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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 photoelectrically through standard
blue and "visual" filters.
Sky & Telescope, 84(3),
273. |
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.
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."
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
"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," note 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°. |
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“Haze, bright moonlight, and aurorae are
known to affect colour perception; thus colour estimates should be reserved for
dark, transparent nights. Stars should be near the meridian, and therefore,
highest in the sky.” |
