary and multiple star systems often contain stars with visibly distinct colors:
- Albireo – A colorful double star with a bright yellow primary and fainter blue companion.
- Mizar – The second star in the Big Dipper’s handle forms a binary with Alcor which has a slightly different white hue.
- Antares – A red supergiant orbited by a much hotter blue main sequence companion star.
- Polaris – The North Star is actually a triple system with a yellow supergiant primary and two fainter companions.
- Almach – A multiple system with a golden-yellow evolved star and two blue-white main sequence companions.
Spectroscopic binaries with two stars of very different colors can even periodically shift between bluer and redder hues as the brighter star is eclipsed. Delta Cephei is a prominent example, fluctuating between red and blue with a period of 5.4 days.
Colorful Star Clusters and Galaxies
Many star clusters and galaxies display a range of stellar colors when viewed through a telescope:
- Jewel Box Cluster – A young open cluster with bright blue supergiants and cooler red supergiants.
- Westerlund 2 Cluster – A dense young cluster with many extremely hot and bright blue O-type stars.
- Omega Centauri – An ancient globular cluster showing the yellow glow of its cooler population II stars.
- Whirlpool Galaxy – A spiral galaxy displaying pink star forming regions along its arms and yellowish central bulge.
The stellar nurseries of emission nebulae like the Orion Nebula contain many extremely young, hot blue stars only a few million years old surrounded by gas and dust clouds lit up in reds, oranges, and yellows.
Conclusion
Stars exhibit a tremendous diversity of colors that reveal key details about their surface temperature, size, age, composition, and environment. Hot blue stars tend to be young and massive, while expanded red giants are older stars nearing the end of their lives. Binary companion stars often display strikingly contrasting colors. And galaxies show color gradients with younger blue populations concentrated in their spiral arms and older yellow stars in their bulges. So while stars may appear white or yellow to our naked eye, closer observation with telescopes and spectrographs reveals a vibrant rainbow of stellar colors across our Universe.
