Stars come in a wide variety of colors, ranging from red to blue and everything in between. But why do different stars have different colors? The answer lies in the temperatures and chemical compositions of the stars.
The Role of Temperature
The main factor that determines a star’s color is its surface temperature. Stars emit light in a broad spectrum of wavelengths, but the wavelength distribution peaks at a certain wavelength that shifts depending on the temperature. Hotter stars peak at bluer, shorter wavelengths, while cooler stars peak at redder, longer wavelengths.
To understand why temperature affects a star’s color, we need to know a bit about the physics of light and heat. Objects that are heated begin to glow, first becoming red hot as their temperature increases. As they get even hotter, they may become white hot or blue hot. This is because higher temperatures allow the object to emit higher energy photons.
The same principle applies to stars. The hotter a star’s surface, the more blue and ultraviolet light it emits. Cooler stars primarily shine in red and infrared light. Our Sun’s surface temperature is around 5800 K (kelvins). This makes it appear yellowish-white to our eyes. But much hotter stars can reach over 20,000 K and appear blue or blue-white.
The Role of Chemical Composition
A star’s chemical makeup also influences its color, but to a lesser degree than temperature. Stars are made mainly of hydrogen and helium, but also contain small amounts of heavier elements like carbon, nitrogen, and iron.
The proportions of these heavier elements affect the wavelengths of light the star absorbs. This changes the color we perceive by subtly enhancing or suppressing certain wavelengths emitted.
For example, very old stars formed early in the universe have fewer heavy elements. They appear bluer for their temperature than younger stars born from gas enriched by the debris of earlier stars.
The Main Sequence
When stars are plotted on a diagram with color or spectral type on one axis and luminosity on the other, they form a band called the main sequence. Hot, luminous stars fall on the left of the sequence and cool, faint stars fall on the right.
| Spectral Type | Color | Temperature (K) |
|---|---|---|
| O | Blue | Over 30,000 |
| B | Blue-white | 10,000 – 30,000 |
| A | White | 7,500 – 10,000 |
| F | Yellow-white | 6,000 – 7,500 |
| G | Yellow | 5,200 – 6,000 |
| K | Orange | 3,700 – 5,200 |
| M | Red | 2,400 – 3,700 |
This table summarizes the range of properties for the main stellar spectral types. The exact temperature and color depend on the star’s luminosity class as well as composition. But in general, O and B stars appear blue, A and F stars appear white or yellow-white, G stars appear yellow like our Sun, and K and M stars appear orange to red.
Giants and Supergiants
When stars expand into red giants and red supergiants late in their life cycles, their surface temperatures decrease substantially. Despite having high luminosities, red giants and supergiants have such cool surface temperatures that they appear red or orange.
The largest known star, VY Canis Majoris, is a red hypergiant with a radius over 1,400 times that of the Sun. Its surface temperature is only about 3,500 K, giving it a deep red color.
Brown Dwarfs
Brown dwarfs are objects intermediate in mass between stars and planets. Unlike stars, they never become hot enough at their centers to fuse hydrogen.
Still, brown dwarfs are hot when they form, with surface temperatures over 2,500 K. As they age, brown dwarfs cool to below 1,500 K, becoming so dim that they emit most of their light in infrared.
White Dwarfs, Neutron Stars, and Black Holes
When stars exhaust their nuclear fuel, their cores collapse. In some cases this leads to a white dwarf, neutron star, or black hole. Though extremely dense, these compact remnants can be quite hot and have high luminosities.
White dwarfs have high surface temperatures, around 8,000 – 25,000 K, causing them to glow white or blue-white. Neutron stars have even higher surface temperatures, in excess of 100,000 K. Their thermal radiation makes them appear blue or blue-white.
Black holes have no surface or emission that could reveal information about their interiors. But the material swirling around a black hole can become millions of degrees hot through viscous heating and friction. This infalling material emits copious X-rays and other radiation before disappearing below the black hole’s event horizon.
Effects of Metallicity
As described above, a star’s metallicity or heavy element abundance can tweak its emitted colors. Populations of stars show this effect statistically.
Elliptical galaxies contain many old, metal-poor stars. Consequently, elliptical galaxies appear redder than spiral galaxies, which contain young, metal-rich stars and appear bluer overall.
Extremely metal-poor stars, remnants from the early universe, are important tools for studying the chemical conditions soon after the Big Bang. Identifying them by color can help astronomers select candidates for further analysis.
Effects of Interstellar Dust
As starlight travels through the interstellar medium, it interacts with gas and dust. Blue light is preferentially scattered while red light continues unimpeded. Distant stars can appear redder than their true color.
Density variations in the interstellar medium can also selectively dim or magnify stars along different sightlines. This effect, called interstellar extinction, makes the determination of intrinsic stellar colors and luminosities more difficult.
Conclusion
A star’s color depends most strongly on its surface temperature, and secondarily on its chemical composition. The hottest stars are blue or blue-white, while the coolest stars are red. A star’s color remains remarkably constant over most of its life, slowly shifting as it evolves.
But interstellar dust and a star’s own expansion late in life can dramatically alter its emission and perceived color. Understanding stellar colors provides insight into the physics, chemistry, and evolution of stars throughout the universe.
