Why i see several postgres server processes sometimes ?

Hi,
I wander why i see several postgres server processes with different PIDs
I don't mean those normal query processes. I see several of these:
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

With different pids. Is this normal ?

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND

27662 postgres 20 0 1623m 14m 10m R 5 0.3 0:00.14 postgres: wmg
wmg 172.18.9.21(43725)
idle

15489 postgres 20 0 1610m 33m 33m S 3 0.7 76:51.46
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

27714 postgres 20 0 1613m 7720 5228 S 1 0.2 0:00.03 postgres: wmg
wmg 172.18.9.12(42434)
idle

27730 postgres 20 0 1611m 2336 1228 R 1 0.0 0:00.02
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

27731 postgres 20 0 1610m 1572 1044 R 1 0.0 0:00.02
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

15494 postgres 20 0 1611m 1.5g 1.5g S 0 30.6 6:10.54 postgres:
writer
process

27711 postgres 20 0 1613m 7752 5248 S 0 0.2 0:00.01 postgres: wmg
wmg 172.18.9.13(53169)
idle

27729 postgres 20 0 1610m 1572 1044 R 0 0.0 0:00.01
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

15495 postgres 20 0 1611m 1020 652 S 0 0.0 0:23.09 postgres: wal
writer
process

15496 postgres 20 0 1612m 1648 564 S 0 0.0 0:24.77 postgres:
autovacuum launcher
process

15497 postgres 20 0 73524 1308 296 S 0 0.0 18:29.63 postgres: stats
collector
process

17480 postgres 20 0 37052 940 936 S 0 0.0 0:00.01 su
postgres

17481 postgres 20 0 19384 1236 1232 S 0 0.0 0:00.01
bash

17510 postgres 20 0 67696 2400 1992 S 0 0.0 0:00.12
/usr/lib/postgresql/8.4/bin/psql
wmg

17537 postgres 20 0 1617m 26m 21m S 0 0.5 0:00.12 postgres:
postgres wmg [local]
idle

27722 postgres 20 0 1614m 10m 7372 R 0 0.2 0:00.00 postgres: wmg
wmg 172.18.9.13(53195)
idle

27724 postgres 20 0 1613m 7700 5208 R 0 0.2 0:00.00 postgres: wmg
wmg 172.18.9.12(42452)
PARSE

27727 postgres 20 0 1613m 4324 2336 R 0 0.1 0:00.00 postgres: wmg
wmg 172.18.9.13(53204)
startup

27732 postgres 20 0 1610m 424 0 R 0 0.0 0:00.00
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf

27733 postgres 20 0 1610m 424 0 R 0 0.0 0:00.00
/usr/lib/postgresql/8.4/bin/postgres -D /database/postgresql/8.4/main -c
config_file=/etc/postgresql/8.4/main/postgresql.conf