NOTES ON DOUBLE STARS : BAIZE COLOUR ARTICLE
LES COULEURS DES ÉTOILES [THE COLOURS OF DOUBLE
STARS]
(1956?)
Written by Paul Baize (1901-1995)
Translated from French by Nick Loveday (1981)
Several Parts Re-Translated by Andrew James (2002)
Edited by Grant Searle (1981) and
Further Updated by Andrew James (2002)
I N T R O D U C T I O N
Written by Andrew James
In my own opinion, the following paper is one of the
best ever produced on the subject of double star colours. It is
unusual because it discusses an avenue of double star observation
that was not very much in vogue at the time. This paper concentrated
on the ones that show the most significant colour contrasts, like the
famous β Cyg / Beta Cygni
(Alberio) and γ And / Gamma
Andromedae, and attempts to explain why these colours appear so
spectacular and so pronounced.
I do not know the origin of this paper, but it was
given to me as a photocopy while I and about two dozen of its members
were conducting the Astronomical Society of New South Wales’ (ASNSWI) “Double Star Estimates Program”
during 1979 to 1982. The original article was in French and was
kindly first translated into English by my friend Nick Loveday in
1981. After many hours searching I cannot find the source of the
paper. As it is now nearly fifty years old, (1)
I’m sure I have no real fears of
breaching copyright and have been very careful to acknowledged this
sufficiently. If do you recognise this paper, I would truly
appreciate if the reader could contact me through the link below, so
I can properly acknowledge the reference and reproduce this article.
If you need to formally reference this article, please kindly refer
to this page in your added notes.
Paul Baize was one the most influential of the
European double stars observers during the 20th Century. French by
origin, he produced many observations and micrometric measures of
double stars and contributed much in promoting the subject. He
started his career as a physician but was a keen amateur astronomy
when he started observing double star in 1924. This new study he
continued to do uninterrupted until 1972, after he had in total
measured some 25,000 pairs. Baize also produced one of the first red
star catalogues, contributed significantly for some 140 new binary
star orbits, and during 1928, was among the first modern observers to
produce a decent history on double stars. (In French) This was after
those old school histories like those of Crossley, et al.
(1870) and Thomas Lewis (1908), but was before the interesting small
histories written by giants like Robert Aitken (1935) and
Wulff D. Heintz (1976). Paul Baize sadly passed
away on 6th October 1995, and the world lost one of its greatest
modern double star observers.
The COLOURS of DOUBLE STARS
Astronomers have in the past described double stars as celestial
rubies, topazes, emeralds or sapphires; while the most interesting
colourful pairs have yellow or orange primaries and blue, green or
violet secondaries. Generally when looking at stars through a
telescope these colours are deceptive. The lively colours, contrasts
and shades described are neither as clear or as frequent as one might
believe from some descriptions. Most stars produce an image quite
lacking in any colour, pale and washed out. The only doubtful
exceptions are double stars and the colours they appear to have are
probably much in error due to the various defects inherent in our
processes of visual perception. These defects include the contrast
phenomenon and colour blindness.
To understand how these defects affect our perception of stars
some knowledge of the structure of the eye is necessary. Rather than
discuss that here, I refer the reader to any respect to the above
encyclopaedia, text on optics (including Amateur Telescope Making) or
texts on photography. The essential photo-receptive surface in the
eye is the retina, the light sensitive elements being known as rods
and cones, due to their shape. The rods provide the overall sensation
of light or dark while the cones detect colour. The retina has an
extra sensitive area called the macula containing mostly
cones, this is the area we use most intensely for precise and acute
vision. Cones however have a poorer response than rods under low
levels of illumination, hence the use of “indirect” vision to see faint objects
in the telescope.
