Planetary nebulae (Clouds ,Nebulae Star Births And Deaths)
Similarities to H+ regions
Like an H+ region, a PLANETARY NEBULA is a cloud of interstellar gas, mainly hydrogen, ionized by a hot star. The important difference between the two types of nebula is generic; while H+ regions are usually a by-product of the birth of highly luminous O stare, planetary nebulae are old objects, formed by the expulsion of gaseous material from the outer layers of a dying star. The name planetary nebula is misleading, and arose historically because of the visual resemblance between certain small circular nebulae and terrestrial planets such as Uranus.
Although in a few particular cases there is some uncertainty about whether a particular object is a planetary nebula or an H+ region there are a number of general observational differences:
(a) Planetary nebulae are usually much more symmetric in appearance than H+ regions.
(b) They occur isolated from each other and from other inter¬stellar clouds and star clusters.
(c) They have a much smaller mass of gas (about 0.2 MQ) than
most H+ regions. The optical spectrum of a planetary nebula is similar to that of
The optical spectrum of a planetary nebula is similar to that of an H+ region in that it consists mainly of bright emission lines such as the Balmer series of hydrogen and the forbidden lines of ionized oxygen, nitrogen, and other such elements. The central star of a planetary is generally hotter than that of an H+ region, with a temperature in the range 50 000-100 000 K rather than 25000-50 000 K; the higher stellar temperatures mean that many more highly excited ions, such as Ne4+ and, in particular, He+ are seen in the ionized shell.
The radio and infrared properties of planetary nebulae are rather like those of H+ regions. Free-free emission produces a radio-wavelength continuum, while dust grains absorb ultraviolet photons from the star and from the nebula, and re-emit the energy as infrared radiation. These dust grains appear to have a different chemical composition to normal interstellar grains, but their concentration and method of formation is uncertain. Emission from carbon monoxide molecules has been detected from some planetary nebulae; this result indicates that there must, be molecular hydrogen associated with at least some planetary nebulae, but we do not yet know whether this neutral gas exists outside the ionized region or in small, dense globules within it.