Barnard 68
Published on Mar 26, 2008 at 7:06 am.
1 Comment.
Filed under nebula.
When I first saw this ESO image of Barnard 68 on the Astronomy Picture of the Day site, I recalled an old Star Trek episode, The Immunity Syndrome, where the Enterprise finds a zone of darkness that looks like a hole in the star field. Naturally, Captain Kirk decides to go inside, and there they find a giant space amoeba. But, thankfully, Barnard 68 is nothing so exotic as a space amoeba coming to destroy life in the galaxy. Rather, Barnard 68 is a perfectly ordinary dark nebula. In particular, Barnard 68 is a dark molecular cloud. It stands out because it is sitting in front of myriads of stars in the direction of the galactic bulge, towards the constellation Ophiuchus.
Dark nebulae like this have been observed for hundreds of years. William Herschel observed a number of these dark nebulae over two hundred years ago. Herschel didn’t know what to make of these objects. He called them “holes in the heavens,” though he realized that that designation did not really fit. The stars are not dots painted on the sky, and these dark objects are not holes in the star field. Instead, they are dark bodies that are between us and the more distant stars. So, they simply block the light of the stars behind them.
Herschel wasn’t the only astronomer to puzzle over these objects. In the early 20th Century, the astronomer Edward Emerson Barnard studied them. In 1927, Barnard published a catalog of 349 dark nebulae. Collectively, the dark nebulae that Barnard studied are called Barnard objects.
Dark molecular clouds like Barnard 68 are “dense” regions in the interstellar medium. By dense, of course, we mean dense relative to interstellar space, which is pretty thin. This region is still much more empty than what you would get with a vacuum pump here on the surface of Earth. But, it is big. It stretches almost half a light-year across. So, even though it is pretty thin, it is big enough that there is enough matter there to block visible light. These dark molecular clouds contain many tiny particles of interstellar dust grains. These dust grains are tiny compared with the dust that most people are familiar with. Interstellar dust grains range in size from a dozens of atoms up to grains that are a tenth of a millimeter across, but most are smaller than half of a micrometer (that makes these dust grains less than 1/2000 of a millimeter in size). It is principally these dust grains that are blocking the light making the dark nebulae dark. But that also makes them cold. The interiors of these dark nebulae are only a few degrees above absolute zero. In such cold places, stars can begin to form.
Barnard 68, in many respects, in pretty typical of a small dark molecular cloud called a Bok globule, named for the Dutch astronomer Bart Bok who studied them. But, if you look at the image above, you’ll notice that there are no stars to be seen in the center of the globule. Of course, the globule blocks the light of the stars behind it, as it would also block any stars that may be forming inside of it. But, with a star field as rich as the one in the direction of Barnard 68, you might expect to find a few stars in front of the globule. You don’t see any. That suggests that Barnard 68 is quite close. Estimates are that it is only about 400 light-years away. Now, 400 light-years might not seem to be exactly nearby, but it is still much closer than most other dark molecular clouds. Barnard 68, like similar Bok globules is a fairly concentrated bit of material. Its own gravity is trying to cause it to collapse further, forming a star (or stars), but it is warm enough in its interior for the gas molecules moving around to provide enough of a pressure to resist that collapse. But, at a temperature of only about 10 to 12 Kelvin, it is only just able to hold itself up against gravity. It is primed to begin its collapse to begin stellar formation. At present, there are no stars in the middle of Barnard 68, and there are none forming yet, either.
But, Barnard 68 does hold some surprises of its own. It has been reported to pulsate. This surprising behavior is possibly due to a shock to the cloud from a supernova explosion. To my knowledge, this is the only globule observed to do this sort of pulsation. However, turbulence in globules is not believed to be an important aspect to star formation, so perhaps Barnard 68 is not so terribly unusual. Only time, and more detailed studies of other globules will tell. At least, we have a very good view of this particular one, and it is beginning to shed its secrets to star formation.
-Astroprof
Image credit: ESO
Alana on August 16, 2008 at 2:43 am: 1
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