Galactic Worm 123.4-1.5 (GW 123.4-1.5) is an H I region with a mass of approximately 10⁵ M_Sun. It has an unusual mushroom-shaped structure that may be the result of having been formed by an infalling cloud slamming through the disc of the Milky Way from the other side. The high-velocity cloud in question is theorized as having hit at around 100 km/s, 5 years ago.

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H I regions Cassiopeia (constellation) {{nebula-stub