Author Topic: Sunday, 6 July 2014: Large Dark Nebulae  (Read 2815 times)


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Sunday, 6 July 2014: Large Dark Nebulae
« on: July 06, 2014, 02:36:45 pm »
Almost everyone here has seen the familiar look of a spiral galaxy with a dust lane in it. For instance look at NGC 6181:

This galaxy has plenty of nebula with star formation, along with quite a bit of gas and dust near the center. This galaxy is surprisingly similar to our Milky Way, actually. We see the mentioned green bits as Ionized Oxygen nebulae, like the nearby Orion Nebula. Then we see some of the brighter hydrogen clouds, such as M78:

But many of these hydrogen nebulae are not so bright, and they are known by name Large Dark Nebulae, or LDNs.

Characterized by their wispy, dark appearance, these nebulae cover our sky if you know how to find them.

There are several ways to find these nebulae. One is, if you're in a starry part of the sky, look for an area lacking as many stars as the area near it. For instance, this LDN near The Cocoon Nebula (IC 5146):


However, many Large Dark Nebulae are in fairly clear parts of the sky. You can fairly easily see the brighter ones, such as this gas cloud near LDN 255, from the similarly-named catalog of Lynd's Dark Nebulae:


However, there are many very dim, very sparse LDNs scattered around, and they're too dim to observe directly, so there is finally a third way to see them. Let's look at the much fainter LDN 207:


While some parts of the nebula are just bright enough to see, many parts are dim and you might not see them. However, if you look closely, you can see that many of the galaxies obscured behind the nebula have a reddish tinge to them, as they are obscured by dust and gas:

2MASX J16593708-1002399

Often these are mistaken for 'red spirals' when, really, they are just normal spirals behind a reddish-brown cloud. I've noticed that often these are the most sensitive indicator for overlying gas clouds, even when there appear to be none. If you see a red spiral or elliptical in galaxy zoo and zoom out, it's likely that you will find a diffuse cloud of gas surrounding it.

So, what exactly are these nebulae that you've spent more than half of your OOTD to explain how to find?

Well, most simply, they are the gigantic gas clouds that coalesced from the remains of hundreds, sometimes thousands of dead stars. Many of these nebulae, although appearing dark and relatively boring from the outside, are beginning to collapse inside with the beginning stages of star formation! They are essentially baby nebulae before they develop into gigantic HII regions, such as the nearby Orion Nebula. Eventually, as more stars begin to form, their formation ionizes and heats up the nebula, turning the reddish-brownish-grey color into a collage of deep reds, bright blues, vivid greens, and dark browns.

While our own galaxy is full of these staforming regions, many other spirals, such as NGC 2, aren't graced with the same such clouds:


As a result, these galaxies consist mainly of populations of older F, G, K, and M stars, as the younger stars have mostly gone supernova at a higher rate than new clouds could replace them. These galaxies, however, have a meager amount of these clouds, just to keep star formation continuing enough to keep the galaxy there.

Well why do so many of these galaxies not have these regions, yet ours does?

Unlike many other galaxies, our galaxy is in the process of merging with Andromeda, but that's another story. These galaxies are slowly (in astronomical terms) heading towards one another, to eventually collide in 3-4 billion years, and some think that it may have already passed near the Milky Way.

When these galaxies passed near one another, the extreme gravitational interaction between the two galaxies triggered many inert nebulae to begin heavy star formation. These nebulae are already beginning to show the fruit of their work. For instance the Pillars of Creation have already begun forming stars, and are even on their way to destruction- the shockwave from a nearby supernova is currently (was) heading towards them, and would have already destroyed them except that the speed-of-light delay will make us have to wait 1000 years to see it.

Unfortunately, the Pillars of Creation in the larger Eagle Nebula are out of the SDSS footprint, so I can't include a picture of them.

When the Milky Way collides with Andromeda, the sky will be covered in these nebulae and HII regions, and they will form billions of new stars for future generations to see in the chaotic night sky of far-future Earth.

Who would have ever thought that some dim wispy nebulae would be so interesting?
I like to find asteroids and galaxy mergers- but all galaxies are still fine to me.


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Re: Sunday, 6 July 2014: Large Dark Nebulae
« Reply #1 on: July 06, 2014, 02:46:15 pm »
Great one! 8)


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Re: Sunday, 6 July 2014: Large Dark Nebulae
« Reply #2 on: July 06, 2014, 02:58:46 pm »
Wowwww ! Thank you PS !  :) :)

Baby star opening its eyes on the Universe.