Case study: the Pipe Nebula
The image of the Galaxy in visible wavelengths from the first post of this series shows many dark regions tightly restricted to the galactic plane. It is not that these are holes in the Galaxy: they are Giant Molecular Clouds, mostly comprised of gas in the molecular form, and of dust that obscures the stars behind the cloud from view. Some of these clouds are so conspicuous on the night sky that they can be seen with the naked eye despite (or because of) their darkness. Slightly above the plane, in the direction of the Galactic Centre (exactly here), lies one such clouds, a V-shaped structure closely resembling a check mark, appropriately called the Pipe Nebula, or Pipe for short.
[Editor note: Bilingual post. Only the English version is shown]