- Collinder 106
Collinder 106 is a large (35 arc-min diameter), sparse open cluster in the constellation Monoceros. In my image, it comprises the group of fairly bright stars lying just to the right of the center of the image. In all, this cluster contains only around 20 member stars spread out over its large size. Collinder 106 would probably be forgettable except that it contains one remarkable star— Plaskett's Star. In my image, this is the bright white star lying to the right of the image center along the bottom edge of the cluster (it's the brightest star in the image). After its discovery in 1922 by John S. Plaskett, this star was thought to be the most massive binary system known (we now know that η Carinae is the largest). The two components of this binary system are massive Class O supergiants with masses equivalent to 42 and 51 Suns. The stars are so close that they almost touch, and rotate around a common center of mass with a period of only 14.4 days. It is believed that some day the two stars will merge, which will result in a supernova explosion.
Even though the components of this binary system are very large and only 5.254 LY from us, they can't be resolved optically because they are so close. The binary nature of this star was discovered spectroscopically. Optically, Plaskett's Star has a visual magnitude of 6.06. It exhibits irregular changes in brightness, so it is also considered a variable star (= V640).
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