Eagle Creek Observatory
"Teaching young minds about the heavens"

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What's a "double star?"
A "double star", or multiple star, is exactly that. It's a set of two or more stars that appear next to each other. There are two types of double or multiple stars. There are "true" binaries or multiples and "optical" binaries or multiples. True binaries or multiples are stars that are actually physically close and are bound together by gravity. Optical binaries or multiples are stars that are lined up just right so that the appear to be physically close.

How do astronomers tell the difference?
The simplest way to tell the difference between optical doubles and binary systems is using parallax. What's parallax? Parallax is the difference in movement between distant objects and close objects. Have you ever noticed this...as you ride down the road objects in the distance move much more slowly than object that are closer. Telephone poles near the road move by much more rapidly than the trees in the distance. This is called parallax.
This is also how your brain uses your eyes to judge the distance to objects. When you focus on an object your left eye is required to gaze in to the right and your right eye is required to gaze in to the left. The angle that your eyes have to gaze in increases as the distance to the object decreases. Your brain uses the angle to determine how far away an object is. This is extremely accurate out to a several feet.

So, if it's only good out to a several feet how do you gage the distance to an object that is many light years away? Astronomers use the parallax caused by the Earth moving around the Sun. This difference in position is about 180 million miles. Astronomers can measure the angles extremely closely, down to milliseconds of arc. (Thousandths of a second of arc). Using this difference astronomers can calculate the distance. This technique works out to several thousand light years.

So if two stars have the same parallax they are close to each other and are most likely a binary system. The second method is to observe what's called "proper motion." A star's proper motion is its movement through the sky in relation to the background stars. If two stars show the same proper motion, they are moving the same speed and direction, then they are most likely a binary system.
A third method is to measure the wobble of a star as it moves through space in its proper motion. Some binary systems are so close together that the two stars can't be seen as separate stars, or one star is so bright that the other can't be seen due to the glare of the primary star. The star Sirius is an example of a star that wobbles. It has a very dim white-dwarf companion that is very close but several thousands of times dimmer than Sirius. It takes a very large telescope and a very steady sky to see Sirius' companion star.
image of parallax example

Here's a listing of some very beautiful color-contrasted double stars.

Here are coordinates, by constellation, of many double stars that you can observe.
Click on the name of the constellation to get the list.

For a Printer Friendly version of these pages click HERE.

For a zip file containing ETX Tours for each constellation and beautiful colored doubles click HERE.

Andromeda Antlia Apus Aquarius
Aquila Ara Aries Auriga
Bootes Caelum Camelopardalis Chamaeleon
Cancer Canes Venatici Canis Major Canis Minor
Capricornus Carnia Cassiopeia Centaurus
Cepheus Cetus Columbia Coma Berenices
Corona Australis Corona Borealis Corvus Crater
Crux Cygnus Delphinus Dorado
Draco Equuleus Eridanus Fornax
Gemini Grus Hercules Horologium
Hydra Hydrus Indus Lacerta
Leo Leo Minor Lepus Libra
Lupus Lynx Lyra Mensa
Microscopium Monoceros Musca Norma
Ophiucus Orion Pavo Pegasus
Perseus Phoenix Pictor Pisces
Pisces Austrinus Puppis Pyxis Reticulum
Sagitta Sagittarius Sculptor Scorpius
Scutum Serpens Sextans Taurus
Telescopium Triangulum Triangulum Astrale Tucana
Ursa Major Ursa Minor Vela Virgo
  Volans Vulpecula  

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2002-2003 Kevin Muenzler, Eagle Creek Observatory.

Last Updated 11/12/2003