Constellation Series: Orion

Since the dawn of human history, we have looked up into the night sky and found patterns in the stars.  Some of us saw animals, others saw gods and heroes, but we all agreed that they were greater than our simple existence. In this blog series, we will take a deeper look into the constellations that Astronomers use to map today’s night sky.  We will look into the history of each of the 88 constellations and the stars and objects that form them, to discover more about our culture, and our connection with the universe.

Our first constellation on the list is bright, large, and visible from latitudes as far as 85 degrees north to -75 degrees south, meaning it can be seen by 99% of the world’s population at some point during the year.  This easy to recognize constellation is a golden opportunity to see contrasting colours in stars, and it hosts a massive nebula that can be seen with the naked eye. Introducing Orion: The Hunter.

History:

The history of Orion is fascinating.  The earliest depiction of the constellation was from a mammoth ivory tablet found in 1979 in a cave in Weiler, Germany.  A figure carved on one side shows a striking similarity to the hunter we know, and a series of dots on the opposite side reveal some striking patterns.

The groups of notches form some interesting coincidences.  There are 88 of them, matching both the number of days in three lunar cycles (88.5) and the number of days between the heliacal rising and setting of the star Betelgeuse, brightest star in Orion.  For reference, the heliachal rising of a star is the first time it can be seen above the horizon just before sunrise, and the heliachal setting is the last time it can be seen just after sunset.

For Betelgeuse, this period is from roughly June 2nd to the following March 7th, nearly nine months.  This timing not only matched the human duration of pregnancy, but gave an indicator for the best time to conceive (June 2nd) so that a child would be born after the worst of Winter (March 7th).  With 6 months of warmer weather to grow, the child would have a better chance of surviving their first Winter.  This shows that the ancient constellation was linked to fertility and used as a measure of time along with lunar cycles.

Once humans learned to write and record information, the first mention of the constellation went back to 1500 BC with the Babylonian star catalogue.  They called it “The heavenly shepherd,” the herald of the gods, who had both a human and bird form.  In Egypt, the stars of Orion represented the god Osiris, who was the god of rebirth and the afterlife.  There is a theory that the great pyramids at Giza match the three belt stars of Orion (Alnilam, Alnitak, and Mintaka), but it has been contested and there is still no conclusive evidence to link them.

Orion’s current name comes from the Greek mythology, where he was a mighty hunter, the son of Poseidon, god of the sea.  There are several mythical stories about Orion, but my favourite one has to do with the Pleiades constellation in Taurus.  In the story, the seven sisters of the Pleiades were bathing in a river in the forest.  Orion saw them and was overcome with lust.  He chased them through the forest for four long years before the sisters pleaded with Zeus to save them.  He placed them in the sky where Orion could not go.  However, Orion was able to negotiate a deal with Hera, queen of the gods, and have himself placed in the sky near the sisters, but Zeus was wise to their plot and placed himself in the form of a mighty Bull (Taurus) between them.  This is the reason why Taurus is charging toward Orion with the Pleiades safely behind.

Cultures all across the world have seen Orion the hunter as both a swordsman or archer, but native Canadian tribes had an interesting take on the belt stars.  The story goes: Four Inuit hunters were chasing a bear, which escaped to the sky.  The hunters decided to follow it, but as they climbed higher and higher, one of them lost a mitten, and had to go back to Earth to get it.  The hunters are still chasing the bear to this day and the story was told by the fourth hunter who came back to Earth.

Characteristics:

Orion the hunter is a Winter constellation in the northern hemisphere, best observed from January to March as it moves from southeast to southwest as it crosses the sky.  Even in heavy light-polluted areas, the seven brightest stars of Orion are visible.  This includes the four stars that make up the shoulders and knees of the hunter, Betelgeuse and Bellatrix, and Saiph and Rigel respectively, and the three belt stars, named Alnilam, Alnitak, and Mintaka.

In darker skies, many more stars are visible, including a head and a bow and arrow shape, as well as a sword or dagger stemming down from the belt.  The darker skies also reveal some of the hidden gems that require keen eyes or a telescope to see.

The three stars that form Orion’s belt are a common asterism.  Often the layperson will know of ‘Orion’s belt’ rather than Orion the hunter.  their similar brightness and simple line in the sky make them a simple pattern to spot and follow.

Brightest Stars:

Betelgeuse and Rigel are the two brightest stars in Orion.  Betelgeuse is an M-Type red supergiant at a distance of 640 light years, that is set to explode as a supernova some time in the next million years or so.  When this happens, the explosion will be bright enough to see in the daytime with the naked eye, and will remain in the sky for weeks or even months.  It is also a semiregular variable star and the eighth brightest star in the night sky.

Rigel is a blue supergiant at a distance of 800 light years, with a distinct blue colour.  It is the sixth brightest star in the night sky.  The contrast between the two allows the observer to determine their colours more easily than if they were alone in the sky, a trick that can help introduce the concept of star colours.

Of the belt stars, Alnitak and Mintaka are closer in distance to Rigel at 800 and 900 light years away respectively, but Alnilam is much further, at a distance of 1359 light years.

Deep Sky Objects:

This is where Orion shines.  The Orion Molecular Cloud complex is a massive interconnected region of active star formation about 1500 light years distant, spanning hundreds of light years across Orion and the surrounding constellations.  It is one of the most active visible regions of star formation in the Milky Way, and has a lot to see.

The jewel of the Orion complex is known as Messier 42, the great nebula in Orion.  Located in the sword of the hunter, it can be seen with the naked eye.  With binoculars or a telescope a much richer view of turbulent gas and dust is visible, and photographs reveal the formation of massive stars that drives the amazing features of this nebula.

Below M42 is NGC 1980, a reflection nebula, while above lies NGC 1973, 1975, and 1977, also reflection nebulae.  At the top of the sword closest to the belt is the open cluster NGC 1981.

Away from the sword there are plenty of other nebulae to investigate.  Near the belt star Alnitak lies the aptly named horsehead nebula.  By the ‘head’ of Orion lies the massive dim bubble of gas known as Lambda Orionis.  The red streak called ‘Barnard’s Loop’ carves its way around the belt and sword and is only seen in long exposures.   Finally, the Witch Head nebula near Rigel reflects the light from the blue supergiant in a ghostly glow.

 

Orion is a favourite for astrophotographers who are hunting for nebulae, the only problem is for northern observers who have to stay out in the cold of winter to observe it.

Other Notes:

Orion serves as the radiant for the Orionid meteor shower in late October, which is associated with debris from Halley’s comet.

In the far future, due to precession of the Earth’s axial tilt, Orion will move to the south, eventually becoming invisible to observers in Canada and Great Britain.  Luckily this won’t happen for 12,000 years or so.

Because the bright stars of Orion lie much farther away than bright stars in other constellations, the Orion shape will remain in tact long after the other constellations have changed shape over time.

Final Thoughts:

Orion the hunter is one of the most easily recognizable constellations in the night sky, visible to the majority of our species at some point during the year, and contains some of the most gorgeous star forming features in the galaxy.  It has captivated our species for millennia, and is culturally significant to history’s great civilizations.  It’s a great way to start the list of the 88 constellations, and almost certainly deserves to be called “#1.”