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