To the naked human eye, most stars appear as white pinpoints of light in the night sky. However, stars actually emit light in a range of colors. The color of a star depends on its surface temperature.
How stars emit light
Stars generate energy through nuclear fusion in their cores. This produces an enormous amount of heat and pressure, causing the core to push energy outwards. The energy propagates through the star and eventually escapes from the surface in the form of light across various wavelengths.
The color and intensity of the emitted light depend on the star’s surface temperature. Hotter stars tend to appear bluer or white, while cooler stars appear reddish. But the relationship between temperature and color is not entirely straightforward for stars.
Blackbody radiation
Stars emit light similarly to a theoretical object called a blackbody, which absorbs all incoming radiation and itself emits radiation across all wavelengths. The distribution of wavelengths emitted by a blackbody depends solely on its temperature, as described by Planck’s law of blackbody radiation.
Specifically, as a blackbody gets hotter, the peak of its emission shifts to shorter wavelengths. For stars, this means the color shifts from red to orange, yellow, white, and eventually blue-white as the temperature increases.
| Surface temperature | Star color |
|---|---|
| Below 3,500 K | Red |
| 3,500 – 5,000 K | Orange |
| 5,000 – 6,000 K | Yellow |
| 6,000 – 10,000 K | White |
| Above 10,000 K | Blue-white |
Apparent star color
However, the relationship between a star’s temperature and observed color is more complex. Two additional factors come into play:
- Stars do not emit as perfect blackbodies. They have unique compositions that cause their emitted light to diverge from the ideal blackbody curve.
- Earth’s atmosphere scatters away much of the blue and ultraviolet light coming from stars before it reaches our eyes.
As a result, stars with surface temperatures above 7,500 K or so will generally appear white to the human eye rather than their intrinsic blue color. And stars with temperatures of 4,000-5,000 K can appear yellowish rather than orange. Only the coolest stars below about 4,000 K clearly show their reddish hue.
Brightness and distance
Additionally, a star’s apparent brightness influences its perceived color. Very bright stars tend to appear white or blue-white regardless of their true color. Sirius, the brightest star in the night sky, is a vivid blue-white yet its surface temperature is only around 10,000 K which would normally appear white.
More distant stars also tend to appear whiter than their actual color. This is because the reddish and bluish components of their light are preferentially scattered away, leaving behind mostly white light.
Categorizing star colors
Nonetheless, stars are broadly categorized into the following color types:
- Blue and blue-white stars: Hot O- and B-type stars with surface temperatures of 30,000 K or more.
- White stars: Mid-temperature A- and F-type stars around 7,500 to 10,000 K.
- Yellow stars: G-type stars like our Sun at around 5,000 to 6,000 K.
- Orange and red stars: Cool K- and M-type stars below 5,000 K down to around 3,000 K.
Notable colorful stars
Here are some examples of colorful stars visible to the naked eye:
- Betelgeuse: A red supergiant star in the constellation Orion. Its distinct red hue is readily noticeable through binoculars or a small telescope.
- Polaris: Our northern pole star has a yellowish tint distinguishable to keen observers.
- Aldebaran: An orange giant star marking the eye of Taurus the bull.
- Rigel: A hot blue supergiant star in Orion and one of the most intrinsically luminous stars in our galaxy.
- Sirius: Appears blue-white to the eye as the night sky’s brightest star.
Viewing colorful stars
Distinguishing the colors of stars with the naked eye requires ideal viewing conditions. Light pollution washes out the subtle hues, so get as far away from city lights as possible. Using binoculars or a telescope gives the best chance of detecting color.
The stars should be reasonably high in the sky, as viewing them near the horizon tends to neutralize their colors. And give your eyes time to adjust to the dark – at least 15-20 minutes – for the best night vision.
With practice and patience, you can pick out the faint reddish, orange, yellow, and occasionally blue stars sprinkled across the heavens alongside the more common white stars.
Measuring star colors
While the human eye has limited color perception for stars, astronomers use instruments to precisely measure starlight across all wavelengths. Special photometric filters allow quantifying the amount of blue, visual, and red light coming from stars.
Photometric measurements show the continuous distribution of star colors that we coarsely group into blue, white, yellow, orange, and red types. This allows properly categorizing the surface temperature and intrinsic luminosity for each observed star.
Causes of star colors
A star’s color stems directly from its surface temperature, which in turn depends primarily on the star’s mass. Here is an overview of what makes different types of stars appear the color they do:
Blue stars
Blue stars are very hot, massive stars. Their high mass means extreme pressure and temperatures in the core, driving a high rate of nuclear fusion. Blue O- and B-type stars have surface temperatures exceeding 30,000 K.
White stars
White stars are moderately hot at around 7,500 to 10,000 K. This includes A- and F-type main sequence stars up to about twice the mass of our Sun. Our Sun is right at the upper end of this temperature range.
Yellow stars
Yellow stars like our Sun emit their peak radiation at green-yellow wavelengths of 500-600 nm corresponding to surface temperatures around 5,000 to 6,000 K. Stars a bit more or less massive than the Sun appear yellow.
Orange and red stars
Cooler K- and M-type stars with surface temperatures below 5,000 K emit lower energy light, giving them orange or red hues. Red dwarfs may be only one-tenth as massive as the Sun.
Evolutionary effects
As stars age, their color can change substantially. Red giants are evolved low-mass stars that have swollen and cooled. Blue supergiants represent an early stage in the lives of ultra-massive stars.
Importance of star color
The color provides valuable insights into the properties of stars. Along with brightness measurements, it allows determining intrinsic stellar properties like temperature, size, mass, and life stage. This underpins much of our understanding of stellar evolution.
Star color also reveals their composition. Unusual colors can indicate the presence of chemical peculiarities or exotic circumstellar environments. Analyzing star colors is a key capability for astronomy.
Conclusion
To summarize the main points:
- Star color is linked to surface temperature, which relates to stellar mass.
- Blue stars are the hottest, while red ones are the coolest.
- Small effects modify the intrinsic colors seen by human eyes.
- With good viewing conditions, the colors of some individual stars are apparent.
- Astronomers use instruments to quantify star colors and temperatures.
- Stellar color reveals fundamental information about the stars.
The myriad of colors among the stars helps bring the static points of light to life, hinting at the diverse properties and life stories encoded in their hues.