The mechanism by which the cones detect colour is not perfectly
understood and the Young-Helmholtz theory will suffice for the
purposes of this article. (2) In this theory
each cone, while appearing to be a single item is really made up of
three fibres. Each of these fibres can detect one of three elementary
colours and there is one of each kind in a normal cone. Suppose the
three elementary colours are Red, Green and Violet [,for example] (R,
G, V) (3)
Unfortunately the perception of colour is also affected by the
overall intensity of the light reaching the eye — the ratios of
the maximum response to light of each colour is not independent of
the level of brightness, and a single colour excites more than one
fibre in the cone. Hence in the case of starlight which is never
highly saturated (the colours are pale and there is a significant
amount of light at all wavelengths in the spectrum) and which is very
much dimmer than the normal level of illumination to which the eye is
accustomed, gross distortions of any colour the star may have are
inevitable. Three phenomena are particularly important:
COMPLEMENTARY COLOURS
1) The complementary colour effect is easily
demonstrated by gazing for a few minutes at a sheet of paper having a
strong colour, eg., red and then looking at a white piece of paper
— it will appear to be a colour which is the complement of the
first colour, in this example, green. Similarly, orange and yellow,
blue and violet are complementary pairs of colours. This effect is
produced by fatigue of the neurons responding to the first
colour — when one looks at the white sheet of paper those
neurons are tired, and the eye perceives the sheet as white minus the
colour of the first sheet, I.e., The complementary colour.
This effect can occur with objects looked at in turn, as above, or
simultaneously in the sense that one turns the eye to receive a star
image of one star at a time on the macula, so that in the process of
looking from primary star to secondary star and back again, the
colour difference in a double star will be exaggerated. This is also
supported by the fact that single stars (if a small enough field is
chosen) rarely show any colours except orangish or white —
certainly not green, violet, blue, etc. In addition, if one component
of a double star is hidden from view by a thick cross hair, the other
component loses all appearance of being brightly coloured. Another
way to demonstrate this is to view combinations of stars and planets
— Saturn appears green when next to Mars.
The PURKINJE EFFECT
2) The Purkinje effect is also interesting: if
a coloured point of light of variable intensity is adjusted slowly
from zero to maximum brightness, the first appearances gives no
impression of any colour at all, and it is only at a certain level of
intensity that any colouration other than white can be detected, and
it must be still higher before the colour can be recognised with any
certainty.
On the other hand, Purkinje also showed that blue and violet were
perceived as colours at lower intensities than other colours at lower
intensities than other colours, White surfaces under low illumination
appear blue-grey.
COLOUR BLINDNESS
3) Colour blindness is the inability to
distinguish between various colours, particularly red and green
— the eye loses the ability to respond to one colour in
particular; in most cases, red. John Dalton was one of the earliest
people to recognise the causes of this problem; he could not
distinguish between the leaves and flowers of geraniums. About 7% of
all men suffer from red-green colour blindness, and a much smaller
percentage of women. Most normal people see light in the range 450nm
to 790nm (4), while others might be able to
perceive light over a range starting well above 450nm going to well
above 790nm in the ultra violet; those people would be unable to
detect red colours. Others might have a range starting in the infra
red, and be unable to see violet.
Without going into the explanations of these effects their
existence is hardly surprising, considering the fact that stars are
seen only in conditions for which the eye is not well adapted. The
result is great variation in the colours of double stars as seen by
various people. For instance Eta (η) Cassiopeiae was
described by J. Herschel as red and green, yellow and blue by Dawes,
yellow and lilac by Flammarion, yellow and reddish by the author,
which corresponds to the spectra, F8 – M1. Σ648 (05h 04m 36s +31° 55′)
was described as yellowish and bluish by Struve, red and red-orange
by Dawes in 1842 and in 1846 white-white again by Dawes,
yellow-yellow by Secchi in 1857 and again white-blue by Secchi in
1858. The companion of Mu (μ) Cygni was seen as blue by
Struve from 1826-1833 and “remarkable
red” in 1836; Dawes noted it as blue, Engelmann red, Duner red,
Perrotin orange, Lewis blue, etc. Since 1921 the author saw it as
white. There are countless other examples. (5)
In addition if one person sees his right eye then the left eye he
will probably also see different colours. (6)
Overall, we may thus conclude that the colours seen are
highly subjective and unreliable.
