A cloud of neutral hydrogen travelling at a velocity far greater than can be explained by the normal rotation of the Galaxy. Some HVCs are debris from a tidal interaction between our Galaxy and the neighbouring Magellanic Clouds. The most spectacular example is the Magellanic Stream, which stretches half-way across the sky, but is visible only to radio telescopes. The distances and origin of other HVCs are still unknown.
Subjects: Astronomy and Astrophysics.