Binary and Double Stars

An interesting visual and telescopic challenge for viewing this spring is binary stars.  Most stars in our galaxy come in pairs or even triples.  Our solar system is unique in that we have only one star, the Sun.  In order to understand this topic there are a few definitions that need to be understood.  First of all, two stars that are close to each other in the sky but are not gravitationally bound are called optical double stars.  These stars are very far apart from each other in distance but are lined up in our field of view and appear close together.  The most famous examples are Mizar and Alcor, two stars in the constellation Ursa Major.  This pair can be seen as the next to the last star(s) in the handle of the Big Dipper. 

Stan Honda – The “handle” of the Big Dipper includes Alioth, Mizar and Alcor and Alkaid. The stars are seen above a tree in Central Park.

Binary-star systems, also called binaries, are two stars in orbit about a common center of mass.  There are visual binaries which can be separated with either binoculars or a telescope.  There are also spectroscopic binaries which can only be seen by observing their changing light spectra due to the their relative motions as their light spectra are shifted toward the red and blue as they move away and toward the observer.  Finally, there are eclipsing binaries, which can only be seen as their combined light is reduced as one star passes in front of the other. 

Before continuing, I need to review the terms that astronomers use to measure angular separation. Our moon is about 0.5 degrees in angular diameter, also called 30 arcminutes, since there are sixty arcminutes in a degree.  Mizar and Alcor are about 12 arcminutes apart, which make them visible to the naked eye for people with good vision.  As a matter of fact, these two stars provide us with an ancient eye exam that has been used for centuries.  For smaller angles of separation, we use arcseconds.  Sixty arcseconds is equivalent to one arcminute and therefore 3,600 arcseconds equals one degree.

Stan Honda – The stars Mizar (top) and Alcor (bottom) shine in the sky above Central Park.

There are some visual binaries that might be fun to try to image this spring or summer.  The first is Mizar itself, which is a binary star system with a 14.4 arcsecond separation.  The two stars cannot be separated with the naked eye but a medium sized telescope should do the trick.  Another visual binary is Albireo, located in the constellation Cygnus.  It rises in the east around 10 PM but will rise earlier as spring turns into summer.  This binary has a separation of 34 arcseconds and the two stars have noticeably different colors.  To find Albireo, a 3.0 magnitude pair, first find the Summer Triangle of Deneb, Vega and Altair, then use free planetarium software such as Stellarium to zero in on it.  A third visual binary is Acrab, located in the constellation Scorpius.  These two stars have a blue tint and are separated by 14 arcseconds.  Acrab is a 2.5 magnitude pair and is located between the bright star Antares and planet Jupiter this spring and summer.  This also does not rise in the east until 11 PM now but will be well positioned during the summer months.

Happy hunting !   

Stars and Planets, a Peterson Field Guide, Jay M. Pasachoff, 2012
Astrononomy Today,  Chaisson and McMillan, 2011