Full Moon Trapped in the Winter Hexagon

This Saturday, a full moon is trapped within the six bright stars that make up the huge asterism known as the Winter Hexagon. Asterisms are easily identifiable patterns of stars, like constellations, but they are not recognized by the International Astronomical Union. Asterisms may be a part of a constellation (like the big dipper), or can be composed of stars from different constellations as is the case with the Winter Hexagon.

Sky chart for the Winter Hexagon
The Winter Hexagon encircling the full Moon on Dec 22nd at midnight as seen from Vancouver.

The six stars that make up the Winter Hexagon include some of the brightest stars in our sky; five of the six are among the top-10 brightest stars visible from the northern hemisphere. Here is a summary along with each star’s visual magnitude.

  • Capella (+0.07), the fourth brightest star in our sky, was the discovered to be a binary in 1899 based on spectroscopy. Photographic plates displayed two superimposed spectrums with different Doppler shifts. This revealed two components that were periodically moving towards and away from Earth and thus orbiting each other.
  • Aldebaran (+0.99) marks the red eye of Taurus the bull. It is the ninth brightest star. It lies close enough to the ecliptic to be occulted by the Moon. In about two million years, NASA’s Pioneer 10 spacecraft will make a fly past of Aldebaran.
  • Rigel (+0.28) is the fifth brightest star in our sky, but it is designated β Orionis because the variable red giant  Betelgeuse (α Orionis) is occasionally brighter. Rigel is a blue-white super giant that shines with the luminosity of 40,000 Suns.
  • Sirius (-1.44), commonly called the “dog star”,  is the brightest star in the sky and is twice as bright as the next brightest. Its brightness is mostly due to its proximity to Earth. At a distance of 8.7 light years, it is the 5th nearest star system.
  • Procyon (+0.40) is another close neighbour with a distance of 11.4 light years.  It ranks as the sixth brightest star in our sky.
  • Pollux (+1.22) lies close to its slighter dimmer twin, Castor, in the constellation Gemini. Pollux is much more massive than the Sun and, despite being younger in age, it is in a more mature stage of its life cycle. Its nuclear furnace has already shifted from fusing hydrogen to helium into fusing helium into carbon and oxygen.  As the 12th brightest star in our sky, it falls just outside the top 10-list.