Bootes

Bootes anchors the spring sky with Arcturus, but far beyond that single bright star, in the same general direction as seen from Earth, lies something the ancient tradition had no way to notice at all: an enormous region of space containing almost no galaxies.

The Bootes Void, identified in 1981 by the American astronomers Robert Kirshner, Gus Oemler, Paul Schechter and Stephen Shectman while mapping galaxy positions and distances, is a roughly spherical region of space several hundred million light years across containing far fewer galaxies than the surrounding volume, so few that it was nicknamed the Great Nothing. Galaxies are not spread evenly through the universe; they cluster along filaments and sheets around still emptier gaps, and the Bootes Void is simply an unusually large example of that emptiness, sitting along the same line of sight as the herdsman without having any physical connection to the stars that give the constellation its shape.

The void's existence says nothing about Bootes the star pattern, since every star in the constellation is many thousands of times closer to Earth than the void itself; the two occupy the same direction in the sky and nothing else. It is nonetheless the reason a professional astronomer hearing "Bootes" is at least as likely to think of a nearly empty patch of the distant universe as of a bear-driving herdsman.

Bootes, the Herdsman or Ploughman, is a large northern constellation, one of Ptolemy's forty-eight, whose kite-shaped figure is anchored by Arcturus, one of the brightest stars in the whole sky. Its name is often taken to mean the ox-driver or the bear-watcher, for the figure appears to drive the Great Bear around the pole, and Arcturus itself means guardian of the bear. In myth the herdsman has been identified with various rustic figures, and the neighbouring hunting dogs, Canes Venatici, are shown at his heel. In astronomical tradition the constellation is a star pattern rather than a zodiac sign. These are the features of Bootes as the classical and modern traditions describe it, distinct from any astrological reading of the region; Arcturus, a golden giant, has long been a guidepost of the spring evening sky.

The hunting dogs at Bootes's heel, Canes Venatici, are not part of the ancient figure at all: Johannes Hevelius formally introduced them in his 1687 catalogue, though similar dogs appear unlabelled in some earlier star atlases, one of several cases where a seventeenth-century astronomer gave lasting official form to an image that had already been floating around informally for some time.