N naked   eye   limit    -   For   observers   in   very   dark   skies,   it   is   possible   to   see   stars   as   faint   as   6th   magnitude.      The   magnitude   system   is   a ranking   system   where   first   magnitude   stars   are   the   brightest.      In   light   polluted   skies   where   I   live,   we   are   lucky   to   see   3rd   magnitude stars. nebula   -    In   the   classical   sense,   a   nebula   is   any   astronomical   object   that   is   not   a   pinpoint   of   light.     At   one   time,   this   definition   included galaxies   other   than   our   own   Milky   Way,   and   clusters   of   stars.      Nowadays,   the   definition   is   limited   to   patch   of   light   caused   by   glowing interstellar gas or dust. neutron   star    - A   condensed   remnant   of   an   exploded   massive   star,   no   bigger   than   a   few   kilometers   in   size.      Spinning   neutron   stars are what we observe as pulsars.
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Carpe Caelum