When individual stars begin to form or coalesce out of the gaseous interstellar medium, more often than not instabilities arise that may cause two or more bodies to form.  It is estimated that 40% or more of all stars form as a binary or multiple star systems. There are currently upward of 140,000 double stars recorded in the Washington Double Star Catalog (WDS) , which is maintained by the U.S. Naval Observatory.  This catalog may be easily accessed through the website Stelle-Doppie.

Double stars are one of the few classes of astronomical objects that are best viewed through a telescope. Photographs just cannot capture the ephemeral quality of a faint and closely bound companion or replicate the vibrant color contrasts that are evident in some pairs.

There are several different classifications of Double stars:

• Physical Doubles – Binary Stars – Stars that are true orbital pairs and share a common evolutionary origin and a gravitational bond, For some of the nearest double stars, complete orbital cycles have been plotted from observations over the past 100+ years   
• Common Motion Pairs (CMP) – Stars that share a very similar trajectory as they move through space which would suggest a common evolutionary origin but the distances separating the stars may be so great as to make any orbital bond unlikely.
• Optical, Coincident or Line-of-Sight Doubles – The chance proximity of stars where the stars appear to be close to one another in the sky but the actual physical separation between the stars is so large as to exclude any common origin or gravitational bond.

The Classification for the systems in question can generally be found on the photos, though in many cases the classification can be very subjective or speculative and subject to change as the star catalogs are updated. Below is an explanation of some of the other terms terms and notations presented on the photos and captions:

Double Star Designation:  Double stars are most commonly referred to by their discoverer designation which consists of a one, two, or three letter abbreviation of the discoverer’s name and a consecutive number for each system cataloged by that individual.  Some early astronomers active from the 1700’s to 1899  cataloged hundreds to several thousand pairs.  Examples of some of the more prolific Double Star observers are:

• Fridrich Willhelm von Struve (1793-1864): STF 1, STF 2 . . .  STF 5018
• Robert Jonckhere (1888-1974): J 1, J 2 . . . J 3182
• Rev. T.E. Espin (1858-1934): ES 1, ES2 . . . ES 3142
• John Herschel (1792-1871): HJ 1 . . . HJ 5732

Component Labels:   The stars that make up a binary or multiple star system are usually sequentially labeled alphabetically – A (usually the brightest star) with the remainder (B, C, D…. etc.) labeled based on order of discovery, separation, or brightness (it can vary). In cases where a newly discovered dim or or extremely close companion is identified, rather than relabeling the existing components, a subscript may be applies such as Aa, Bb etc. This is especially common when new spectroscopic components (see below) are identified.

Separation (Sep):  The distance between the components of the binary star or multiple star system measured in seconds of arc (“) (arc-sec) which is equivalent to 1/3600 of a degree. Amateur telescopes of 4″ to 6” of aperture can easily resolve brighter pairs down to separations of 2 arc-sec or so.

Position Angle (PA):  The angle between  the components of the binary star measured in degrees from celestial north through east (a counter-clockwise direction). Unless specifically noted on the my photos, Celestial North is UP and Celestial EAST is to the Left side of the Photo. Over time both the Separation and the PA will change which will help establish whether the pair is gravitationally bound or merely an optical association.

Magnitude (Mag, M, m):  The brightness of a star – The lower the number the brighter the star.  Each whole magnitude difference represents a 2.5x difference in brightness.  For example if we were comparing a magnitude 2 star to a magnitude 5 star this would be a 3 magnitude difference. The perceived difference in brightness between the two stars would be:  2.5 x 2.5 x 2.5 = 15.6 times brighter or fainter.  Magnitude 6 is about as faint a star that can be viewed by the naked eye under dark skies.  Stars or objects fainter than this (a larger magnitude number) will require binoculars or a telescope to view.  A moderate sized telescope will let you visually view stars down to about 10 or 11 magnitude.  With my current imaging setup I can photograph stars down to about magnitude 18 – very faint indeed.

Distance:  Light Years (LY):  The distance a beam of light can travel in the course of one year:  At 180,000 miles per second = 6 trillion miles per year.   This measure is typically used to define the distance of the stars or galaxies from our solar system.

Distance:  the Astronomical Unit (AU):  The average distance from the Earth to the Sun; 93 million miles or 0.000016 light year.  This measure is typically used to define distances within our solar system or the orbital separations of double star components.

Proper Motion (arc-sec per year):  The movement of a star relative to our solar system measured in seconds-of-arc per year.  Most people assume the stars are stationary but every astronomical body does exhibit some slight movement over time.  This is especially true of stars that are very close to our solar system.   Barnard’s Star which is only 6 light years away has a proper motion of 10.3 arc-sec/yr; a movement that is easily noticeable year-to-year in a small telescope.

Spectral Class (Sp):  Stars are classified based on the characteristics of their electromagnetic radiation.  The temperature, mass and and  atomic makeup can be deduced from a stars spectral characteristics.  For more information see:  Stellar Classification.

Spectroscopic Doubles:  Many double stars have companions that are either too dim or too close to be resolved optically but which can be detected through shifts in the absorption and emission lines of a stars spectra due to the orbital motion of an unseen companion.

Recommended Books and Atlases for Double Stars:

Here is a LINK to Bruce MacEvoy’s wonderful website detailing the finer points of Double Star Astronomy.

Double Stars for Small Telescopes by Sissy Haas, Sky Publishing, 2007. A listing 2,100 of the best double stars best suited for amateur Telescopes. This book does not have any pictures, maps or diagrams but does give written descriptions and ratings of what to expect when viewing the objects. This resource is best used in conjunction with a Star Atlas or Planetarium App on your tablet or Smart Phone.