The Big Dipper is one of the most distinctive asterisms in the sky. Hold your mouse cursor over the image to see the names of the stars that make up the asterism and some of the prominent deep-sky objects nearby.
The familiar and easy to spot bowl and handle of the Big Dipper are composed of bright stars that are visible even from the city. It is part of the constellation of Ursa Major, the Big Bear.
The second star from the end of the handle of the Big Dipper, Mizar (Zeta Ursa Majoris), is an interesting multiple star system made up of four stars.
To the keen unaided eye, two stars can be seen, the brighter 2nd magnitude Mizar, and a fainter 4th magnitude nearby star, Alcor. They are separated by about 11.8 minutes of arc, and being able to detect the two as separate is considered a test of good eyesight. However, Alcor is not part of the Mizar multiple star system.
Mizar, at a distance of 78 light years from the Sun, and Alcor at 81 light years, while appearing to be a double star, are actually only an optical double. An optical double star is not a true double star where the components are gravitationally bound together and orbit around each other, but rather a chance alignment along our line of sight. However, Mizar and Alcor are members of the nearest star cluster to the sun, the Ursa Major moving cluster, Collinder 285.
The components of the Mizar multiple star system are Mizar A (Zeta 1 Ursa Majoris at magnitude 2.27) and Mizar B (Zeta 2 Ursa Majoris at magnitude 3.95), located 14.42 arc seconds away.
Each of these stars is in turn a binary star, however the companions are so close to their primaries they can only be detected by spectroscopy.
When examining the field near to Mizar and Alcor in a telescope, four stars can easily be seen. The brightest is Mizar at magnitude 2.27. Alcor is about 12 arc minutes away at magnitude 4. Closer to Mizar at just 15 arc seconds away is Mizar B, the 4th magnitude companion. Between Mizar and Alcor is a fourth star, Sidus Ludoviciana, shining at magnitude 8, which is a field star that is not even part of the Ursa Major cluster. |