Following this conclusion regarding colour effects the next step
is to try and find an objective method of determining stellar
colours. A brief glance at the sky shows that some colours are real.
Aldebaran and Betelgeuse are definitely different to Rigel and
Sirius. On the whole stars are yellowish, with some white and a still
fewer orange or reddish. Various devices called colorimeters were
devised by Secchi and Zellner but they have been abandoned because
they did not compensate properly for the personal effects. In fact
there are still two rigorous methods that can be used : the
determination of effective wavelength and the measurement of the
colour index.
(a) Most readers will be familiar with, direct vision
spectroscopes, and the kinds of spectra that can be seen ; line
spectra such as those produced by elements or simple molecules
and continuous spectra, such as those produced by filament
lamps. In addition there are dark line spectra, due to a cold
gas or vapour being located between a source of a continuous spectrum
and the observer — the gas absorbs radiation at the frequencies
it would normally produce bright lines if it were excited.
Through a small spectroscope of low resolution a star shows a
continuous spectrum and while the stars emit radiation across the
whole of the spectrum. Cooler stars emit more strongly in the red and
feebly in the blue, while hotter stars emit more strongly in the
blue-violet part of the spectrum. If one studies the starlight from
any star with the aid of filters and bolometer using precise
photometric standards, that star will have a particular wavelength
which is more intense than any other in the visible spectrum —
the dominant colour. This wavelength can be determined by Wien’s Law, which states that the most intense
wavelength is a function of only the temperature of the star.
However, the star also radiates at all other wavelengths by
varying amounts and so the colour is not saturated, in fact it is
very washed-out by the presence of all the other wavelengths in the
spectrum. The only astronomical objects for which this is not true
are nebulae, which are composed of gas and dust particles and produce
brighter spectra (emission nebulae) or dark line spectra (absorption
nebulae and reflection nebulae). Emission nebulae are often strongly
coloured because they emit light in only a few bands of the
spectrum.
Franks has done a study of stellar colours spectra; the results
are summarised below:
O |
White, sometimes a bit yellowish |
12 |
B |
Very White |
242 |
A |
White |
1190 |
F0-F5 |
White-yellowish |
545 |
G-K |
Yellow |
1238 |
K2-K5 |
Yellow-orange |
109 |
M |
Orange |
150 |
N |
Red-orange |
11 |
(b) Needless to say, because of the differences between the
response of the eye to the various colours it, the spectrum and the
responses of photographic emulsions, stars often appear on
photographs as having magnitudes distinctly different to their visual
magnitude. The colour index is defined as the visual magnitude minus
the photographic magnitude. As examples Betelgeuse and Aldebaran have
photographic magnitudes of 3 or 4.
The reference for the colour index is selected in the middle of
the spectrum (white stars) see that white stars (e.g. Sirius, Vega)
have a colour index of zero, and the range of spectra B, A, F, G, K,
M correspond to the colour index range -0.4m to +1.6m in 0.4 steps
:
Spectral Class |
B |
A |
F |
G |
K |
M |
Colour Index (CI) |
−0.4 |
0.0 |
+0.4 |
+0.8 |
+1.2 |
+1.6 |
This has been refined: (King, Parkhurst & Jordan,
Schwarzschild, etc.)
Spectrum Colour Index Description
B0 |
-0.42 |
White-Blue |
B2 |
-0.21 |
|
A0 |
0.00 |
White |
A5 |
+0.21 |
|
F0 |
+0.42 |
White-yellow |
F3 |
+0.63 |
|
G0 |
+0.84 |
|
G5 |
+1.05 |
Yellow |
K0 |
+1.26 |
|
K5 |
+1.47 |
Orange-yellow |
M0 |
+1.68 |
Reddish |
The description of colours among double stars is interesting. W.
