DOUBLE STARS
Neil Webster
NEIL WEBSTER 03/2015
William James Observatory
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1) Definition and types of double/binary stars
2) History and early catalogues
3) Present day sources
4) Important Results from Double Star/Binary System observations
5) Recommended reading
6) Observing plans/tips
7) Observing tips
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Definition and types of double/binary stars
Two stars in close proximity as seen through a telescope
Either:
1)
OPTICAL DOUBLES: chance alignments and not physically related
2)
VISUAL BINARIES: physically related orbiting around a “Common
Centre of Mass” (barycentre)
Can be separated with a telescope
3) NON-VISUAL BINARIES: usually not possible with a visual
telescope
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VISUAL BINARIES
divide into:
1) COMMON PROPER MOTION PAIRS (CPMs)…………that are drifting
through space like a single body
They are physically in orbit around a common centre of mass
(barycentre) but over long orbital periods of 1000+ years
2) VISUAL BINARIES that are seen to orbit over shorter time periods of
a few years to 100s of years
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Types of NON-VISUAL BINARIES:
1) INTERFEROMETRIC BINARIES ………ultra close. Less than 0.15 seconds of arc.
Measurements down to 0.01 arc seconds!!
2) ASTROMETRIC BINARIES……..detected using CCD astrometry through anomalies in
proper motions. Hipparcos (1989 – 1993)
3) SPECTROSCOPIC BINARIES…revealed through spectroscopy
Mizar: all three stars are spectroscopic binaries (6-star system)
Castor: visual triple but six star system
Third star is eclipsing binary as well
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4) ECLIPSING BINARIES (extrinsic) revealed through occultation and associated light
curves
Algol (John Goodricke 1782)
5) CONTACT/CATACLYSMIC BINARIES….mass exchange…..Novae, Dwarf Novae,
Supernovae (Type 1a)
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History and early catalogues
Early telescopic observations/discoveries:
Nu SGR
Observed/described as a double over 2000
years ago by Ptolemy
MIZAR
Giovanni Battista Riccioli (1650)
Earlier by Benedetto Castelli (1578 – 1643) and
Galileo (1564 – 1642)?
GAMMA ARIETIS
Robert Hooke (1664) whilst following Comet Hevelius
ACRUX (Southern Cross)
Fontenay (1685)
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All stars in close proximity were thought to be mere line of sight chance alignments
(Copernicus 1543, Galileo 1610)
But JOHN MICHELL (who also predicted black holes) in 1767 argued that close stars
were connected “under the influence of some general law”
First observed by
WILLIAM HERSCHEL (1779+) used powerful reflectors and home-made micrometers
to measure (with considerable skill) small stellar parallaxes of
alpha Geminorum (Castor)
gamma Leonis (Algieba)
epsilon Bootis (Izar)
zeta Herculis
delta Serpentis
gamma Virginis (Porrima)
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but by 1802 concluded that they must be
gravitationally connected and in mutual orbits
due to similar:
Proper motions, parallaxes, radial velocities
Herschel first used the term “Binary
Stars”
Outlined in Philosophical Transactions 1803:
“Account of the Changes that have happened
during the last Twenty-Five Years in the Relative
Situation of Double Stars; with an Investigation
of the Cause to which they are owing”
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CATALOGUES
repeated observations allow orbit calculations and classification
of systems as “optical” or “binary”
EARLY CATALOGUES
1779 Christian Mayer
80 entries (led by Herschel, Struve)
1782 William Herschel
269 entries
1784
484 entries
1821
145 entries
1824 John Herschel / James South
380
1822 Wilhelm Struve
795
1827
3112
1874 John Herschel
10,300 (post.)
1878 Otto W. Struve
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547
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1906
Sherburne Wesley Burnham A General Catalogue of Double Stars
(BDS)
within 121 degrees of the North
Pole
13, 665 systems
1932
Robert Grant Aitken (ADS)
New General Catalogue of Double
Stars within 120 degrees of the
North pole
17, 180 entries
26, 650 (by time of death)
Further work by Hamilton Jeffers (1893 – 1976), Willem van den Bos, Albert Edward
Whitford (1905 – 2002), Kaj Aage Strand (1907 – 2000), Charles Edmund Worley (1935 –
1997) and Brian D. Mason (1961 - ) lead eventually to the
WASHINGTON DOUBLE STAR CATALOGUE
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Read in detail
KEEPERS OF THE DOUBLE STARS
Joseph S. Tenn (2013)
http://arxiv.org/ftp/arxiv/papers/1304/1304.5494.pdf
An excellent paper outlining the history of double star catalogues, containing brief
biographies of the main astronomers from Castelli to the present day contributors of the
Washington Double Star Catalogue
Also of interest:
THE WILLIAM HERSCHEL DOUBLE STAR CATALOGS RESTORED
Bruce MacEvoy
http://www.handprint.com/ASTRO/Herschel.html
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Present day catalogues:
The WASHINGTON VISUAL DOUBLE STAR CATALOGUE
http://ad.usno.navy.mil/wds/wdstext.html#intro
The definitive catalogue covering the whole sky compiled from many
earlier lists (see website) and containing
132,120 entries (2000)
20437 physical doubles, 4584 optical doubles and 107099 unknown (!!!!!!)
