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Star colours

Published: 2009 December 12.

Colour index: the higher the colour index, the more orange the star appears. Negative values are blue. Colour index is defined as B-V, where B and V are magnitudes measured photometrically through standard blue and 'visual' filters.

Looking for colours in stars can be a rewarding exercise, demonstrating not only the variety found in stars, but also highlighting the differences between observers.

John Mullaney writes:

"The subject of star colours is fascinating as well as controversial. It is well known that hues assigned to double stars are often widely discordant and even bizarre. Involved are such complex factors as telescope optics, atmospheric conditions, contrast, colour perception, and the observer's 'preconditioning' from published descriptions."

Paul Merrill (1955) writes:

"The stars exhibit a beautiful range of colour, even to the naked eye. The ruddy Betelgeuse presents a fine contrast to Rigel, its bluish neighbour in the constellation Orion. The ancients gave the name Antares to the fixed star Alpha Scorpii on account of its resemblance to the red planet Mars."

An excellent overview of colour perception in astronomy may be found in Philip Steffey's (1992) article. He writes:

"To the unaided eyes about 150 stars, mostly giants or supergiants of late spectral type (G, K and M), exhibit colours other than plain white or grey. Among the brightest 30 or so, in which colours appear the most distinct, the only definite hues are yellowish oranges, yellows, and pale blues. True reds appear only in a few objects visible with optical aid, principally carbon stars."

The language of star colours

I've found it useful to use a rating scale to record the colour in any stars that I see. The scale I use is:

blueblue-whitewhiteyellow-whitelight yellowdeep yellowlight orangedeep orangeorange-redred

Different people may assign stars to different categories but this is to be expected given the individual differences in human physiology.

Once you've rated a large number of stars according to this scale, you could calibrate your estimates by comparing your categories with the corresponding B−V index of the stars.

Glimpsing elusive star colours

While looking for colour in brighter stars, try shaking your binoculars. The vibration spreads out the image across the field of vision, blurring it and making the colours more easily visible. And, of course, don't use averted vision! Similar advice is offered by Mitton and MacRobert (1989), who say that colours in the brighter stars may become more apparent if you defocus your eyes to turn the stars into small disks.

Colours in double stars

"Every tint that blooms in the flowers of summer, flames out in the stars as night."

John D Steele, 1869

"Double stars have long been appreciated for their often gemlike colours. When two stars of contrasting hues are seen side by side, colour difference becomes very plain," notes Mitton and MacRobert (1989). Check out their table 'Some Vivid Double Stars of William Henry Smyth' for observing suggestions.

Colours in single stars

The table below, arranged by RA, lists stars brighter than 4th magnitude, that have a B−V greater than +1.6, and that lie south of declination +6°.

Bright and colourful

StarVB−VRA (2000)Decl       StarVB−VRA (2000)Decl
alpha Cet 2.53 +1.64 03h 02m 16s+04° 05.4' lambda Vel 2.21 +1.66 09 07 59 -43 25.9
tau4 Eri 3.69 +1.62 03 19 31 -21 45.5 sigma Lib 3.29 +1.70 15 04 04 -25 16.9
gamma Hyi 3.24 +1.62 03 47 14 -74 14.3 alpha Sco 0.96 +1.83 16 29 24 -26 25.9
omicron1 CMa 3.87 +1.73 06 54 07 -24 11.0 zeta Ara 3.13 +1.60 16 58 37 -55 59.4
sigma CMa 3.47 +1.73 07 01 43 -27 56.1 beta Gru 2.10 +1.60 22 42 40 -46 53.1
pi Pup 2.70 +1.62 07 17 08 -37 05.8 lambda Aqr 3.74 +1.64 22 52 36 -07 34.8

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