Outline of the Winter Hexagon

Sunday, March 6, 2022

In the waning days of winter, we're still able to see all of the stars of the Winter Hexagon after astronomical twilight ends. There are three gradations or tiers of twilight: civil, nautical, and astronomical. Civil twilight is when the sun (more precisely, the center of the sun) is between 0 and 6 degrees below the horizon and there's still enough sky light that artificial light is not needed. Nautical twilight is when the sun is between 6 and 12 degrees below the horizon and both bright stars and the horizon are visible for the altitude of the stars above the horizon to be measured. Astronomical twilight is when the sun is between 12 and 18 degrees and there's still some skyglow, so faint, diffuse objects (galaxies and nebulae) and the faintest stars can't be detected.

The Winter Hexagon is a large asterism (pattern of stars) that encompasses several constellations. It is formed by connecting in clockwise order: Capella in Auriga, Aldebaran at the eye of Taurus, Rigel at the knee of Orion, Sirius at the neck or nose of Canis Major, Procyon at the hindquarters of Canis Minor, and Pollux or Castor at the heads of Gemini. Capella is actually a quadruple star system made of binary systems orbiting each other. One of the binary systems is responsible for practically all of the light we see and is made of two yellow giants orbiting each other every 104 days. The giants are 13.6 and 8.3 times larger in diameter than the sun and about 3 and 2.6 times as massive as the sun. They spent most of their lives as much hotter blue stars but are now swelling in size as they begin to fuse the helium in their cores to make carbon and oxygen while the outer layers puff up to make red giants. The other binary is made of much fainter red dwarf stars that are about 250 times farther away from the yellow giant pair than Pluto is from the sun. At this distance the red dwarf binary takes about 400 years to orbit the yellow giant pair.

Other giants in the Winter Hexagon are red giant Pollux at about 9 times the diameter of the sun and a mass 1.8 times the sun and red giant Aldebaran at 44 times the diameter of the sun and a mass 1.7 times that of the sun. Aldebaran is expected to get even larger in diameter as its helium core contracts heats up. Sirius, Procyon, Rigel, and Castor are all normal adult stars still fusing hydrogen in their cores, though they are all larger, hotter, and more massive than the sun. 

Sirius is 1.75 times larger in diameter and 2.1 times more massive than the sun. It has a very small but very dense companion orbiting it, a white dwarf just 90% the diameter of the Earth but slightly more massive than the sun. The white dwarf is the dead, exposed core of star that was once 5 to 7 times the mass of the sun. Besides having that white dwarf orbiting it, Sirius' other claim to fame for Bakersfield observers, is that it is the closest star outside the solar system that we can see without a telescope. Observers a bit further south of us (Mexico, etc.) can see Alpha Centaurus that is even closer to us. 

Procyon is about 1.4 times the mass of the sun and is 2.1 times the diameter. It too has a white dwarf companion that is about 60% the mass of the sun and is 1.35 times the diameter of the Earth. Castor is actually a sextuplet with three pairs of binaries orbiting each other. 

The last of the Winter Hexagon is definitely not the least. Rigel appears slightly dimmer than Capella but that is because of its large distance of 860 light years. Rigel actually produces about 85,000 times as much light as the sun and is a blue-white supergiant about 73 times the diameter of the sun (about the size of Mercury's orbit) with nearly 18 times the sun's mass. Rigel is swelling as it turns into a red supergiant that will be about as large as Betelgeuse on the other side of Orion. Rigel has a binary system orbiting it made of two blue-hot stars that take about 25,000 years to orbit Rigel A. Around those three is a cool orange star that takes about 250,000 years to orbit all three. To finish the asterisms in the east, you can also connect the bright red star inside the Winter Hexagon, Betelgeuse, to Sirius and Procyon to make an equilateral triangle called the Winter Triangle (as opposed to the Summer Triangle of Deneb, Vega, and Altair visible in our evening sky high in the west). 

Most of the stars you can see without a telescope are bigger and brighter than the sun. They turn out to be rare in the galaxy. Most stars are smaller, cooler, and fainter than the sun and require a telescope to see, even if they're close to us. Fortunately, just about every one of them has at least one exoplanet orbiting them, so there's all sorts of real estate for life to arise.

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com