Has lists of “neglected doubles”
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www.webbdeepsky.com/double-stars/
WEBB DEEP SKY SOCIETY has info on many double star systems (“double
star of the month” - Rob Argyle ( Also Astronomy Now))
46150 measured systems
https://www.astroleague.org/
AMERICAN ASTRONOMICAL LEAGUE Contains both binocular and
telescopic observing program listings…..which can lead to a
reward/badge!!
www.britastro.org
BRITISH ASTRONOMICAL ASSOCIATION buried in the Deep Sky section
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Important Results from Double Star/Binary System
observations:
1) Determinations of stellar masses
Possible using Newtonian Mechanics (1686-87) and Kepler’s Third Law (1609-1618)
Semi-major axis/average separation during elliptical orbit (a), Period of revolution
(P) lead to
𝑀1 + 𝑀2
4π2 𝑎3
=
𝐺𝑃2
If distances of stars from common centre of mass (𝑎1 , 𝑎2 ) can be measured
Then
𝑀1
𝑀2
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=
𝑎2
𝑎1
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The individual stellar masses
𝑀1 , 𝑀2
can then be calculated
This is the only direct way of determining stellar masses but is very time
consuming and only carried out for a few systems!!
2) Mass - luminosity relationship (Eddington) / H-R diagram
Both follow from previous mass determinations
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3) Masses of Pre Main Sequence Stars
1990s+: most PMS stars are binary (observations HST: curved
motions of individual components…….orbits!!)
Therefore used for mass determinations
PMS binary numbers are often greater than those in MS
Implies possible disruption of early multiple star systems (later:
OCs)
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4) Type 1a supernovae
Form from binary system (White Dwarf +……). Accretion/mass
transfer onto WD up to maximum Chandrasekhar mass of 1.4 solar
masses…..supernovae!!
Consistent peak luminosity and therefore
“Standard Candles”
Vital for refined distance measurements, re-evaluation of Hubble
constant and…………………
accelerating expansion of the universe (1998: Supernova
Cosmology Project/High Z supernovae search team)
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5) Evidence for gravitational waves??
Studies of the binary pulsar PSR 1913+16 using the Arecibo 305-m radio telescope in
1974 by Alan Russell Hulse (1950 - ) and Joseph Taylor (1941 - ) showed:
A decaying orbit due to energy loss by gravitational waves (as predicted by Einstein’s
GR)
However, gravitational waves have, as yet, to be directly
detected!!
6) Astrophysical phenomena
Dwarf Novae, Novae, X-Ray Binaries, Type 1a Supernovae, Contact Binary/Accretion Disk
systems
All at the forefront of current research and are incredible
Sources of energy
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7) Star Formation Theories
Many theorists believe stars always form in multiples. Future
single /double stars form by breaking away from a former group?
Binary stars are “fossils” of early formation
Also, Massive stars could be formed from a binary system:
Forming star with accretion disk…………..companion passes
close……..tidal effects cause episodes of rapid increased
accretion…………leading to larger star forming??
“Implications of Binary Properties for Theories of Star Formation” Richard
B. Larson: Yale Astronomy Department (2001)
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8) Planetary Nebulae (Astronomy Now: June 2015)
Observed shapes show huge variety that would not be expected form single
red giant ejecting its outer atmosphere
SO
interactions from orbiting companions
magnetic field interactions
Could be plausible mechanisms (binary is favoured)
Although, no binary observed in planetary nebulae (yet)
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BUT
Very Large Telescope (VLT) has recently captured onset of a planetary
nebula from an evolving red giant star (L2 Puppis) using the SPHERE
instrument
With a companion!!
Nebula is already seen to be “carved out” due to assistance from
companion
Binary stellar wind interactions, interactions between dust disc and
stellar winds
Binary orbit is only a few years……………..to be monitored
Astronomy Now July 2015
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9) Exoplanets
Stable planetary orbits thought unlikely around binary systems
BUT
19 binaries with determined stellar/orbital parameters exist less than 31pc from Sun
(Solar Neighbourhood)
17 could retain stable Earth-like planets in HZs in S-type configurations
Atmospheres/Oceans can buffer against fluctuating radiation levels and eccentric
orbits
Binary systems surprisingly may not inhibit potential planets with life
“Circumstellar Habitable Zones of Binary Star Systems in the Solar Neighborhood”
Eggl, Pilat-Lohinger, Funk, Georgakarakos, Haghighipour (2012)
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SO:
Binary stars play significant roles in many areas of
astronomy/astrophysics
The Universe would be a very different place
without them!!
