OBSERVING GREEN STARS
A small handful of stars appear to glow emerald in the night sky. Astronomer Phyllis Lang reveals how to spot them
Technically, they may not exist, but green stars still make for a great observing challenge.
For many years I’ve observed all kinds of deep-sky objects. I have been most interested in planetary nebulae, partly because they’re most likely to show colour. Within the last few years I’ve also become interested in the colours of stars, particularly colour-contrasting double stars. A long-time observing friend of mine brought up the idea of observing green stars, and a social observing project began.
Stars seem to appear in almost every colour of the rainbow. As described in Star of the Month in our May issue, both eye physiology and the spectrum play a part. The novice observer may detect no colour in stars but, in fact, any star has an apparent colour that is detectable with photometry. The star’s light can be filtered so that separate colour components (wavelengths) can have their brightness measured. The brightness of a star’s light also determines whether the human eye can detect a colour. If a star’s light isn’t bright enough to stimulate the eye’s cones, we see no colour. The rods in the eye are more sensitive to light, but only detect shades of gray. Thus, a bright star may appear to have colour, while a dimmer star may appear only whitish or greyish.
Bright stars may appear red, orange, yellow, blue or white. Noticeably absent from this palette is green, even though the human eye is most sensitive to green light. Instead, a pair of double stars with different-coloured components interact in such a way they may give the impression that one of the stars is green or bluish green. This small handful of ‘green’ stars are worth seeking out and make for a fun observing challenge.
Observations in this feature were made with an 8-inch reflector and rather clean eyepieces. Visual impressions at different magnifications are included. Many of the stars on our list are bright so they can be examined successfully with binoculars or small telescopes, but colour may be difficult to tease out without higher magnification and moderate aperture. A steady mount is required for binoculars and steady seeing is a must.
Your results may not match those of another observer, but it is an enjoyable exercise to compare results with your observing companions. Don’t be surprised if there is great debate among participants! You may find that telescope aperture, magnification, local light pollution and age of the eye affect your findings. Also, your first impression of colour may be a green hue, but the longer you look at a star, the less green and more blue or yellow it might appear. Any of these results should be noted among your observations.
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