What determines the color of a star other than temperature?

Omega Centauri
Colorful Stars Galore Inside Globular Star Cluster Omega Centauri. Credit: NASA, ESA, and the Hubble SM4 ERO Team

The stars are one of the wonderful objects in the sky which can be seen with naked eyes. When you look at the sky some stars appears brighter than others which may be either due to closeness of one star as compared to other or due to its bigger size. You also have observed that mostly stars appear white but some stars appear reddish and even bluish.

Although this colored appearance of stars is not easily visible with naked eyes but if you look at them with telescope, you may easily get their colored view. What determines the color of a star? Which factors are responsible for the colored appearance of stars?

The main factor responsible for color of stars is the surface temperature of Star. The other factors responsible for color of stars are following:

  • Human Vision
  • Interstellar Reddening and Extinction
  • Metallicity in the star

The surface temperature of a star

The color of a star is basically decided by the surface temperature of the star. Any electromagnetic radiation is generated when an atom comes from excited state to lower energy state. The excitation of atom depends on the temperature; the atom will be excited to higher energy state when the temperature is higher. So the energy and frequency of emitted radiation will depends upon the temperature causing the excitation.

The cooler star with less temperature emits electromagnetic radiation of low energy which corresponds to larger wavelength for example red or infrared radiation. Similarly the hotter star with high temperature emits electromagnetic radiation of high energy which corresponds to smaller wavelength for example blue, violet or ultra violet (UV) radiations.

The temperature of a star depends on the size of star. The smaller stars have low temperature due to low gravity and pressure. The massive stars have larger gravity and pressure, the burning rate of fuel is also larger which corresponds to higher temperature. Now we can infer that the smaller stars will be reddish and bigger stars will be bluish in color.

Classes of Stars

Color
Class
solar masses
solar diameters
Temperature
bluest
O
20 – 100
12 – 25
40,000
bluish
B
4 – 20
4 – 12
18,000
blue-white
A
2 – 4
1.5 – 4
10,000
white
F
1.05 – 2
1.1 – 1.5
7,000
yellow-white
G
0.8 – 1.05
0.85 – 1.1
5,500
orange
K
0.5 – 0.8
0.6 – 0.85
4,000
red
M
0.08 – 0.5
0.1 – 0.6
3,000

You will be surprised to know that the star which appears blue or red in color is not only emitting blue or red radiation but also radiation of other colors. The stars do not emit single wavelength radiation; the emission has range of wavelength. The intensity of one wavelength is highest and intensity is lower for higher as well as lower wavelength. The wavelength of maximum intensity corresponds to the surface temperature of star. The radiation from star is similar to the blackbody radiation. So this peak of intensity shifts towards higher wavelength as the temperature decreases and peak shifts towards lower wavelength as the temperature decreases.

Intensity distribution color of a star
The intensity of only wavelength is highest and decreases both side of the peak at constant temperature. Credit: astro-interest.com

Some people have questioned, “Why do the stars never appear green?” Since the stars emit radiation in a range of wavelength, so the appearance of star is the average of all the radiations. All the colors add together to give white color, so the star which have maximum intensity peak at green color will appear white due to equal contribution from red and blue region.

Human Vision

Human eyes are extremely sensitive detectors for visible radiation. Only few photons are required to trigger the stimulation in the eyes. Then why do we not see the color of stars readily? The reason for are following:

  1. Cones and Rods: Our eyes are made up of two types of photoreceptors rods and cones. The rods are present in the peripheral region of retina and cones are present in exactly center of retina in front of eye lens. The rods are more abundant (120 million) and sensitive than cones (7 million). The rods cannot see colors and they only sense grey scale. They are nearly unresponsive to red colors. The cones are of three types red, green and blue. Since the cones are less sensitive they require more photon of correct frequency to trigger them. That’s the main reason we can’t see colors in the night.
  2. Stars as point sources: The stars are present at very huge distance from us, so they appear point source. The human eyes are insensitive to color of point sources. This effect is called small field tritanopia. This effect can be easily understood from the photographs of stars in colored film. The color is hard to distinguish when the stars are focused but if the camera is defocused so that the stars’ light is spread into discs their colors become more apparent as is seen in the photo below by David Malin of the constellation Orion. Here the untracked camera has been increasingly defocused over time so that instead of narrow star trails the starlight is spread out.

    Color of stars
    Our eyes can detect color of extended objects not point object. Credit: Anglo-Australian Observatory
  3. Defocused star trails showing the colour of prominent stars in Orion. Compare the colours of the M-class Betelgeuse with the B-class Rigel. Notice the distinctive pinkish colour of the emission nebula M42.
  4. Pupil size: In the nights, the intensity of light is less so our pupils dilate to allow the more photons come into the eyes. This resulting large aperture of eye lens degrades the quality of image which is called chromatic aberration. Due to this we see colored haloes around the objects.
  5. Dark adaptation – the Purkinje effect: Our photoreceptors become more sensitive in faint light over time. Cones adapt in about 7 minutes whilst rods take about half an hour to reach maximum sensitivity. Cones still remain far less sensitive than rods but as rods are much more sensitive to blue light than red light we perceive dim light as bluer than it actually is. This effect is called the Purkinje effect. One implication for visual astronomy is the tendency to underestimate the brightness of red stars.

Interstellar Reddening and Extinction:

The interstellar space in not empty, it contains neutral gas and ionized plasma located in the plane of galaxy. The cosmic dust contains small grains like silicates, carbon, iron, frozen water and ammonia ice having size in the range 0.1 to 0.01 micron (μm). Although the cosmic dust is just 1 % of the mass of interstellar medium but it absorbs and scatters the radiation coming from stars. This decreases the intensity of the light coming from the stars. The decrease in intensity is called extinction. The stars present at the farther distance suffer more extinction than the closer stars.

The extinction is not same for all wavelengths because scattering depends on the size of the particles of the medium. The wavelength which is comparable to the size of particle get scattered more effectively. If the wavelength of radiation is larger than the size of particles then the radiation passes the medium without being scattered. So the smaller wavelength gets scatter more than the larger wavelength. The blue light get scattered more effectively than red light so the final radiation have more intensity in red wavelength then blue wavelength. Thus the stars appear more reddish than they actually are. This is called interstellar reddening.

Metallicity

The Red giants like Betelgeuse are not actually red instead they are orange in color. There is a group of stars which appears deep red like19 or TX Piscium. These are the carbon stars because they have abundance of carbon molecule such as C2, CH and CN in their outer layers which absorb most of photon in the blue and violet region of spectrum. Thus the light coming from the stars appears deep red in color. They are collectively referred to as type C (Carbon) stars.

Conclusion:

The color of a star is mainly decided by the surface temperature of the star but other factors also affect color of the star. Human eye is more sensitive to blue light than red in the night which gives bluish appearance to the stars. The interstellar medium decreases the intensity of blue light more than the red light which causes the reddening of light coming from the stars. The presence of carbon molecules gives the deep red colors to many stars.

If you want to know more you may read the following references. If you find this article helpful then you may share this on facebook, twitter, google plus or any other social media using the button given below. If you have not liked our social media pages then you are free to like or follow us. You can also get email notification by subscribing us.

References:

http://www.atnf.csiro.au/outreach/education/senior/astrophysics/photometry_colour.html
http://www.astronomynotes.com/starprop/s12.htm
http://en.wikipedia.org/wiki/Extinction_%28astronomy%29
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

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