DOUBLE STARS

Most, if not all astronomers know what double stars look like & where to find them, but the details of exactly what double stars are can remain a mystery.  They are noted in all star atlases, and there are regular updates as to where to find them in monthly astronomy magazines.  What I shall do here is explain in basic terms what double stars are and how they are formed.  

Double stars are also known scientifically as binary systems because they consist of two stars (sometimes more) orbiting around a common centre of mass.

There are two types of classic double stars.  The visual double star & the actual double star.

•  Visual binaries i.e. line of site double stars, are very simply two stars which lie roughly in the same line of site.  We therefore see two stars very close together even though they may be many hundreds of light years apart.

•  Actual double stars are the proper binary systems in which two stars do actually orbit each other.  It is estimated that around 50% of all stars in the Universe are binary.

Actual double stars are themselves split into three categories.  Visual binaries, spectroscopic binaries & eclipsing binaries.

•  Visual binaries are double stars that are optically visible through telescopes.

•  Spectroscopic binaries are double stars in which the two or more components are too close together to be seen visually. To understand the nature of the binary system, periodic doppler shifts are used.  An object (star) receding would exhibit a frequency shift toward a lower frequency (RED end of the spectrum), while the second star in the system moving toward the observer would exhibit a higher frequency shift (toward the BLUE end of the spectrum).  From this information the orbital elements & distances between the components can be worked out.

•  Eclipsing binaries can be both visual & spectroscopic.  Instead of optical methods or orbital motions use to find out the properties of the system, the magnitude of the star is used to determine the information required.  A basic graph can be drawn plotting magnitude v's time.  As the stars orbit each other, the luminosity of one of the stars (usually the biggest) decreases as a smaller star passes in front of the primary member.  This is graphically illustrated in the image below, note however the brighter star in the diagram is the smaller blue star..

The orbital period, orbital radii, masses, and stellar radii of both stars can be determined from this type of information.

Below is a short list of some of the best double stars to view in the night sky.

Below is a list of 54 of the best double stars to observe in the night sky

SKY & TELESCOPE July 2001 Page:112.