Double Stars

When you look up into the night sky you might think that the stars you see were all solitary objects. Well you'd be wrong. In fact many stars have a companion (some have multiple companions) and the stars share a center of gravity and orbit around each other. Orbital periods can vary from a number of hours to millions of years. Occasionally the stars will pass in front of each other or eclipse each other, this can cause the magnitude of a star to dim. Stars like this are known as eclipsing binaries - they're not as common as you might think.

Some of the more well known binary systems in the northern hemisphere are listed below:

Constellation

Star

Magnitude / Separation

Comments

Ursa Major Mizar & Alcor 2.2 & 4.0 /
2.1 & 4.0 / 14.4"

An easy naked eye double if conditions are clear. Telescopically mizar itself is a double with a 3.8 magnitude companion. I could barely make it out using the 25mm eyepiece, if you didn't know to look for it you wouldn't see it! With the 9mm eyepiece the two stars are cleanly split and make a good target.

Orion Mintaka 2.3 & 6.7 /
An easy star to locate in Orions belt, I used the 25mm eyepiece to split the pair, it wasn't a brilliant sight. Using the 9m eyepiece the separation was very prominent and the 6.7 magnitude companion though faint showed up clearly.

Lyra Epsilon 5.0 & 5.1
5.2 & 5.5

Epsilon Lyra lies East of Vega, the brightest star in Lyra. Epsilon is a double star system and is easy to see with a telescope. However, with higher magnification the double stars each have a companion star - a double double system! I managed to split the pair using a 6mm eyepiece and a 2X Barlow. I had difficulty making out the split between the stars without the Barlow.

Gemini Castor 1.9 & 2.9 / 2.5"
A good example of a binary system with a period of 420 years with the angle of separation reducing all the time. It's an easy telescopic double. There is a third member in the system which itself is an eclipsing binary.

Cassiopiea h eta 3.4 & 7.5 / 12.6"
A wide, easy telescopic double.

Perseus Algol 21. to 3.4
This star is an eclipsing binary with a period of 2 days, 20 hours and 48 minutes. When the companion star passes in front of Algol the magnitude of Algol drops to 3.4 and then rises back to 2.1 as the star moves along its orbit. As the companion star is so close to Algol its not visible with a small telescope.

Multiple star systems can be fun to seek out especially when the moon is full and prevents you from searching for other deep sky objects. A list of good target stars systems can be found at The Astronomical League.

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Constellation	Star	Magnitude / Separation	Comments
Ursa Major	Mizar & Alcor	2.2 & 4.0 / 2.1 & 4.0 / 14.4"	An easy naked eye double if conditions are clear. Telescopically mizar itself is a double with a 3.8 magnitude companion. I could barely make it out using the 25mm eyepiece, if you didn't know to look for it you wouldn't see it! With the 9mm eyepiece the two stars are cleanly split and make a good target.
Orion	Mintaka	2.3 & 6.7 /	An easy star to locate in Orions belt, I used the 25mm eyepiece to split the pair, it wasn't a brilliant sight. Using the 9m eyepiece the separation was very prominent and the 6.7 magnitude companion though faint showed up clearly.
Lyra	Epsilon	5.0 & 5.1 5.2 & 5.5	Epsilon Lyra lies East of Vega, the brightest star in Lyra. Epsilon is a double star system and is easy to see with a telescope. However, with higher magnification the double stars each have a companion star - a double double system! I managed to split the pair using a 6mm eyepiece and a 2X Barlow. I had difficulty making out the split between the stars without the Barlow.
Gemini	Castor	1.9 & 2.9 / 2.5"	A good example of a binary system with a period of 420 years with the angle of separation reducing all the time. It's an easy telescopic double. There is a third member in the system which itself is an eclipsing binary.
Cassiopiea	h eta	3.4 & 7.5 / 12.6"	A wide, easy telescopic double.
Perseus	Algol	21. to 3.4	This star is an eclipsing binary with a period of 2 days, 20 hours and 48 minutes. When the companion star passes in front of Algol the magnitude of Algol drops to 3.4 and then rises back to 2.1 as the star moves along its orbit. As the companion star is so close to Algol its not visible with a small telescope.