February 2014 – Orion the Celestial Hunter


Hi everyone, I’m Darrell Heath with the UALR College of Science, welcome to The Night Sky.

If I had to choose my favorite from among the 88 officially recognized constellations, I’d have to pick Orion. Orion is widely regarded among stargazers as the brightest and most conspicuous of our constellations Visible in the northern hemisphere evening sky from late autumn to early spring Orion shines at his very best during January and February. If one of your New Year’s resolutions was to learn the constellations, then Orion is surely the best place to start. Orion is also a great constellation to use for learning about the lives of stars and as we take our tour of the constellation we’ll briefly examine stars from infancy to old age.

According to Greek legend Orion was the mortal son of the god Poseidon and was possessed of both extraordinary good looks and great skills as a hunter. He was also very boastful and had a tendency to irritate anyone around him. After bragging once too often that he could hunt down and kill any creature alive, the Earth-mother Gaia caused a giant scorpion to rise up out of the Earth and kill Orion after a fierce battle. Zeus took pity on Orion and placed him in the sky. Zeus also memorialized the scorpion but to keep the two apart he placed them at opposite ends of the sky. This is why, according to legend, as we see Orion slowly setting in the west we can turn to the east and see the constellation of Scorpius rising.

Orion is often depicted as a man holding a club over his head in his right arm while his left is extended outwards holding a shield (perhaps to fend off the giant bull Taurus which is adjacent to Orion in the sky). One of the great things about Orion is not only that it looks like what it’s suppose to be (in this case a man) but that it can also be clearly recognized from even light polluted city skies.

Ok, so where should you look to see Orion and what can you see within the borders of this constellation? During January and February just step outside around 10PM and look towards the southern sky. Start by looking for three stars forming a straight diagonal line, each spaced an equal distance apart. These three stars shine at an apparent magnitude of 2.

Apparent magnitude is a numerical scale that astronomers use to denote how bright a celestial object looks when viewed from here on Earth. Perhaps somewhat counter intuitively the brightest objects have the lowest apparent magnitude while the dimmer ones have a higher apparent magnitude. Some of the brightest objects in our sky will have negative numbers associated with them. For example: the Sun shines with an apparent magnitude of -26.78 while the Moon is at around -12.7 and the planet Venus has a maximum apparent magnitude of -4.7. The dimmest apparent magnitude that can still be seen with the unaided eye is 6.

Now, back to those three stars forming a straight diagonal line, as I said, these stars are shinning at a magnitude of 2 and collectively they form what is known as the Belt of Orion. Their names, going from east to west are: Alnitak, Alnilam, and Mintaka. The names are Arabic and in fact just about all of the brightest stars that we see have names derived from Arabic. Why? Well, during western civilization’s decline into the Dark Ages the Middle East became the center for astronomical learning and Arabic culture kept alive the body of celestial knowledge that had been compiled by the Ancient Greeks.

Once you’ve located the Belt of Orion draw an imaginary line from the eastern belt star, Alnitak northwards by about ten degrees (ten degrees being the width of your fist held out at arm’s length). At the end of this imaginary line you will find a rather orange-red looking star, this is Betelgeuse (if you are a fan of film director Tim Burton you might pronounce it “Beetlejuice”). Betelgeuse marks the hunter’s right shoulder. Looking around the night sky you will notice that stars have different colors. Hotter stars have bluer colors while cooler stars range from yellow to red and orange. Betelgeuse, the 10th brightest star visible from Earth and located some 430 light years away is what is known as a “red giant”, a star that is fast approaching the end of its life. Betelgeuse has nearly expended all of its available fuel within its core, as a result the core is contracting in upon itself and becoming ever denser and hotter. As the core heats up it causes Betelgeuse’s outer layers to simultaneously expand outwards and cool down into the bloated red giant we see in our night sky. Betelgeuse has become a truly monstrous star, around 1,000 times the radius of our own Sun. Sometime the next million years Betelgeuse will eventually collapse in upon itself triggering a massive supernova explosion. But if it were to happen today we wouldn’t know about for another 430 years due to the star’s distance and the finite speed of light.

