The equatorial disk of a spiral galaxy is never so apparent as when viewed edge on. NGC 891 (inclined 88.3 degrees) gives the viewer an appreciation of the exceedingly thin and flat nature of the disk and its dense, dusty contents. In fact the dust content and gas/dust ratio of NGC 891 is very similar to that of the Milky Way. Extraplanar dust structures are a distinguishing feature of NGC 891. These huge filaments of dust are aligned perpendicular to the disk and extend above and below as much as 7200 light years from the central plane of the galaxy. At higher galactic latitudes some of the dust filaments assume a peculiar parallel alignment with the galactic plane. The filaments are enormous and some of the more prominent ones contain some 100,000 solar masses of material.
An important question is what drives the ejection of matter away from the thin disk and into the galactic disk-halo boundary without destroying it? Several galaxies exhibit the phenomenon of anomalous neutral gas found several thousand light years out of their galactic disk plane. The ejection of gas out of the disk, called a galactic fountain, is thought to be powered by stellar winds from massive stars and supernova explosions. After cooling, this gas most likely falls back into the disk. This process may exist in many galaxies and may be responsible for some of the neutral gas detected at high latitudes within our own galaxy.