Galaxies contain several billion to hundreds of billions stars, along with gas, dust, and a great deal of mass the nature of which nobody knows, and that goes by the term, "dark matter."
Galaxies are of three main types: spirals, ellipticals, and irregulars. Since only one Messier object is an irregular galaxy, M 82, I have not devoted a seperate page to the irregulars.
After each galaxy's M and NGC numbers I have listed its Hubble type.
E denotes ellipticals. They range from E0 to E7. E0 appear circular, and E7 are most highly elliptical in appearance. These numbers do not necessarily correspond to the galaxy's actual degree of ellipticity.
S means spiral, SB barred spiral. Lowercase a, b, and c describe the spiral or barred spiral from large nucleus and tightly-wound arms ( Sa or SBa) to small nucleus and loosely-wound arms (Sc or SBc). The Milky Way was once thought to be an Sb, but is now thought more likely to be an SBb.
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