About 1000 planetary nebulae are known. The best-studied ones have angular diameters between about 10 arc sec and several arc minutes, but planetaries can range in size from giant objects such as NGC 7293. the Helix Nebula , which is about one degree in diameter, to objects so small that they appear starlike even on the best optical photographs. These so-called stellar planetaries were discovered by searching the sky with a very low dispersion spectrograph and looking for objects which, though otherwise resembling stars, give out most of their light in narrow emission lines.

Most planetary nebulae lie within a few degrees of the galactic plane, with the greatest concentration of objects lying towards the Galactic Centre. Planetary nebulae have a typical disc distribution rather than that of Population I or II; they show concentrations towards neither spiral arms, interstellar clouds nor young stars, but occupy instead the region of the Galaxy populated by evolved. stars such as novae and RR Lyrae variables. The implication of this distribution is that planetary nebulae are associated with the later rather than the earlier stages of stellar evolution.

It is now generally believed that planetary nebulae are a normal stage hi the late evolution of single stars with masses of between 1 and 4M0. Pulsations of such a star during its helium-burning, red-giant stage can lead to the swift expulsion of essentially the whole of its hydrogen-rich envelope to form the shell of the nebula. This gas expands outwards at 20-30 kms-1, leaving behind the extremely hot burnt-out core as a very compact blue-hot star. This star is the source of the ultraviolet ionizing radiation for the nebula, but cools over a Period of 105 years, becoming an ever-fainter white dwarf and,finally, a black-dwarf star. Long before the star has cooled this far, however, the ionized gas shell will have become so diffuse that it has blended with the general interstellar medium, eventually to be¬come embroiled in a subsequent round of star-formation. The total amount of matter returned to the interstellar medium by all the planetary nebulae in the Galaxy is about 5M0 per year, which amounts to perhaps 15 per cent of all the matter expelled by all sorts of stars. Planetary nebulae therefore play a significant role in the evolution of the whole Galaxy.

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