Starship Asterisk*

The Dark Tower in Scorpius

Explanation: In silhouette against a crowded star field toward the constellation Scorpius, this dusty cosmic cloud evokes for some the image of an ominous dark tower. In fact, clumps of dust and molecular gas collapsing to form stars may well lurk within the dark nebula, a structure that spans almost 40 light-years across this gorgeous telescopic portrait. Known as a cometary globule, the swept-back cloud, extending from the lower right to the head (top of the tower) left and above center, is shaped by intense ultraviolet radiation from the OB association of very hot stars in NGC 6231, off the upper edge of the scene. That energetic ultraviolet light also powers the globule's bordering reddish glow of hydrogen gas. Hot stars embedded in the dust can be seen as bluish reflection nebulae. This dark tower, NGC 6231, and associated nebulae are about 5,000 light-years away.

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Well... i don't seem to be one of the *some* spoken of in the first sentence of the description.

That's a great picture!

Note how blue stars seem to "cluster" around the "head" of this cometary globule. At the very top of it, two stars seem to be emerging from the dust. This clearly suggests that star formation is going on in this "dark tower".

But the stars that form are almost certainly not very massive. If they are indeed blue then they are definitely more massive than the Sun, but it seems certain that they are not massive enough to ionize a red emission nebula. The red glow surrounding the Dark Tower is caused by the fierce stellar wind and ultraviolet emission from cluster NGC 6231 slamming into the dust and gas of this tall dust structure. This link from today's APOD beautifully demonstrates the connection between the cluster and the Dark Tower. The stellar wind and ultraviolet emission from this great open cluster not only ionizes the outer rim of the Dark Tower and makes it glow red, it also compresses the gas and dust "inside" and triggers star formation there.

So the Dark Tower is a "product" of NGC 6231. But fascinatingly, NGC 6231 may itself be a product of an even greater cluster, globular cluster NGC 6397, which may have made such waves in the disk of the Milky Way when the globular passed through it that the "splash" led to the formation of NGC 6231!

Govert Schilling wrote:
Richard F. Rees Jr. (Westfield State College) and Kyle M. Cudworth (Yerkes Observatory) propose that when the 6th-magnitude globular NGC 6397 passed through the plane of the Milky Way some 5 million years ago, it triggered the formation of the open cluster NGC 6231, which marks the core of the large Scorpius OB1 Association.

Read the full text here.

Anyway, it's interesting to consider the possibility that the Dark Tower represents a second wave of triggered star formation, where the first trigger may have been born not long after the universe came into existence. Wikipedia claims that NGC 6397 may be 13.4 ± 0.8 billion years old.

Ann

http://apod.nasa.gov/apod/ap110427.html
http://asterisk.apod.com/discuss_apod.php?date=110427

When the word "dust" is used is there and earthly analogy. Are we talking a desert sand storm or something more or less dense?

Big Dave wrote:When the word "dust" is used is there and earthly analogy. Are we talking a desert sand storm or something more or less dense?

Cosmic dust is very, very different from a desert sand storm on the Earth.

http://en.wikipedia.org/wiki/Cosmic_dust wrote: Cosmic dust can be taken to be all dust in the cosmos, as its name implies, or limited to space dust in our solar system, as scientists who study dust in the solar system prefer. It is for the most part a type of small dust particles which are a few molecules to 0.1 µm in size.

1 µm is one micrometer, or one millionth of a meter. It is similar to one millionth of a yard. This is the typical size of cosmic dust particles. A cosmic dust particle may look like this.

Even those parts of space which are typically described as "dusty" are really extremely, extremely empty by Earthly standards. If we had been inside a typical cosmic dust cloud we wouldn't have been able to see it at all, because it would have been so tenuous. What makes dust clouds important in space is not generally that they are dense, but that they are large. Imagine a volume with a diameter equivalent to the average distance between the Earth and the Moon, some 450,000 kilometers or 1.5 light-seconds. In fact, for the sake of simplification, let's imagine a volume whose diameter is only 1.0 light-second. Imagine that there are a million tiny dust particles in this space. The volume of space is so large and the dust grains so tiny that we absolutely wouldn't be able to spot them. But imagine that we had a cosmic dust cloud that was a light-year in diameter. If you are mathematically inclined, you should be able to calculate how many cubic light-seconds you could fit into one cubic light-year. There would be a lot of them, that's for sure. And there would be incredible numbers of dust particles in this cubic light-year.

Ann

APOD Robot wrote:The Dark Tower in Scorpius

Super looking picture. Right up there with the Eagle Nebula.

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Ominous? Hardly. This apod is glorious.

Case, thanks for the maps. It helped a lot to put this image into proper perspective.