Sh2-140 is an H II region in Cepheus. An H II region is formed by interstellar atomic hydrogen that is ionized. That’s pretty much what most emission nebulae are. There are other things in the cloud besides hydrogen but hydrogen is by far the largest component. In this case there was a fair amount of sulfur but very little oxygen. There are definitely other elements are perhaps molecules present, but most amateur astronomers are limited to hydrogen alpha, oxygen III and sulfur II filters. These filters let in specific emission lines.

In this case, the lack of oxygen made processing more challenging. I opted to use the HOOSY palette – hydrogen in red, oxygen in blue and green with sulfur overlayed in yellow. Using the Hubble palette (sulfur In red, hydrogen in green and oxygen in blue) led to an overwhelmingly green image and while the sulfur emission could balance that out some, there were still large green patches that just didn’t look nice so I opted for HOOSY even though in practice it was. more like H–SY since the oxygen was very faint (and that was after devoting four times as much capture time to oxygen as hydrogen to try to get more faint signal).

The nebula is about 3,000 light years away. For all the technical details, see astrobin.