Double Stars

Double stars and Binary stars

A double star is two stars that are very close together. There are actually two different types of these. The first type is called a "double star" and in this type the two stars appear to be close together because of our line of sight. In reality the stars could be very far from each other. But because of our position in space they seem to be close together.

To understand this concept put your left hand up about a foot in front of your face. Point your index finger up. Now take your right hand and extend it as far in front of you as you can, pointing your index finger up. . Close one eye and look at your fingers. See how they appear close together? Yet they are far apart. This is what a typical double star is.

But, there is also something called a binary star. This is where the two stars are close together and often in orbit of each other. They will appear close together because they are close together!

The Most Famous Double Star

The Most famous double star is the Mizar and Alcor pair in the big dipper. Observations and stories about this pair go back for many centuries and it was often used as a vision test for people. A person with normal eyesight could see the bigger Mizar and its smaller companion Alcor. But if a person had reduced vision they could only see the bigger Mizar.

And there is a real irony in this because the two stars are three light years apart and not a true Binary star. But, Mizar itself is a double star but it can only be seen with a telescope.

This development has caused some speculation as to whether or not the "eye test" is really true when it comes to these stars. Maybe a few hundred years ago the real Mizar double was visible with good eyes. Or maybe alcor was dimmer. Interesting speculations.

The most beautiful Double star

Beauty is of course objective but there are a couple of things to look for when judging double stars. First off, stars that are doubled and very close to equal in brightness often make for a very attractive pairing. And secondly there is a distinct beauty when the double star is composed of two stars of very differnt colors. The double star Alberio is a great example of this

<--- Albireo



And here is a star map of the location of Albireo in the constellation cygnus. The arrow points it out in the lower right corner. To the naked eye it appears to be a single star. But with binoculars or telescope it is easily resolved into it's beautiful double.

A couple of other Remarkable double stars with contrasting color include Delta Cephei in Cepheus and Gamma Andromedae in Andromeda.





A Peculiar Double Star

There is a 2nd magnitude star called Algol in the constellation of Perseus (Algol is arabic for Prankster) that changes in brightness. A star like this is called a variable star. And usually this variability is a result of the star itself changing in brightness. But, we now know that algol is actually a double star. The second star is a large red dwarf that we cant see. It is on a roughly three day cycle and for ten hours every three days the big red star passes in front of Algol and eclipses it, reducing its brightness. This type of binary is called an eclipsing binary and there are many of them but Algol was the first to be discovered and it is the most famous. Because of its peculiar variability it is also called the Demon star.

Double Stars and Telescopes

with a bigger telescopeThere is a whole science to using double stars to test telescopes. It is all about the resolving power of the scope and how good the optics are .The closer the two stars are together the more challenging it is to resolve them. When a double star is separated by between one and two arc seconds it will be a challenge for telescopes with an aperture of 4 inches. And stars of 1 arc second or less will be a challenge to an eight inch telescope. Anything under .5 arc seconds is not resolvable by any telescope.

The drawing here shows a double star being resolved with a telescope. This is quite a fascinating thing and I have a whole article about this which shows wavelength and the resolution of telescopes: The Resolving Power of telescopes



Here are some of the well known and easy to find double stars:

  • Gamma Andromedae - ten arc seconds of separation
  • Beta Monocerotis
  • Iota Cancri 31 arc seconds
  • Gamma Virginis 4 arc seconds
  • Beta Scorpii 14 arc seconds
  • Beta Cygni (albireo) 35 arc seconds
  • Alpha Centauri varies between 2 and 22
  • Polaris (the North Star)

There are lots of different Compendiums and Catalogs of Double Stars


Picture Credits:

Ursa Major: Original uploader was Shawngano at en.wikipedia Author: Shawn E. Gano, http://www.nd.edu/~sgano/astronomy/diagrams/dipper.jpg

Albirio credit: Hewholooks


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