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Why Are Stars Never Green In Colour?

by Madonna Watts D'Souza
Why Are Stars Never Green In Colour?

January 18, 2021

Stars are some of the most beautiful treats for our eyes and even our minds. Yet, of all the colours of stars we know and see, none has ever been 'green' in colour. The answer may lie deep within the radiation of the stars.

A bright starry night far away from the city is a leisurely pursuit many of us yearn for. Our skies are filled with stars of varying brightness and different colours. The wonderful constellations made in the skies with the stars’ position and the frequent appearances of planets make the interstellar space a living treat for the inhabitants of the earth.

But, if someone asked you to remember the colours of the stars you’ve witnessed so far, you most probably will have seen all your stars as white. Some might even have seen stars that glowed of yellow, blue, orange and red hues. But one specific colour of stars would not have been seen by anyone- that colour is green.

Why Aren't Stars Green?

Theoretically, the glow of stars can reflect the entire spectrum – VIBGYOR – of the rainbow. However, the colour green is never observed in stars. In the quest for finding out why stars never look green, you can (and should if you are as geeky as us about stars) perform a safe and straightforward experiment. For relativity purposes, remember that stars are massive heavenly bodies with sizzling temperatures that can melt metals in seconds.

Take a tiny piece of an iron wire. With protective and heat resistant forceps, expose the wire to high temperatures from a blowtorch or a burner. Now focus on the wire. Initially, the metal will emit a bright red colour, then orange and finally a bluish-white shade will be cast before the wire melts off.

scoolbuzz-stars-sky-colors-space-universe
Although you can spot many colors of stars in the night sky, purple and green stars aren't seen because of the way humans perceive visible light.   (Image: © Mike Read (WFAU), UKIDSS/GPS and VVV)

Now you may wonder, why does the wire glow in different colours in such progression? Answer- any form of matter above absolute zero temperatures (-273 Celsius) will emit a glow (even humans!). The wavelength of light, however, is dependent on the source object’s temperature. The warmer the item, the shorter the wavelength, and vice versa.

Science of Glowing Colours

Practically, colder objects emit radio waves while hotter objects radiate the visible spectrum of light. Extremely hot objects even emit X-rays or Ultraviolet light. Objects never usually emit a single wavelength of light. Instead, they emit an aesthetic combination of photons at different wavelengths—the colour changes with light wavelength.

So, why do stars not glow green? 

Stars emit a combination of lights, where every wavelength has a spectral peak. However, when the colour green gets its time to shine, the spectral peak is already achieved by other colours. This can be explained by the graph of black body radiation, where it can be observed that the spectral peak shifts to the left with the increase in temperature.

In effect, it means that when the star’s temperature rises in the green region, it cannot glow green, since the other wavelengths of light have already been emitted and we observe all the different colours. This can even apply to why one cannot see violet coloured stars in the sky. However, thanks to an optical illusion caused by red coloured objects, the nearby stars ‘can’ appear green. But reality sense, stars can never appear green in colour.

sirius-colors-UK-scoolbuzz
Amanda Cross in the U.K. caught these images of Sirius on December 11, 2017. She wrote: “The color flashes are picked up by the camera as the atmosphere splits the light from the star. No color enhancements were made to this image. This is how the camera picked up the colors.”
Four Cool Things About Stars
Stars Are Black Bodies
A black body is an object that receives 100 per cent of all electromagnetic radiation (that is, light, radio waves, etc.) that comes on it. In the case of a star, it consumes all radiation that falls on it, but it also radiates back into space much higher than it absorbs. Hence, a star is a black body which shines with superb brilliance.
Our Sun is a Green Star
The sun is a "green" star, or much accurately, a green-blue star, whose top wavelength lies clearly in the development sector on the spectrum within blue and green. This is not just an inert fact, but it is crucial because a star's temperature is related to the colour of its most predominant wavelength of emission. (Whew!) In the sun's case, the surface heat is about 5,800 K, or 500 nanometers, a green-blue.
You Can Witness 20 Quadrillion Miles, at least.
On a good night, you can notice about 19,000,000,000,000,000 miles, easily. That's 19 quadrillion miles, the estimated distance to the bright star Deneb in Cygnus, which is raised in the evening skies of Fall and Winter.
Black Holes Don't Suck
In the case of black holes, the matter is pulled into it by a strong gravitational attraction. It is a bit like falling into a hole in one way of visualising it, but not like being hoovered into the entity. Gravity is a central force of Nature, and all matter is subjected to it. When an object is pulled into a black hole, the process is more like a fish being reeled in by an angler than being driven along like a rafter inevitably dragged over a waterfall.
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Galaxies Have Green, Just Not for the Stars

Galaxies are abundant with hues of green because their formation differs from that of stars. One of the astronomical phenomena which are green in colour is the Aurora Borealis or the Northern Lights. But when one observes the space, the green light is relatively abundant in the area apart from stars. 

Comets, gas clouds, nebulae and galaxies are full of green hues. The answer to these phenomena again is in the heat of the gas particles. When gas is superheated to unimaginable temperatures, its electrons are lost and become ionised. When these electrons recombine with the ionised particles, they radiate wavelengths of different colours.

For emitting the colour green, oxygen must be ionised twice by reaching fiery temperatures of 50,000 K or above. And hence, during memorable astronomical events, like starbursts or nebulae, the colour green is the compulsory aftermath for these events.

So, the next time you look up to the sky and wonder why the stars don’t emit green colour, you can look out for the Northern Lights to help complete the VIBGYOR palette!

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