Monday, October 7, 2013

Classifications of Objects in Space Part 4: Dark Nebulae

Because astronomy is an observation-based science, we need to carefully categorize the objects we see. As we explore the cosmos, we discover that not all objects are the same, and even then, some objects that appear the same at first glance differ greatly in the details. Here, we will be discussing the classifications we give the objects we find.

A nebula is basically a large cloud of gas – in that, there are several different categories that define the type of nebula we are viewing.

Today, I wanted to talk about dark nebulae.

In itself, the name is pretty descriptive, a dark nebula is a dense and or dark cloud of dust that blocks light from behind it. Many dark nebulae are silhouetted against the objects around it (such as the Horsehead nebula). Dark nebulae are usually seen in conjunction with reflection and emission nebulae, but they can be found by themselves as well.

When a dark nebula is isolated within a reflection/emission nebula (as in, it appears to be ‘free floating’) it’s called a bok globule. These are normally found within H II regions and are usually a tell tale sign of star formation. These globules form the cocoons of new star systems and are usually tens to thousands of solar masses.

So, what are some examples of dark nebulae?

To the right is none other than the Snake Nebula. It is found in the constellation Ophiuchus and is about 650 light-years from Earth. This is a part of the much larger Dark Horse Nebula (or Great Dark Horse) – not to be confused with the Horsehead nebula (which is also pictured in this article). Even though this nebula isn’t connected to a reflection or emission nebula you can see it through a telescope. That being said, more times than not, you need pretty good viewing conditions to see dark nebula.

Here, we have bok globules. This particular set can be found within IC 2944 or the Running Chicken Nebula. It is located within the constellation Centaurus and can be found about 6520 light-years from Earth. This particular image is going to make a liar out of me, because whereas bok globules usually denote star formation, such star-forming activity has yet to be observed in this particular region.

Last, but not least, we have the Horsehead Nebula pictured above. The Horsehead is silhouetted against the rest of the Orion Molecular Cloud Complex which it calls home. This is a stellar nursery and is comprised of both organic and inorganic molecules.

References, sources, and further reading can be found here:

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