double stars neil webster william james observatory neil webster 03/2015

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DOUBLE STARS

Neil Webster

William James Observatory

NEIL WEBSTER 03/2015

http://myweb.tiscali.co.uk/cmhas/index.html


www.itslifejim.btinternet.co.ukWilliam James Observatory

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



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



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



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



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)



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)



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)



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”



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 547



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



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

http://arxiv.org/ftp/arxiv/papers/1304/1304.5494.pdf

http://www.handprint.com/ASTRO/Herschel.html



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”





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

http://www.webbdeepsky.com/double-stars/



http://www.britastro.org/



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

If distances of stars from common centre of mass () can be measured

Then



The individual stellar masses

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



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)



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)



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



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)



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)



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



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)



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



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



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



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-and-science/

5) Others: chronometer, filar micrometer, mask diffraction, CCD. Occultation………






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!



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!



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



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)



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?)



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!!



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)



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!!

http://www.handprint.com/ASTRO/bineye1.html


double stars neil webster william james observatory neil webster 03/2015

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