Zone of Avoidance

As a result, optical galaxy catalogues are usually incomplete close to the galactic plane.

Many projects have attempted to bridge the gap in knowledge caused by the Zone of Avoidance.

The dust and gas in the Milky Way cause extinction at optical wavelengths, and foreground stars can be confused with background galaxies.

Even so, approximately 10% of the sky remains difficult to survey as extragalactic objects can be confused with stars in the Milky Way.

Projects to survey the Zone of Avoidance at radio wavelengths, particularly using the 21 cm spin-flip emission line of neutral atomic hydrogen (known in astronomical parlance as H I line), have detected many galaxies that could not be detected in the infrared.