IC 342 - The Hidden Galaxy
IC 342 is a large spiral galaxy in the constellation Camelopardis located relatively close to the milky way plane. Regarding the fundamental cosmological principles of homogeneity and isotropy, galaxies should on average be evenly distributed in the sky. However, close to the galactic plane, less bright and a lower number of galaxies are observed.
This effect called "Zone of Avoidance", puzzled astronomers for decades and contributed to our understanding of cosmology at the time. However it is our milky way itself, moreover the dust and gas in the galactic disk that obscures our view. When light from distant objects, such as IC342 passes clouds of gas and dust, it is scattered out of the line of sight by the particles of the cloud, which are then not detected in the telescope. Blue light is more strongly affected by this phenomenon, hence this effect called "extinction" causes sources to appear both dimmer and redder. While making observations more difficult or even infeasable, once can use this effect on the other hand to determine the amount of gas in the way, when the prperties of the source behind is known. This has important implications for observations regarding molecular clouds and the mechanisms of star formation. For photometrical measurements, one has to use predetermined extinction maps to correct for the effect.
Because of extinction, this galaxy is extremely difficult to observe, hence the name "The Hidden Galaxy". If it wasn't to be obscured by dense clouds of dust, this galaxy which has the same size as M 101 would be visible by naked eye and be the 3rd brightest galaxy in the northern sky! It is part of a small galaxy group adjacent to the Local Group, called Maffei-Group. It is the birghtst member of this group, other members are also strongly affected by extinction and might be an interesting, yet difficult and uncommen target of observation as well.