Thursday, February 7, 2008

The Sword of Orion

Last week we took a naked-eye tour of the constellation that owns the winter sky: Orion. Our heavily-armed hunter's most impressive weapon is his sword, dangling from the diagonal three-star belt that's recognizable even to novice stargazers. The sword also appears to be composed of three stars, but upon closer examination, we see this is a trick of light.

If you have binoculars or a telescope, go outside tonight and look at the Sword of Orion to discover just how much more is going on there than meets the (naked) eye.


1) Yes, we pay through the nose for these crisp, clean winter night skies: with face-freezing temperatures. But do try to dark adapt before observing by avoiding all white light for at least 20 minutes. It will make a big difference in the acuity of your night vision and how much detail you can see. You can always isolate yourself with the lights off in a heated room that's adjacent to an exit, or in a heated car with the dash lights turned all the way down.

2) Begin observing at least one hour after sunset, for good sky darkness. The Moon will be a waxing (growing) crescent this week, culminating in a First Quarter Moon (aka "half moon") on February 13. It will be easier to observe the Sword of Orion earlier in our observing week, while the Moon is still small and sets fairly early. The Moon will set around three hours after sunset on the evening of Saturday the 9th and four hours or so after sunset on the evening of Sunday the 10th. After Sunday, the Moon may set too late for comfortable evening viewing of the sword, unless you're a night owl and very cold hardy.

3) Here's a nifty tip for targeting sky objects with binoculars, so that when you look through your binos, the object you’re seeking is in the field of view or very close by. This becomes especially important when you’re wandering around in dense star fields where there are lots of distractions.

Look directly at the point in the sky that you want to view through binoculars. Don't take your eyes off that point as you slowly bring the binoculars up to your eyes. I repeat: do NOT take your eyes off that point as you move your hands up toward your face. If you look at the binoculars, you will lose your focus on the target in the sky. Keep your eyes glued to your sky target until the bino eyepieces touch your face. Your target should be in the field or very close by. This takes a little practice, because the impulse is to glue your eyes to the binoculars first and then sweep around until you (maybe) find the object. You'll be amazed at how efficient your targeting becomes when you master this little trick.

Bottom of the Sword

4) The bottom sword 'star' that you see naked eye is actually the combined light from three stars. In my petite 10x24 binoculars, I can see two of those stars, which are the brightest in the field of view. The dimmer of the two is actually two stars, but they are so close together I can't split them with either my binoculars or a friend's beefier 10x50s.


The Running Man Nebula


5) The top sword 'star' that you see naked eye is actually the combined light from two stars, amplified by the nebula (cloud of gas and dust) that surrounds them and reflects their light. This cloud is known as the Running Man Nebula, because of its whimsical shape. Amateur astronomers with large telescopes and very good sky conditions can see the Running Man Nebula visually, and he's a favorite target of astrophotographers. Can you pick out the two brightest stars in your field of view?

With my 10x24s, I see a faint smudge above the two stars, in line with the rest of the sword. When I switch to the 10x50s, which have much more light-gathering capability, I see that the smudge is really a star cluster, also known as an open cluster. An open cluster is a loose grouping of stars that formed around the same time in the same nebula. You may want to think of them as a 'family unit.' This open cluster has the unromantic name of NGC 1981, but I like to call it the "Sword Hilt Cluster," to give it a little more flair.


Open cluster NGC1981
Image from
Sloan Digital Sky Survey


6) The middle 'star' of the sword isn't a star at all. It's one of the finest objects in the Milky Way: the Great Orion Nebula, a massive dust and gas cloud where stars are born. In binoculars, it reveals itself as a fuzzy glowing blob with some stars in it. In the 10x24s, the blob looks rather homogenous, but in the larger 10x50s, I begin to see tendrils of hazy material fanning out from the bright core.

What illuminates this bustling stellar nursery is a combination of the huge, hot stars being born there and their impact on the surrounding material. Ultraviolet radiation from these stars excites the cloud's atoms, which then emit energy in the form of light. It is the nebula's distance from Earth, 1350 light years, that makes it appear so small and star-like when we gaze at it naked eye. After all, just one light year, the distance light travels in one Earth year, is nearly six trillion miles!


The Sword of Orion


7) Let's switch now to a telescopic view. In a modest refractor telescope at 40x magnification--a significant bump up from the 10-power binoculars--more is revealed. You can now split the two clingy stars at the bottom of the sword that looked like one dim star in binoculars. You should also begin to see nebulosity (foggy-looking haze) around the brightest star of the trio.

Moving up the sword to the Great Orion Nebula, you should now see three bright stars near the dense core of the nebula. If you increase magnification to at least 65x, you’ll see that the brightest of the three is actually a tight cluster of four stars. This famous group is known as the Trapezium (a trapezium is a four-sided geometric figure). It was our old friend Galileo who first discovered the Trapezium was a star cluster, using what we'd call a primitive telescope.



Compared to our five-billion-year-old Sun, the 300,000-year-old Trapezium stars are young whippersnappers. They were born from the material of the Orion Nebula, just as stars have been born throughout the history of the universe--with the ignition of nuclear fusion in ultra-dense, ultra-hot clumps inside a hydrogen-rich nebula.

8) No matter how limited your equipment, you can see that even a modest bit of magnification on the Sword of Orion reveals far more than you can see naked eye. The more aperture (diameter of the lens or mirror) and magnification you can muster, the more you will see. Go as deep as you dare.

The most amazing view I ever had of the Orion Nebula was through my friend Gordon's 20-inch (diameter) reflector telescope, a 'cannon' that requires a tall ladder and a nosebleed to get to the eyepiece. He pumped up the magnification to eleventy-eleven-gazillion, and I felt like I was floating inside the nebula. Pinpoints of light--newborn stars--were embedded everywhere I looked, like sequins scattered in the folds of fabric. The more I looked, the more I saw. It was a cosmic experience.

The moral of my story? If you don't have a big telescope, make friends with someone who does!



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