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Our Star, The Sun

Our brilliantly incandescent golden Sun is a solitary star; a lonely fiery ball in the daytime sky. But it probably was not always so bereft of stellar companionship. The Sun might have been born as a member of a dense open cluster with thousands of other glittering sister stars. Astronomers think that the newborn Sun was either tossed out of its birth-cluster or it drifted away from its sisters about 4.5 billion years ago. The missing sisters of the Sun have long since wandered off to more distant regions of our Milky Way Galaxy–and there well may have been as many as 3,500 of these nomadic stellar siblings, according to computer simulations administered by Dr. Simon Portegies Zwart, a computational astrophysicist of the University of Amsterdam in the Netherlands. According to Dr. Zwart, the Sun’s birth-cluster began with approximately 500 to 3,000 solar-masses and a diameter smaller than around 20 light-years–which is typical for open clusters. Evidence for the cluster’s size and mass, Dr. Zwart writes, is preserved in the anomalous chemical abundances and structure of the Solar System’s Kuiper Belt–the realm of tiny and not-so-tiny icy objects that circles our Sun beyond the planet Neptune. Some of the Kuiper Belt’s denizens are dynamically “hot”–that is, they were shaken up and dispersed by the gravity of at least one neighboring cluster star zipping by in a close pass very long ago. Like other open star clusters, the Sun’s birth cluster fell apart over time. The Sun’s lost sisters have by now wandered so far away that many of them are probably lost to us forever. There are only two candidate solar-siblings that are known to dance around in our Galaxy. Astronomers discovered the pair by using the spectrum of light shining from candidate stars of the same age as our Sun, and then determining their chemical compositions, which can then be compared to the Sun’s. A kindred chemical composition points to the likelihood that a solar-sibling has been spotted.

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