What an interesting APOD!
Me being me, it goes without saying that the stars interest me because they are blue. The brightest of the stars is Mu Serpens, a relatively hot and blue A0-type star with a negative B-V index and a visible-light luminosity of about 86 times solar. Two of the other blue stars in the picture are lesser A-type stars, cooler and fainter than Mu Serpens, but still a lot hotter and brighter than the Sun. The blue star at bottom center is more distant than the three others and does not belong to the same group or cluster.
But the other three must have been born at more or less the same time. According to today's caption, the distance to LDN 183 is 325 light-years, too far away to have anything to do with the formation of Mu Serpens and its blue siblings, which reside at about 170 light-years. But guess what? The blue odd-on-out at bottom center of today's APOD, HD 141569, is a B9 star located at about 380 light-years, clearly farther away than LDN 183, but still possibly connected with it. This star just might have been born from LDN 183, which, in that case, would have shrunk in size. And it would appear that the far side of LDN 183 is the one that has retreated, if HD 141569 was indeed born from it.
The Lagoon Nebula with cluster NGC 6530.
Photo: Adam Block.
I think you can see the same phenomenon in other regions of star formation. One of more stars are born from a dust cloud, and the nebula that they were born from shrinks accordingly and retreats from the stars.
One example might be the Lagoon Nebula. In the eastern (left) part of the nebula you can find a star cluster, NGC 6530, that was recently born from the nebula. I believe that NGC 6530 is located in front of the remaining nebula, which has shrunk and retreated in response to the formation of the star cluster.