Struve found the following from a catalogue of 596 pairs :
Both Components Same Colour
White |
295 |
Yellow-White |
75 |
Reddish-orange |
5 |
Components of Different Colours
White /yellow |
43 |
White / blue |
58 |
Yellow / blue |
104 |
Green / blue |
16 |
In the past some observers have used rather exotic descriptions of
colours. eg. W. Struve described Alpha (α) Orionis as divine — deep ruby and
Webb used a large range of rather subjective terms — indigo,
grey-white, olivine, pale greyish-rose, bluish-red, brownish, etc.
etc., and more bizarre ones are included.
Personally, my studies of many double star spectra (the 3919 stars
in the Draper Catalogue) confirm that the table above made by F.G.W.
Struve is quite close to describing the kinds of double stars visible
quite accurately. Orbiting doubles are particularly frequently
advanced types of F5-M5 and occasionally M (i.e. yellow or perhaps
reddish) and on the whole the secondary is the same colour as the
primary. Lick Observatory Bulletin No. 343 contains spectral
measurements for 238 double stars, measured by Leonard, who found
that in the vast majority the spectra of primary and secondary were
almost identical. In the cases where the two spectra are not alike,
the companion is almost always more advanced (redder) than the
primary. For example, Eta (η)
Cassiopeiae (F8 and M1), Xi (ξ)
Bootis (G5-K4), 70 Ophiuchi (K0-K4), 61 Cygni (K5-K8), Alpha Centauri
(G0-K5), Sigma (σ) Serpentis (A0-A5),
Zeta (ζ) UMa / Mizar (A0-K5) etc.
Also interesting are double stars with identical spectra where
observers see the components as having different colours. In this
class are, Phi (φ) Draconis (yellow /
lilac, F8-F8) and Alpha (α) Canes
Venaticorum. (yellow / lilac, A0- A0).
Doubles where the companion is less advanced than the primary
include Epsilon (ε) Bootis (G8-A1),
ο Ceti (Mira, M0-A0) Epsilon (ε) Hydrae (F9-F5), but in most of these
cases the primary star is two magnitudes difference making them giant
stars. Under these conditions, this resulting and relatively uncommon
situation, is especially when the stars are of very unequal
brightness. This produces the phenomena of contrast which I have
insisted on at the beginning of this article. It is the
companion’s vicinity to the primary that produces the deep
colours and the nuances like green or bluish. These are sometimes
very beautiful but are to be stripped of any objective reality. Such
are the pairs are well-known by amateurs.
The following doubles are well known to most amateurs
and should prove interesting. Try looking at the double with both
stars in the field of view, then with only one star visible at a
time.
Table 1
Design. Name Con |
R.A. h m (2000) |
Dec o ′ (2000) |
Primary v mag. |
Col. |
Second. v. mag. |
Col. |
Sep. Yr. |
Primary Spectral Class |
Secondary Spectral Class |
Σ163 AB Cas |
01 51.4 |
+64 22 |
6.80 |
or |
9.13 |
bb |
34.7 / 00 |
K5IaO-a |
B5 |
Σ205A-BC / γ And |
02 03.9 |
+42 20 |
2.31 |
jj |
5.02 |
ve |
9.6 / 00 |
K3IIb |
B9 |
Σ307 AB / η Per |
02 50.7 |
+55 54 |
3.76 |
