Association of Falkirk Astronomers

By Graeme McLeod

The question was raised at the April Star Party and it seemed to be a difficult question to answer. We can see Red stars, Yellow stars and Blue stars, why not green? The coolest stars are red, and as the temperature of stars goes up, yellow light, then green and finally blue light are added to the light which is already emitted so that the colours present in very hot stars includes the colours in cooler stars and the spectrum shows an increasing range of colours.

We normally see the colour of the star is that of the strongest colour or combination of colours emitted. Green in the middle of the continuous spectrum will be accompanied by yellow and red, or by blue yellow and red. The apparent colour is affected by the way we detect the star light; the eye, photographic film and CCD and the degree to which they are colour sensitive and how we interpret colour.

The eye detects colour by cells sensitive mainly to blue, green and red light although the colour sensitive ranges overlap ⁽¹⁾. The eye is constantly in motion and scans the light falling on adjacent cells. The brain combines the outputs from the cells to give the sensation of colour. Film and CCD cameras process colour similarly although the colour sensitivity and balance may be different from that of the eye. If there are green stars then they should be detectable by one system or other.

Further there are emission nebulae, which are mainly red and reflection nebulae, which are blue. Emission nebulae depend for their colour on the nature of their constituent gases (mainly Hydrogen) and the energy of radiation they absorb which makes them glow just like the Aurora ⁽²⁾ but usually red. Levy suggests there can be green as well as red within emission nebulae. Reflecting light from nearby stars lights reflection Nebulae. The Trifid Nebula (M20) in Sagittarius shows both types of nebulae.

Are there any possible candidates for green stars? One mention of an apparently greenish star is in NGC 5128 within Centaurus where a Supernova was obscured by a dust lane. The green was prominent only "when the green image of the red/green blue composite was photographed" ⁽³⁾.

This may be due to red and blue light being scattered and/or absorbed by the dust leaving green. It is suggested in Astronomy Now ⁽⁴⁾ that Antares in Scorpius has "a greenish companion 6.5 magnitude, B class*" 3 arc sec away. This however is apparently due to the redness of Antares ⁽⁵⁾. Another example from a different issue of the same magazine is Albireo in Cygnus ⁽⁶⁾. Rasalgethi in Hercules also has a green companion (5^th magnitude) 4.6 arc sec away ⁽⁷⁾.

Unfortunately, this is a contrast effect due to the extreme brightness of the accompanying red star and probably the saturation of the colour receptors in the eye to red. The saturation remains due to the persistence of vision. A similar effect is usual after looking at highly coloured paper and looking at white paper. These are a few examples of apparently green stars, all of which are the minor partners of a binary system. The existence of intrinsic green stars is therefore not certain but we can keep looking! This may lead to an interesting project.

Notes

• Coordinates: Antares or Alpha Scorpius (R.A. 16h 29.4 m, dec -26^o 26C)
• Albireo or Beta cygni (R.A. 19h 28.7 m, dec 27^o 52C ) Rasalgethi or Alpha Herculis (R.A. 17h 12.4 m, dec 14^o 27C )
• * B class stars are white or bluish-surface temp ~ 25 000 K

References

1. Best and Taylor The Human Body p600
2. David Levy Skywatching p42
3. David Levy Skywatching p153
4. Astronomy Now June 2001, "Sky Scene": Features of Scorpius p 43
5. Patrick Moore Mitchell Beazley New Concise Atlas of the Universe p166
6. Astronomy Now August 2001 "Two Small Refractors "p69
7. Patrick Moore Mitchell Beazley New Concise Atlas of the Universe p126