Why Are There No Purple and Green Stars?

The stars are amazing and beautiful to look at. But they also have color limitations.

Copyright : NASA, ESA, and T. Brown (STScI)

Have you ever looked at stars on a clear night? Surely they look very much. In our galaxy alone, the Milky Way galaxy, it is estimated that there are around 100 – 400 billion stars. Yes, a very large number. But why do we never find green or purple stars?

Basic Things

Stars also have a layer structure like Earth. Sequential from the inside out is the sun’s core, radiative zone, tachocline, convective zone, photosphere, chromosphere, corona, and heliosphere.

Layers of the Sun.
Copyright : NASA/Goddard

One of the outermost layers is called chromosphere (from the word chroma + sphere, meaning color ball). As the name implies, the color of the stars that we observe comes from this layer.

The color emitted from the chromosphere is determined by the temperature. Consider the following color temperature diagram.

Temperature color

The higher the temperature, the color will be bluer, and the lower the temperature, the color will be redder. But stars (or any object) will only be seen emitting light if the temperature is above 700 Kelvin.

From red to blue. Maybe what comes to our mind about the color spectrum in general, “red, yellow, green, blue, purple”, in the middle is green, and at the right end, there will be purple. But in reality in the middle of the temperature diagram is white and at the blue’s end, there is no purple.


Why Is There No White Color in the Middle of the Temperature Diagram and at the End?

This happens because the star emits light in almost all available spectrum, including colors appearing green and purple, only each color has a different intensity according to its temperature.

Low temperature stars

Here is a spectrum of 3750 Kelvin stars.

It can be seen that red has the highest intensity among other colors. While the lowest intensity is around the colors purple and blue. This makes the star has a color that tends to be red.

Medium temperature stars

Then consider the following 5500 Kelvin star spectrum.

It can be seen that the highest intensity is in green. But why do the colors of the stars not tend to be green? Because the intensity of the other opposite lights is almost balanced, like dark red with purple, red with blue, and cyan with yellow. Do you remember that white is a combination of all colors? That’s what makes these stars tend to be white.

High temperature stars

Then consider the following 6500 Kelvin star spectrum.

It can be seen that the peak intensity is in blue, while the lowest point is in red. This makes the star blue.

Very high temperature stars

And finally, consider the following 7700 Kelvin star spectrum.

If you pay attention, the highest intensity is in purple. So why is the color of the stars blue instead of purple? This is because there is not enough purple light to dominate the other light colors. It can be seen that purple light has a very thin spectrum range. Thus, the blue color which incidentally has a wider spectrum dominates the color of stars. This makes the star blue.

Conclusion

Stars emit all colors of visible light, only with varying intensities. Cold stars tend to be red, because they are dominated by red. Whereas hot stars tend to be blue, because the intensity of purple light is not enough to dominate the spectrum so the blue color dominates. And medium-temperature stars tend to be white, because they emit all light in a balanced intensity.

Sources and References:

  1. Why are there no Purple Stars? or Green Stars? – The Science Asylum
  2. Blackbody Spectrum – PhET Interactive Simulations
  3. Chromosphere – Wikipedia
  4. Sun – Wikipedia

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