Stars come in a wide range of colors and temperatures. The color of a star depends on its surface temperature – hotter stars tend to appear bluer or whiter, while cooler stars appear redder. A star’s temperature gives clues about its size, age, and other properties.
The Electromagnetic Spectrum
To understand stellar colors, we first need to understand electromagnetic radiation. Objects like stars emit light across a broad spectrum of wavelengths. The rainbow of colors we see represents only a small sliver of this spectrum, known as visible light. Beyond the colors our eyes can see are radio, microwave, infrared, ultraviolet, x-ray and gamma radiation.
Of these, human eyes are sensitive to wavelengths from about 400 nanometers (violet) to 700 nm (red). Other parts of the spectrum, like radio waves or x-rays, require specialized telescopes or detectors to observe.
Blackbody Radiation
Stars glow because their hot cores emit electromagnetic radiation. The intensity and wavelength distribution of this radiation depends on the object’s temperature. Hotter objects emit more high-energy radiation overall, with a greater proportion in the ultraviolet, blue, and white part of the spectrum. Cooler objects emit more low-energy, redder radiation. Physicists call this relationship between temperature and electromagnetic radiation blackbody radiation.
For example, a hot metal rod pulled from a furnace glows red. As it cools, it transitions from red to orange, yellow, and eventually black as its temperature drops. The sun’s surface temperature is about 5800 K, giving it a yellowish white glow to human eyes. A red dwarf star might have a surface temperature of 2000-3000 K, giving it an orange to red color.
Stellar Classification
In the early 1900s, astronomers developed a system to classify stars according to temperature. This system divides main sequence stars into seven categories, from hottest to coolest:
O (30,000 – 60,000 K) – Blue-white
B (10,000 – 30,000 K) – Blue-white
A (7,500 – 10,000 K) – White
F (6,000 – 7,500 K) – Yellow-white
G (5,000 – 6,000 K) – Yellow
K (3,500 – 5,000 K) – Orange
M (2,000 – 3,500 K) – Red
This sequence is known as the OBAFGKM spectral classification. Each category also has 10 subdivisions, so a star might be classified as A5, F2, G8, etc. This gives a more precise indication of the star’s surface temperature and color.
Color and Temperature
Below is a table relating stellar classes to approximate temperature and color:
| Spectral Type | Temperature (K) | Color |
|---|---|---|
| O | 30,000 – 60,000 | Blue-white |
| B | 10,000 – 30,000 | Blue-white |
| A | 7,500 – 10,000 | White |
| F | 6,000 – 7,500 | Yellow-white |
| G | 5,000 – 6,000 | Yellow |
| K | 3,500 – 5,000 | Orange |
| M | 2,000 – 3,500 | Red |
As the table shows, O and B type stars are the hottest, with temperatures of 10,000 K or more. They appear bright blue-white or blue. At the other end, M type red dwarfs are the coolest at less than 3,500 K. They glow deep red.
Between these extremes, A stars are white, F and G stars are yellowish, and K stars have an orange tint. These colors correspond directly to the star’s blackbody radiation curves according to the Stefan-Boltzmann law. The hotter the star, the more blue light it emits.
Size and Color
In general, hotter stars also tend to be physically larger. O and B type stars are extremely luminous and massive, with up to dozens of solar masses. K and M dwarfs are the smallest stars, with masses under 60% of the Sun’s.
This ties into a star’s evolution. More massive stars burn through their fuel faster, exhausting their core hydrogen in billions or even millions of years. Low mass stars can burn hydrogen for tens or hundreds of billions of years, meaning they stay on the main sequence appearing mostly unchanged.
So the largest, hottest stars are short-lived. The smallest, coolest stars can persist for long cosmic timescales. In between are stars like our Sun, with lifetimes of roughly 10 billion years.
Other Factors in Stellar Color
A few other factors can modify a star’s apparent color:
- Metallicity – Stars with more heavy elements (“metals”) tend to appear bluer.
- Gravity – Higher gravity redshifts a star’s radiation.
- Viewing angle – We see stars most directly at their poles, affecting color.
- Space reddening – Interstellar dust scatters blue light, reddening distant stars.
Nonetheless, temperature and blackbody radiation remain thedominant factors in stellar color for most main sequence stars.
Unique Stellar Colors
A few types of stars have distinctive colors unrelated to blackbody temperature:
- Brown dwarfs – These “failed stars” are red-brown or magenta from trapped heat.
- Red giants – Swollen, dying red stars are cool but very luminous.
- Blue stragglers – These are hot merged stars appearing young and blue.
- Pulsating variables – Stars like Cepheids pulsate in brightness and color.
But most stars visible to optical telescopes adhere closely to the OBAFGKM spectral sequence. This makes stellar colors a handy measure of their temperature and other physical properties.
Observing Stellar Color
The naked eye usually isn’t sufficient to discern the color of most individual stars. Only the very brightest stars like Sirius, Rigel and Betelgeuse appear bluish, white or orange. But even modest telescopes and binoculars can reveal the primary colors of many brighter stars.
Specialized astronomical instruments and techniques allow much more precise measurements:
- Photometry – Compares stellar brightness at different wavelengths.
- Spectroscopy – Spreads out light to measure intensity by wavelength.
- Filters – Isolate specific colors like red or hydrogen-alpha.
With photometry and spectroscopy, astronomers can classify stars by color, often adding a numerical subscript like B2V or G8III. This gives a wealth of information about its properties and life cycle stage.
Star Colors and Temperature
In summary, a star’s color directly reflects its surface temperature and radiation according to the laws of blackbody radiation. O, B and A stars are searing blue-white, while K and M stars glow deep red. Our yellow Sun lies between them around 5800 K.
A star’s color correlates with mass, size, luminosity and age. Hot, blue stars are young and massive. Smaller red dwarfs can persist for billions of years. Stellar color is a key to understanding the many properties, origins and fates of stars.
Conclusion
A star’s color is primarily determined by its surface temperature. Hotter stars appear bluish or white, while cooler stars are redder. The OBAFGKM spectral classification categorizes stars from 30,000 K blue-white O stars to under 3,500 K red M dwarfs. Stellar color correlates strongly with size, mass, luminosity, composition, age, and other factors that astronomers can determine through photometry and spectroscopy. With proper instruments, even amateur astronomers can discern the colors of many stars.