Orion and the Winter Triangle

“Out here in the perimeter there are no stars.”

—Jim Morrison, The Doors, The WASP (Texas Radio and the Big Beat) 

During the summer months, the evening sky is filled with countless stars of the Milky Way, our home galaxy. Because our little yellow dwarf star, the Sun, is located about two-thirds of the way out toward the edge of the Milky Way galaxy, we see far more stars in the summer when we are looking back, toward the center of the galaxy. On winter evenings we are looking out through the outer third of the galaxy and the stars are much more sparse.

Orion, the Hunter, is probably the most recognizable constellation in the winter sky. Because we are located in the Orion arm of this cosmic pinwheel we call The Milky Way, the fewer winter stars we see are generally bright and relatively close.

To find Orion in January, go where you have a good view of the eastern sky, the opposite side of the sky from the sunset. Look for three stars that are about the same brightness and spacing in the southeast sky. This, of course, is Orion’s belt.

Now, imagine a line that is roughly perpendicular to the center star in the belt and follow that line out both sides of the belt about three belt lengths. Above the belt is a very bright reddish star: Betelgeuse. Below the belt, about the same distance is a very bright blue-tinted star: Rigel.

Betelgeuse is a red supergiant star. If this massive star were at the center of our solar system, its surface would extend past Mercury, Venus, our Earth, Mars and beyond the asteroid belt. It is that big and one of the largest stars we can see. This star is also called Alpha Orionis because it is occasionally the brightest star in Orion. I say occasionally because it is a variable star. As the name implies, a variable star is a star that changes in brightness.

Rigel is a blue-white supergiant star. Also known as Beta Orionis, Rigel is yet another variable star that is, on average, the seventh brightest star in the entire sky after the Sun.

So to see the Hunter, imagine that Rigel is the Hunter’s left foot. Betelgeuse is the Hunter’s right shoulder. Bellatrix is the left shoulder and Saiph is the right foot. (see diagram) A small triangle of stars above Betelgeuse and Bellatrix marks the Hunter’s head. He is holding up a shield in his left arm and a club in his right.

Orion is faithfully followed through the heavens by his two dogs, Canis Major and Canis Minor. The brightest star in Canis Major is Sirius, appropriately called The Dog Star. Sirius is the brightest star in the entire sky, second only to the Sun. To find Sirius look below Orion and slightly off to the left.

Above Canis Major is Canis Minor. Another bright star, Procyon, marks the little dog. Sirius and Procyon are both very close to Earth at 8.58 and 11.40 light years, respectively.  This makes them the sixth and thirteenth closest stars to Earth.

Together Betelgeuse, Sirius and Procyon make up the Winter Triangle asterism.

Orion’s sword hangs from the left star of the belt. The middle “star” of the belt is actually one of the most magnificent deep sky objects in our skies, the Great Nebula in Orion. The Great Orion Nebula is a giant cloud of hydrogen gas that has been excited to luminance by the stars that are forming within the nebula. This nebula is a stellar nursery—the birthplace of stars. The Orion Nebula is so bright that it is visible to the naked eye, even in light polluted areas around our cities. Here in the Santa Monica Mountains it appears as a fuzzy star.

A good pair of binoculars or a small telescope will reveal a glowing cloud of ionized gas with many young stars embedded within it. On a moonless night when the atmosphere is steady a small telescope will resolve an asterism of four stars very close together called the Trapezium in the center of the glowing gas cloud. A larger telescope will reveal six stars. Galileo Galilei first sketched the Trapezium on February 4, 1617. These baby stars are all very close together, within 1.5 light years of each other.

In 1771 the French astronomer Charles Messier, published a catalog of 103 astronomical objects that were not comets, but not seemingly individual stars either. This was an effort to aid in identifying comets in their mercurial visits to our solar system. Observing the entire Messier catalog makes a wonderful challenge for the Backyard Astronomer. The great Nebula in Orion is Messier object number 42, or M42 for short, and is classified as a diffuse nebula that is forming an open cluster. The stars of the Trapezium are the beginning of that open cluster and provide most of the ionizing energy for the gasses in the diffuse nebula.

Viewing the Great Orion Nebula in a large telescope far from city lights is a formidable experience. It is among the most vividly colorful wonders in the sky.

So, on one of these chilly winter eves soon, put on your wooley warmies and venture outside to scan the southeast skies for the intrepid hunter, Orion. See if you can find his belt and the bright red and blue stars, Betelgeuse and Rigel. Look for the wondrous Great Nebula in Orion’s sword and witness the birth of stars.