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Measuring
double stars
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Double
stars
Although
to the naked eye many stars appear to be single points of light, many
of them have one or more companion stars forming a gravitationally locked
dynamic system. Of the many double, binary or multiple star systems
that exist, some are simply too close together or too faint to be seen
without optical aid; but with the telescope their duplicity becomes
apparent. An important class of these double stars are the visual double
stars and it is these that amateur astronomers often turn their attention
to.
While a
moderate sized telescope is often required for serious visual double
star observing, many of the wider double stars are well within the range
of binoculars or small telescopes. The amateur astronomer is often attracted
to double stars finding suitably difficult stars to separate or 'split',
but also in comparing their brightness and colours. More serious work
lies in accurately measuring position angles and angular separations,
thereby providing invaluable data to complement that used by professional
astronomers for core work in astrophysical research.
The drawings
of double stars shown below have been compiled from the logbooks of
Christopher Taylor, and show what an experienced amateur astronomer
using a moderate sized instrument at the site, can expect to see. These
drawings were done at the eyepiece of the 12.5" f/7 Newtonian.
Particular
attention is drawn to the high magnification and seeing recorded which
is typical of the Hanwell site. Note too the definite change of position
of z Cancri, which shows quite conclusively
that the detection of the dynamic nature of these binary star systems
lies well within the reach of any amateur astronomer prepared to record
their observations in this simple manner. Also of note is the detection
of duplicity of Albireo A in which the elongation and noted change in
colour of the Airy disc at the point of elongation verified not only
the existence of a very close companion star, but its spectral class
too.
 (All
drawings © 1999, Christopher Taylor)
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Rigel
12.i.72, X176, X352
scale bar = 10 arcsec |
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z
Cancri
10.iii.94, X820, Seeing II, period AB = 59.6 yrs
scale bar = 5 arcsec |
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z
Cancri
10.iii.97, X820, Seeing I - III, Note orbital motion of AB since
1994 |
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h
Orionis
17.i.75, X352, Seeing II
scale bar = 5 arcsec |
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72
Pegasi
4.xii.93 and 12.x.94 (averaged), X820, Seeing II - I, period = 241
yrs
scale bar = 2 arcsec |
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l
Cassiopeiae
11.i.95, X820, Seeing II, period = 640 yrs
scale bar = 2 arcsec |
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g Andromedae
various dates 14.xii.94 - 17.x.96, X238, X352 and X820, Seeing II
or I, period = 61.1 yrs
scale bar = 5 arcsec |
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i
Cassiopeiae
14.xii.94, X352, Seeing II, period AB = 840 yrs
scale bar = 10 arcsec |
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b
Monocerotis
29.xii.74, X122, Seeing III
scale bar = 10 arcsec |
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f
Andromedae
19.x.95, X820, Seeing II, period = 372 yrs
scale bar = 2 arcsec |
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t
Cygni
25.ix.72, X352, Seeing ~II, period = 49.8 yrs
scale bar = 2 arcsec |
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t Cygni
27.ix.96, X820, Seeing II, note orbital motion since 1972 |
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Albireo
17.x.96, X820, Seeing II - I, AB not to same scale as Aa (AB =34
arcsec, AA = 0.29) |
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d
Equulei
8.x.95 and 15.x.95, X820, Seeing II, period = 5.7 yrs
scale bar = 1 arcsec |
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d
Equulei
27.ix.96, X820, Seeing II - III, note motion since 1995 (but
p.a. unreliable). |
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b
Delphini
4.xi.95, X820, Seeing II - III, period = 26.65 yrs
scale bar = 1 arcsec |
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b
Delphini
13.xi.96 and 23.xi.96 (averaged), X820, Seeing III - II, note motion
in last year |
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g
Persei
4.xii.97, X820 and 26.i.97, X820, Seeing I - II, period = 14.65
yrs, round in 1995
scale bar = 1 arcsec |
Measuring double stars
Visual
impressions are, of course, not to be trusted unless adequate precautions
are then taken to eliminate observer bias - the common phenomenon of
seeing what you expect to see. To reduce this to workable limits, a
rigorous approach to the pursuit of objectivity has been adopted. No
pair having a current separation below about 1.5 arcseconds for which
consequently there may be any doubt regarding true detection of its
duplicity, is ever observed after prior knowledge of its 'correct' or
predicted position angle. In every case the observation is conducted
'blind' and only afterwards is an ephemeris consulted to see how well
theory and observation compare.
In this
way (and no other) do we have an independent objective criteria by which
to test the reality of any resolution or image-elongation seen. This
procedure, though contrary to that frequently recommended in amateur
observing handbooks, is, of course, nothing more than standard scientific
experimental method; without which such objective methods visual observation
of sub-arcsecond binaries becomes a meaningless exercise in verifying
observer prejudice. A project is currently planned to carry this method
of objective testing to these rigorous levels, by use of an image-rotator
to randomize the position angles seen at the eyepiece, then subjecting
the repeated values recorded to full statistical correlation analysis
(observed vs expected).
In the
context of what has just been said, it is worth noting that more than
half the observations depicted here are of pairs whose separations at
the relevant epoch were ~0.5 arcsec or less, the closest here being
g Persei at 0.25 arcsec. Broadly shown in order of decreasing separation,
these observations show excellent agreement with the empirical 'Dawes
limit' for this 12.5" aperture (0.37 arcsec), the telescope showing
discs tangent at 0.36-0.37 arcsec as in d
Equ 1995.78 and b Delpini 1996.88, the image
blending into a single oval of pronounced elongation at 0.31 arcsec
(b Del. 1995.84). All of this is for pairs
whose contents differ in brightness by less than 1 magnitude, a fact
which often makes a 0.3-0.4 arcsec equal binaries far easier telescopically
than a widely contrasted pair at 1 arcsec or more, and which makes possible
fairly confident estimates of PA for these systems from their oval blended
images, well down into the 'sub-Dawes' regime.
The current
record for the Hanwell 12.5" is held by k
Pegasi 1996.88 at 0.23 arcsec and a Comae
1998.41 at 0.20 arcsec; down to this level of separation, an informal
examination of results shows a correlation of observed with expected
PA's of something like 90% with the errors of the visual estimates,
the cases of clear discordance (showing the observation to have been
spurious) accounting for only a tiny minority of the whole.
This type
of observing undeniably places severe demands on sky-conditions, optical
quality and adjustment of instrument, and training of the observer,
but it is hugely rewarding to see a number of the sacred cows of astronomical
mythology demolished; that a Newtonian is not capable of this high-resolution
performance even if the seeing were perfect (the Hanwell 12.5"
routinely shows the full Airy pattern of disc and 3 rings at full aperture
on good nights); that magnification above 30X or 40X per aperture-inch
are useless (this is one of course, that no real double star observer
takes seriously - noting that none of the appearances shown here at
separations below 0.5 arcsec are visible at powers less than 500X on
the 12.5") and that the amateur cannot see the orbital motion of
physical pairs over a reasonable period of time-span (Both d
Equ and b Del. showed very obvious orbital
motion in the observations in just 12 months!). Perhaps the most remarkable
feature is the truly minute change in separation of a sub-arcsecond
pair which will transform its appearance completely: the two images
of b Delphini differ in separation by only
0.06 arcseconds, which is about the limit of resolution attained by
the Hubble Space Telescope. Serious high-resolution can be done even
in England from 400 feet above sea-level!
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