What Chris pointed out about the thinness of this dark cloud is important.
Making new stars isn't easy. The cloud in today's APOD isn't up to it. The cloud really is far too thin and spread up to do that job. It is no wonder that the infrared photography of this cloud didn't reveal any pre-stellar cores in it.
EDIT: Okay, I just read bystander's link to the ESO page where some astronomers claim that Barnard 68 is on the verge of becoming unstable and contracting to the point that it will give rise to "a hot, low-mass star", according to the ESO page. That is a contradiction in terms, since low-mass stars aren't hot.
My impression, as I'm looking at this dark cloud, is that it looks too small to make any stars. The cloud isn't really elongated at all. My impression is that star formation often takes place along long dark thick "filaments" of dust. Take a look at
this Spitzer Space Telescope infrared image of Orion, where you can see how new stars are being born along dusty filaments above the Orion Nebula. Barnard 68, by contrast, is too round in shape to look like promising star formation material, and it ends too abruptly.
If you change the number Barnard 68 into
Barnard 86, you'll get a molecular cloud that is somewhat similar to this. What is so interesting about Barnard 86 is that you can see that this cloud is the more or less "dead" dusty remnant of previous star formation. The cluster to the lower left of Barnard 86, NGC 6520, was probably born out of "the mother cloud" that was spent in the process and left stars and the small dust cloud Barnard 86 behind.
Admittedly, cluster NGC 6520 has a thin dusty remnant "below" it, and it may be the this remnant that used to be the actual cloud that gave rise to NGC 6520. But if so, it is still likely that the dusty core that gave rise to NGC 6520 was once connected with what is Barnard 86 today.
You can see that NGC 6520 contains a red giant in the middle, so the cluster is at least old enough to have seen its most massive member use up all the hydrogen in its core and expand to a red giant. According to my software, which may or may not be absolutely reliable here, all the bright stars in this cluster may have used up all or most of their central hydrogen, which again suggests that the cluster isn't brand new. However, the blue star which is located between NGC 6520 and Barnard 86, and which appears to be surrounded by a swarm of small stars (but is not), may or may not be a younger star that was born slightly later out of the same dust cloud, and is still fusing hydrogen in its core.
What about the bright yellow-orange star on the other side of Barnard 86? I find it unlikely that this star was born out of the larger cloud that Barnard 86 is the remnant of. You can see that Barnard appears to end pretty abruptly on the side where the bright yellow star is, but remnants of it seem to extend quite far on the side of NGC 6520.
There are many small "barren" dust clouds like Barnard 68 and Barnard 86 in our galaxy. When you find them near young bright stars or clusters, you are probably right to think that these little clouds are the remnants of once mighty dense clouds that gave rise to huge stars and mighty clusters.
Ann
What Chris pointed out about the thinness of this dark cloud is important.
Making new stars isn't easy. The cloud in today's APOD isn't up to it. The cloud really is far too thin and spread up to do that job. It is no wonder that the infrared photography of this cloud didn't reveal any pre-stellar cores in it.
EDIT: Okay, I just read bystander's link to the ESO page where some astronomers claim that Barnard 68 is on the verge of becoming unstable and contracting to the point that it will give rise to "a hot, low-mass star", according to the ESO page. That is a contradiction in terms, since low-mass stars aren't hot.
My impression, as I'm looking at this dark cloud, is that it looks too small to make any stars. The cloud isn't really elongated at all. My impression is that star formation often takes place along long dark thick "filaments" of dust. Take a look at [url=http://www.spitzer.caltech.edu/uploaded_files/graphics/fullscreen_graphics/0008/2667/sig10-003_Sm.jpg]this Spitzer Space Telescope infrared image of Orion[/url], where you can see how new stars are being born along dusty filaments above the Orion Nebula. Barnard 68, by contrast, is too round in shape to look like promising star formation material, and it ends too abruptly.
If you change the number Barnard 68 into [url=http://apod.nasa.gov/apod/ap020722.html]Barnard 86[/url], you'll get a molecular cloud that is somewhat similar to this. What is so interesting about Barnard 86 is that you can see that this cloud is the more or less "dead" dusty remnant of previous star formation. The cluster to the lower left of Barnard 86, NGC 6520, was probably born out of "the mother cloud" that was spent in the process and left stars and the small dust cloud Barnard 86 behind.
Admittedly, cluster NGC 6520 has a thin dusty remnant "below" it, and it may be the this remnant that used to be the actual cloud that gave rise to NGC 6520. But if so, it is still likely that the dusty core that gave rise to NGC 6520 was once connected with what is Barnard 86 today.
You can see that NGC 6520 contains a red giant in the middle, so the cluster is at least old enough to have seen its most massive member use up all the hydrogen in its core and expand to a red giant. According to my software, which may or may not be absolutely reliable here, all the bright stars in this cluster may have used up all or most of their central hydrogen, which again suggests that the cluster isn't brand new. However, the blue star which is located between NGC 6520 and Barnard 86, and which appears to be surrounded by a swarm of small stars (but is not), may or may not be a younger star that was born slightly later out of the same dust cloud, and is still fusing hydrogen in its core.
What about the bright yellow-orange star on the other side of Barnard 86? I find it unlikely that this star was born out of the larger cloud that Barnard 86 is the remnant of. You can see that Barnard appears to end pretty abruptly on the side where the bright yellow star is, but remnants of it seem to extend quite far on the side of NGC 6520.
There are many small "barren" dust clouds like Barnard 68 and Barnard 86 in our galaxy. When you find them near young bright stars or clusters, you are probably right to think that these little clouds are the remnants of once mighty dense clouds that gave rise to huge stars and mighty clusters.
Ann