Binary systems are important
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Recommended reading:
Double and Multiple Stars…..and how to observe them
James Mullaney (Springer) A good first book…but expensive!!
Cambridge Double Star Atlas
James Mullany/Wil Tirion
Contains about half of Mullany’s book and is an excellent general atlas.
Over 100+ “showcase doubles” at the front and over 3000+ doubles at the end.
Highly recommended!!
Observing and Measuring Visual Double Stars (The Patrick Moore Practical
Astronomy Series)
R. W. Argyle
An in depth survey of the field and measurement techniques
(both professional and amateur). Very detailed and theoretical in places
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Double Star Measurements
Separation: distance in arc seconds between the primary (brighter)
and the secondary stellar surfaces
Position Angle: of the secondary with respect to the primary
Observations of these quantities by many astronomers (professional
and amateur) over many years can lead to future orbital trajectory
calculations
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Measurement Techniques
1) John McCue (BAA journal: June 2014)
Uses crosshair/reticle eyepiece and stopwatch
2) Meade Astrometric Eyepiece (manual)
Meade eyepiece plus stopwatch
3) Rob Argyle (described in Observing and Measuring Visual Double Stars (The
Patrick Moore Practical Astronomy Series)
Meade eyepiece plus stopwatch (drift method)
4) Ronald Charles Tanguay Observing Double Stars for Fun and Science
http://www.skyandtelescope.com/observing/observing-double-stars-for-fun-andscience/
5) Others: chronometer, filar micrometer, mask diffraction, CCD. Occultation………
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Observing tips:
Dawes limit
R = 4.56/D
( D equals telescope aperture in inches)
R = 116/D
( aperture in millimetres)
Defines the minimum separation achievable with a given aperture
Separation: a slither of black between the stars. A double with the stellar surfaces
touching is not split!!
For example
An 80mm refractor can theoretically split double stars down to 1.45 arc seconds
But, in practise, this implies perfect seeing and little light pollution
I can usually split down to 2 arc seconds but anything less is a challenge!
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Navigation: SW Synscan systems
To date (2015) the object catalogues contain very few double stars
To find most doubles navigation using R.A./ Dec coordinates is required
This is easy but not obvious!
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1) Select all doubles to be observed and have the coordinates to hand. I make a
list to save time
Plan them so they are in the same part of the sky
2) Start by slewing the scope to the nearest named object (star, M.., NGC…)
3) From this position you can slew using the directional arrows but the
coordinates will not change on the handset
So precise slewing to set coordinates can not be achieved with any accuracy
4) But: hit EXIT (more than once maybe)
5) Go to UTILITIES…………….and then SHOW POSITION
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6) Now you can slew and watch the coordinates change. Set the rate fairly low
(4,5)
7) Use the directional arrows to slew. If your polar alignment is good when
you are moving in R.A. the DEC coordinates will hardly change and vice
versa
8) When you approach the coordinates of the target object take the RATE
down and carefully slew in precisely to the target coordinates
I then use a low power eyepiece (20x) to look for the star…….and then centre
precisely in the eyepiece. Spotting the candidate double star becomes easier
with practise!!
Then zoom in and enjoy
(Zoom lenses are ideal for this purpose as the best magnification can then be
selected to view)
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An observation plan (NW: using the Cambridge Double Star Atlas)
Decide on a constellation that will be easily accessible all night
Pick an easy named star (a “showcase” double?) to begin the tour
From the back list select a series of doubles in order of increasing RA from this star
(Try to keep to small changes in DEC between doubles if possible. The aim is to spend
the session looking at doubles and not performing long slews between the targets)
Select doubles that will be possible……not too great a magnitude difference and not
too close
(note: it may be tempting to choose very wide separations but these can often prove
difficult to identify in a general field of stars: which two stars are actually the
double?)
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Start with easy doubles for the first few sessions…………challenges come later!!
After a few sessions:
The eye becomes practised at seeing separation distances and estimating
magnitudes at each session
Gradually closer doubles can be targeted
Doubles with larger magnitude differences can be attempted
Planning becomes more efficient with less time wasted……………and more
doubles are seen and logged!!
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Observing
Take time to let the eye adjust to the double star and everything else in the eyepiece. I let
my eye wander around the field of vision for about a minute allowing any faint stars to
“pop out”. Play with the focuser control knob to bring out the details
Using the focuser controls focus on the primary star (the brightest). When the best focus
is achieved let the eye settle
Repeat for the secondary star (usually at a different focus especially if there is a
noticeable colour contrast)
Now find the best focus point to view both stars as a double…………and split them if
they’re very close
Very soon an eye to focus control knob coordination is achieved and more detail will be
seen (subtle colourations for example)
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Following on from the above is a fascinating paper (very detailed) on “training
the binary eye”:
www.handprint.com/ASTRO/bineye1.html
Finally:
Sketch, make notes, measure??.............and
Log the Double Star!!
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