The Soul Nebula (Sharpless 2-199)

The Soul Nebula is a large emission nebula in Cassiopeia. Inside the nebula some stars formed. The wind from the most massive stars carved out large cavities in the nebula. This process pushes the gas on the outside of the cavities together - and ignites new stars. You can see several such cavities in this image. Analysis has indeed shown that the stars in these cavities get younger with distance from the center of the cavities.

(click on the image for full resolution)

This image consists of 5.1 hour integration time. It was also acquired on my recent trip to Richard's ranch near Yosemite.