A small hand­ful of stars ap­pear to glow emer­ald in the night sky. As­tronomer Phyl­lis Lang re­veals how to spot them

Sky at Night Magazine - - CONTENTS -

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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