Planetary nebula

A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude. A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term 'planetary nebula' is arguably a misnomer because they are unrelated to planets or exoplanets. The true origin of the term was likely derived from the planet-like round shape of these nebulae as observed by astronomers through early telescopes, and although the terminology is inaccurate, it is still used by astronomers today. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, 'very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet'. All planetary nebulae form at the end of an intermediate massed star's lifetime. They are a relatively short-lived phenomenon, lasting perhaps a few tens of thousands of years, compared to considerably longer phases of stellar evolution. Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus (PNN), ionizes the ejected material. Absorbed ultraviolet light then energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula. Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role. The first planetary nebula discovered (though not yet termed as such) was the Dumbbell Nebula in the constellation of Vulpecula. It was observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus. William Herschel, discoverer of Uranus, perhaps coined the term 'planetary nebula'. However, in as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, 'a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet'. Whatever the true origin of the term, the label 'planetary nebula' became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today. The true nature of these objects was uncertain, and Herschel first thought the objects were stars surrounded by material that was condensing into planets rather than what is now known to be evidence of dead stars that have incinerated any orbiting planets. In 1782 (237 years ago) (1782), William Herschel had discovered the object now known as NGC 7009 (the 'Saturn Nebula'), upon which he used the term 'planetary nebula'. In 1785, Herschel wrote to Jerome Lalande: Herschel assigned these to Class IV of his catalogue of 'nebulae', eventually listing 78 'planetary nebulae', most of which are in fact galaxies.