Let’s return to Orion’s belt stars. This time we are going to extend an imaginary line from the westernmost star, Mintaka, southwards by about 10 degrees to the blue-white giant star known as Rigel. Rigel, which marks Orion’s left foot, is located some 860 light years away, shines at a magnitude of 0.2 and is normally the brightest star in Orion. I say “normally” because Betelgeuse varies in brightness due to its inherent instability. If you were able to visit Rigel in a futuristic space ship you’d see that the star’s actual brightness is some 40,000 times that of our Sun and emitting 100,000 times as much energy. Interestingly Rigel is in fact two stars, not one. Beyond its intense heat and brightness, Rigel hides a secret; it is actually two stars rather than the single star that it appears to be. It has a closely orbiting companion star that shines at an apparent magnitude of 6.8, the primary star shines far too bright for us to see the companion with the naked eye. You’ll need a telescope and some very fine observing conditions to spot it. Hot, massive stars like Rigel use up their fuel supplies very quickly and do not live for very long compared to other stars. One day, Rigel will become a bloated red giant like Betelgeuse.

Now that we have Orion’s right shoulder and left foot stars located let’s take a look at two other stars that round out the hunter’s frame. Opposite Betelgeuse you can make out the star that forms Orion’s left shoulder, this is Bellatrix. And if you are a fan of the Harry Potter series then you may already be familiar with the name in the form of the evil true-blood witch Bellatrix Lestrange. Bellatrix is the third brightest star within Orion and is a hot blue giant located 240 light years away. Looking opposite Rigel, you will find the hunter’s right knee counterpart in the form of a star named Saiph. Saiph is the 6th brightest star in Orion and is a hot blue supergiant located 720 light years away.

If you are a parent of a small child then you probably live in dread of being asked that most awkward of questions: “Mommy/Daddy where do baby stars come from?” No need to be worry, you can answer that question with a quick look at Orion’s Sword. Stars are born within vast interstellar clouds of gas and dust composed primarily of cold molecular hydrogen. One of the closest of these star birthing factories to Earth can be seen with just the naked eye. To find it lets return to the Belt of Orion.

Looking again to the easternmost belt star, Alnitak, we are going to let our eye drift down a little ways to see a faint row of stars that appear to be hanging in a slightly curved line. These stars collectively form what is known as the “Sword of Orion”. About midway down the sword’s length you will notice a faint and fuzzy “star”. This isn’t a star though, it’s a cloud of gas and dust some 1,300 light years away and some 30 to 40 light years wide. It’s called the Orion Nebula or, more officially as Messier 42 or M42 for short. It is here that stars are being born. The Orion Nebula is one of the grandest celestial showcases the night sky has to offer and it is without doubt the most photographed by amateur astronomers. Through a pair of binoculars with a minimum aperture of 50mm you will see a glowing cloud of gas. This glow is created by the radiation from newborn stars energizing the cloud of gas and dust. Through a telescope you see even more detail. The most obvious feature will be four bright and very young stars that are collectively referred to as the “Trapezium”. A long exposure photograph like this one will of course reveal the nebula in all its glory. You won’t see this kind of color with your eye when looking through the telescope although every now and again, if your eyes and seeing conditions are good enough you might make out some greens and blues.

Recent studies of the nebula have revealed the presence of what astronomers call “proplyds”, which is shorthand for “protoplanetary discs”, a rotating disc of gas and dust that surrounds infant stars and may one day give rise to planetary systems all their own.

New studies of the motions of the stars within the nebula also suggests that there may be a stellar mass black hole lurking somewhere within. Stellar mass black holes are formed out of the gravitational collapse of the most massive stars.

M42 is just part of an even bigger region of nebulosity in this part of the sky. Right below Alnitak there lies another famous star forming region: the Horsehead Nebula. The iconic horse head is actually a dense, dark cloud silhouetted against the glowing cloud of gas that lies behind it. Sadly, this is a nebula that only shows up best in long exposure photographs than it does through a telescope view.

Orion is famed for his hunting prowess and now I’m going to show you a few hunting skills of my own that will allow you to track down a few other constellations that are adjacent to Orion. Extending an imaginary line through Rigel and Betelgeuse you will come to a pair of bright stars that form twins of Gemini, their names are Castor and Pollux. Right now you can also find the planet Jupiter within Gemini. Take the belt stars and extend a line towards the southwest and you will run into the brightest star in our night sky, Sirius. Sirius is part of the constellation Canis Major which, along with Canis Minor, form the pair of Orion’s hunting dogs. Extend the line from the belt stars towards the northeast and you encounter the fiery red eye of the bull, Aldebaran, which of course lies within the constellation of Taurus. Draw a line from Bellatrix through and just beyond Betelgeuse and you will come across Procyon, a prominent star in the constellation of Canis Minor, Orion’s other hunting dog.

In this episode we have only scratched the surface of what all Orion has to offer but it should be enough to encourage you to get outside, look up, and wonder.

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