What's the difference between globular clusters and dwarf spheroidal galaxies?
GREG COMEGYS, CALEDONIA, MICHIGAN
In a word: size.
Dwarf spheroidal galaxies are much bigger than globular clusters. The Pegasus dwarf spheroidal galaxy, which orbits the famous spiral galaxy M31 in Andromeda, spans 2,000 light-years. A newly discovered object, the Ursa Major dwarf spheroidal (see "New Milky Way satellite," in News, July 2005) is about half this size.