The colors of the stars indicate their surface temperatures. There are five star colors: blue, white, yellow, orange, and red. The hottest stars are blue, with temperatures around 25,000 K. Red is the color of the coldest stars, which have surface temperatures of approximately 3,000 K.
The 5 Colors of Stars
While the five star colors are blue, white, yellow, orange, and red, there are in-between colors. The color classes are O (blue), B (bluish), A (blue-white), F (white), yellow-white (G), orange (K), and red (M). Remember the order with the mnenomic “Oh Be a Fine Girl, Kiss Me”.
Annie Jump Cannon devised the Harvard spectral classification, which orders stars according to their apparent color as viewed from Earth. At the time, astronomers had not made the connection between star color and surface temperature. The modern classification system (Morgan-Keenan) uses the same classes, but approaches colors based on spectra and luminosity.
|Star Color||Class||Approx. Temperature||Examples|
|Blue||O||25,000 K||Rigel, Spica, Bellatrix|
|White||F||10,000 K||Sirius, Vega|
|Yellow||G||6,000 K||Proxima, the Sun|
|Orange||K||4,000 K||Aldebaran, Arcturus|
|Red||M||3,000 K||Antares, Betelgeuse|
The bluest stars are not only hotter than the Sun, but also much larger (12 to 25 solar diameters) and more massive (20 to 100 solar masses). Meanwhile, red stars are cooler and smaller (only 0.1 to 0.6 solar diameters and 0.08 to 0.5 solar masses). As stars go through their lives, they consume fuel, decrease in size, and shift in color and temperature. So, star color indicates its temperature and also its age.
Factors That Affect Star Color
The star colors look different once you get outside the Earth’s atmosphere. From Earth, most stars appear white or bluish because they are too dim for the human eye to perceive color. So, many people assume photographs taken from Hubble or other space telescopes are colorized. In reality, stars really are much more vibrant and colorful than what we see at night.
But, even the space telescopes don’t see a star’s true colors. Stars moving away from us are red-shifted because of the Doppler effect. Another factor that affects color is extinction. Extinction occurs when cosmic dust between the star and the viewer absorbs and scatters light. This decreases a star’s apparent brightness and also its color. The dust scatters blue light more than red light, so stars appear redder than their true color. This effect is interstellar reddening. A star’s chemical composition or metallicity also affects its color. For example, carbon-rich stars contain molecules that absorb blue and violet light, turning the stars deep red.
Of course, the most significant factor influencing star color is human vision. Even though a telescope sees many reddish stars, the human eye is more sensitive to blue light than red light. So, we see blue stars, but miss most of the red stars. Our eyes do a poor job of distinguishing color of point light sources, making stars mainly appear white. Also, human color vision perceives the net effect of a star’s color and not its spectral peaks. Again, this leads to stars appearing mostly white.
Are There Green Stars?
There are no green stars because star colors come from their black-body spectrum. In other words, the color depends on temperature, much like a candle flame or heated bar of metal. The black-body spectrum does not include all of the colors of the rainbow.
That being said, there are stars which have peak intensity in the green portion of the spectrum. These stars don’t look green because they also emit other colors of light. The way the human eye works, the only way to see green is if it’s the only color.
But, if you photograph stars or look up at the night sky, you might see a green-looking star. This happens when the atmosphere distorts light before it reaches our eyes. The atmosphere is also what makes stars “twinkle”.
One other way of seeing a green star is when the eye sees separate blue and yellow stars as one single point of light. For example, Albireo A (yellow) and Albireo B (blue) appear as a double star. To the naked eye or unmounted binoculars, the pair appears as a single green star. A telescope resolves the individual stars as yellow and blue.
What About Violet Stars
The black-body spectrum allows for violet, which occurs at a temperature around 39,700 K. That is quite a bit hotter than a blue star (~25000 K). However, the Morgan-Keenan (MK) classification system allows for Class O (“blue”) stars that emit significant ultraviolet radiation. While humans can’t see this light, these extremely hot stars are essentially violet.
Purple stars are another matter entirely. Purple is the eye’s interpretation of a mix of red and blue. Stars that emit red and blue light also emit other colors of the spectrum, so they appear white. The only time you’ll see a purple star is when the atmosphere shifts the star’s true color.
What Color Is the Sun?
Our Sun is an example of a star that emits peak light in the green region of the spectrum. But, the Sun appears white as viewed from space because its apparent color is an average of all emitted wavelengths (which include red and blue). From Earth, sunlight is yellow because the atmosphere scatters blue light. Near sunrise and sunset, scattering is more pronounced and the Sun appears orange or even red.
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