1. A recent review of the Local Group is given by S. van den Bergh [ Astron. Astrophys. Rev. 9 273 (1999)].
2. The common length unit used in astronomy is the parsec (pc) derived from the (unit) parallactic second of arc angle subtended by the semimajor axis a of Earth's orbit (around the sun) as seen from a distance of 1 pc. Because a = 1.496 × 10 13 cm it follows that 1 pc = 3.086 × 10 13 km = 3.2615 light years; 1 kpc = 10 3 pc.
3. A dwarf satellite galaxy in Sagittarius
4. Here we consider only those components of the Galaxy which consist of luminous stars formed from gas and dust of ordinary matter. The dark matter component is discussed in other papers in this issue (11);
5. The Baryon Halo of the Milky Way: A Fossil Record of Its Formation