Orion the Hunter rises, the Summer Triangle sets. These two opposing groups of stars lie on opposite sides of the sky from one another and represent the great seasonal divide of summer and winter. Nowhere is this more obvious than in the stance of the Northern Cross.
I noticed it Friday night while out walking the dog. Cygnus the Swan a.k.a. the Northern Cross stood exactly upright in the northwestern sky around 10 o’clock. Meanwhile Vega, the brightest star in the Summer Triangle, required some neck twisting to see, hidden as it was by tree branches to the lower right of the Cross. In the exact opposite corner of the sky Orion and his tri-star Belt led a charge of bright winter stars laying waste to the dim constellations of the fall southern sky.
Despite their seasonal differences, the “alpha” or brightest stars of Cygnus and Orion – Deneb and Betelgeuse – are both supergiants with diameters so large they make the sun look point-like in comparison. Before continuing, you must know that Betelgeuse, is not Orion’s brightest star despite its designation as Alpha Orionis. It’s bested by a few tenths of a magnitude by Rigel in the hunter’s foot. Since Betelgeuse is a pulsating variable star with a diameter and brightness that changes over the months and years, it may at times wax brighter than Rigel.
Astronomers estimate Betelgeuse varies between about 550 and 920 times the size of the sun as it expands and contracts, unable to reach stasis like our more dependable star. These ups and downs are all part of Betelgeuse’s evolution from a red supergiant to potential supernovadom – one winter evening we may look up to find it’s blown its top, outshining every star in the nighttime sky.
Betelgeuse’s counterpart Deneb in the Northern Cross is likewise a supergiant but a younger, much hotter blue-white star 100-200 times the sun’s diameter. Both these great giants and their respective constellations draw our eye this time of year, when the leaves are down and the stars sparkle between tree branches like stellar tinsel.