jj |
8.50 |
b |
28.3 / 98 |
K3Ib |
C/M3Ib-IIa |
OΣ67 Cam |
03 57.1 |
+61 07 |
5.25 |
or |
8.06 |
b |
1.7 / 91 |
K3I-II |
A0 |
OΣ72 Tau |
04 08.0 |
+17 20 |
6.10 |
jj |
9.71 |
b |
4.6 / 98 |
K5IIIb |
|
BU 87 Tau |
04 22.4 |
+20 49 |
6.21 |
jj |
8.60 |
b |
2.0 / 83 |
B3V |
K3II |
Σ654 /
ρ (3) Ori |
05 13.3 |
+02 52 |
4.62 |
or |
8.50 |
b |
7.0 / 95 |
K0 |
|
Σ997 /
μ CMa |
06 56.1 |
-14 03 |
5.27 |
or |
7.14 |
b |
2.8 / 99 |
G5III |
A2 |
HJ 3945 |
07 16.6 |
-23 19 |
5.00 |
or |
5.84 |
b |
26.8 / 91 |
K3Ib |
dF0 |
Σ1268 /
ι Cnc |
08 46.7 |
+28 46 |
4.13 |
j |
5.99 |
b |
30.5 / 00 |
G8Iab |
A5 |
Σ1441 AC Sex |
10 31.0 |
-07 38 |
6.51 |
j |
10.14 |
b |
62.4 / 95 |
K5 |
|
Σ1657 Com |
12 35.1 |
+18 23 |
5.11 |
or |
6.33 |
b |
20.3 / 96 |
K2III |
A7m |
Σ1877 AB /
ε Boo |
14 45.0 |
+27 04 |
2.58 |
jj |
4.51 |
bb |
2.9 / 01 |
K0II-III |
A1 |
α Sco / Antares |
16 29.4 |
-26 26 |
0.96 var |
or |
5.4 |
bve |
2.8 / 96 |
M1Ib |
B2.5V |
Σ2140 /
α Herculis |
17 14.6 |
+14 23 |
3.48 |
or |
5.40 |
ve |
4.8 / 00 |
M5Ib-II |
F9 |
Σ43 Aa-B /
β Cygni |
19 30.7 |
+27 58 |
3.37 |
jj |
4.68 |
ve |
34.4 / 98 |
K3II+ |
B8V |
H 84 Sge |
19 39.4 |
+16 34 |
6.38 |
or |
9.46 |
bv |
28.4 / 00 |
K4Ib |
A? |
HJ 599 AC / 54 Sgr |
19 40.7 |
-16 18 |
5.42 |
jj |
7.65 |
b |
45.6 / 91 |
K3III |
F8V |
Σ2727 /
γ Del |
20 46.7 |
+16 07 |
4.36 |
j |
5.03 |
bb |
9.2 / 00 |
K1IV |
F7V |
Σ2877 AB Peg |
22 14.3 |
+17 11 |
6.65 |
or |
9.23 |
ve |
21.0 / 00 |
K4 IV |
G5 |
Σ58 AC /
δ Cep |
22 29.2 |
+58 25 |
4.21 var |
j |
6.11 |
b |
40.9 / 94 |
F5Iab |
F7V |
A quick examination of the preceding list shows that
in the majority, the companion’s
stellar spectrum could be determined. Star in isolated cases would
have their corresponding colour matching their spectral class, and it
is this that brings together the changes like the yellow stars,
giving them sometimes remarkable appearances by their complementary
colours. That this does not prevent the enthusiastic observers from
turning their telescopes towards these pairs; and importantly the
objectivity or the subjectivity of phenomenon. From the point of view
of the artist, is this phenomenon not beautiful? However, the
astronomer has the right and the duty to say that they are only by
appearance.
j = jaune, yellow
ve = vert, green
b = bleu, blue
or = orange, orange
v = violette, violet
Double letters mean particularly strong colour
NOTES
- (1) I have looked at the French, Ann.
D’Astrophys. Nd. It
isn’t in this.
- (2) Comment: The information is a little
out-of-date. (Nick Loveday)
- (3) The information here does not describe
the correct mechanism but principles do work in similar ways.
- (4) Baize expresses the units for nanometres
(nm) as ×1012 Hz
- (5) Comment: As demonstrated by Andrew
James’ DSCE I, II and III. (Nick
Loveday)
- (6) Comment: This is quite true in my case.
(Nick Loveday)
REFERENCES
1.) "Universe" 28, 3 (1981) [Jan] and 28, 4 (1981)
[Mar]
Last Update : 4th October 2012
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(2012)
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