PLANETARY NEBULAE : Part 1
WHAT ARE PLANETARY NEBULAE ?
Planetary Nebulae (PNe), singularly, Planetary Nebula (PN) are among the most magnificent, interesting and sometimes challenging objects for amateur observers. Often simply referred to as planetaries, they are visually small gaseous nebulae, appearing as circular, ring-like or oval-shaped patches of gossamer light.
Photographically, the brighter PNe are distinctly very pleasing to the eye and quite colourful. All planetaries are found scattered equally throughout both hemispheres of the sky, but are primarily concentrated along the celestial pathway of the Milky Way. Some also congregate towards the galactic centre in Sagittarius. Planetaries often have the appearance of bubbles in space, smoke-rings, symmetrical disks or strange mirrored shapes oriented along one axis in space. A few appear simply as irregular amorphous masses of light. Others show stellar or disk-like appearances, being similar in size to the two outer planets of the solar system Uranus and Neptune — with the similarity even extending to having a bluish or greenish colouration. Such small disks are only several seconds of arc across.
PNe can appear almost featureless but in some common cases they portray elusive or very subtle features. Others are shaped like annular rings. The various observed shapes are primarily caused by the chance perspective of the nebulosity boundaries — or, at least, as they are projected upon the celestial sphere.
Classification of all the roughly 3,000 known galactic planetaries is difficult, as they customarily display some significant individuality. Telescopically, some are easily observed while others are extremely difficult to find — either because of their faintness or because the elusive size of their tiny “starlike” disks. Visually, some do have obvious structures or mottling across their disks. Regrettably, the vast majority have only round or elliptical featureless disks, making them more similar to the many scatterings of the fainter galaxies.
Apparent diameters also vary considerably. Among the “bread-and-butter” planetaries, the tiniest stellar pinpoints are one to five seconds of arc, whilst the largest of the bright amateur-observable PNe, is the Helix Nebula in Aquarius (NGC 7293) covering half the Moon’s apparent diameter!
Application of astrophotography or CCD imagery reveals far more detail than can be seen with the eye. Often, we see structures containing irregularities such as filamentary material or “mottling” of brightness across the disk or uneven odd-shaped patches — each suggesting turbulence or chaotic disorder. Since the early 1990s, the images of these beautifully coloured objects from the Hubble Space Telescope (HST) have continued to show intriguing complexity, with each new release of images revealing increasingly delicate and unexpected features, or even new types of intricate structures. Yet all planetaries have one thing in common — a central hot luminous star causing the nebulosity to shine so brightly.
We once theorised that planetaries were new stars — “novae stellarium” but paradoxically, they are now known to be very near the end of the stellar evolution process. Current theories suggest great age — among the elderly stars undergoing true metamorphosis as they converting from huge red giant stars i nto planet-sized white dwarfs.
Often we use the general abbreviation of PN or preferably PNe instead of writing out the full term. Those objects about to change into planetaries are the proto-planetary nebulae or PPN or PPNe, but unfortunately nearly all are invisible unless observed in the near-infrared wavelengths.
Any partly exposed stellar core is called the Planetary Nebula Nucleus or PNN. At one stage during the star’s life, an atmosphere of hydrogen and other elements surrounds the small PNN — an initial sign of the prerequisite for white dwarves. Red giants do eventually shed their outer layers away from the central stars and into regions of the surrounding interstellar space. These expulsions have been observed to extend more than one parsec (about 3.3 light-years). The continuous mass loss continues unabated until the stellar core is revealed some 10,000 to 20,000 years later. Such stellar cores are ultra-hot, so the PNN gives the illusion of being new-born stars. Yet this fresh burst of life does not last very long because relatively little hydrogen fuel is available to burn. Compared to the entire stellar lifetime, this short phase exists for just a fleeting moment.
If modern theories are correct, planetaries are formed when the outer atmosphere of a red giant is ejected. This forms into a nebula several thousand astronomical units across (1 A.U. being the mean distance of the Earth to the Sun — about 150 million km.) The expelled material is similar to the solar wind and travels away from the red giant at speeds of 10 to 20 kilometres per second. One might think that the overall outburst of material is some single gigantic ejection, but the atmosphere is expelled by a process involving many irregular ‘puffs’ interspersed with periods of slow, steady mass loss — but how this period of mass loss works is not well understood by astronomers.
After a time, the material is expelled at a more pedestrian rate. Gravity has continuously driven the ejection by crushing the ultra-hot core and blowing the thinning stellar outer atmosphere violently away from the star. The boundary region between the core and the lower atmosphere may sometimes reach thermonuclear temperatures, igniting only fractions of the available hydrogen, or in the more massive progenitors, helium. In these instances, the liberation of energy undergoes one or more thermal pulses. In the last few centuries before the star’s thermonuclear energies finally expire, dozens of thermal pulses may occur.
Following this phase, the outer atmosphere is feed by gases diffusing more slowly through the white dwarf’s bulk which causes the stellar core to shrink very slightly. The mounting energy crises is short lived lasting about 0.01% of the whole stellar lifetime. Earlier theories advocated that the very final stages were not peaceful, ending in one sudden catastrophic outburst of the stellar outer atmosphere.
The existence of particularly violent ejections during PNe formation is believed to be possible but only occur under very rare circumstances. This may explain the double rings seen in about forty known PNe. Such objects would be more akin to the nova phenomena, or perhaps as a close binary systems. Irrespective of the mechanism causing the ejection, the mass of the visible ejecta within the planetary disk is very small and is roughly estimated to be only 0.1% to 0.01% of the original stellar mass.
At the end of the PNe formation era comes the post-planetary nebulae phase (PostPNe). Now the nebulosity has dispersed and faded away, however the central star still has the characteristics of its original PNN. For a short time, these pre-white dwarfs show characteristics of unusual richness in either hydrogen or helium, evidenced sometimes by spectral emission lines. It is presumed this is caused either by material leaking up to the dwarf’s surface or from being swept up by the stellar core’s strong gravity. Needless to say, all show evidence of extremely hot surface temperatures. Little is known about these stars, but some forty-seven candidates were suspected in 2002.
OBSERVING PLANETARY NEBULAE
Planetary Nebulae are interesting objects for the deep-sky observer, offering a differing and challenging task for the study by the novice or advanced observer. In the telescope, planetaries show great variety in regards brightness or appearance. Often advanced amateurs use the so-called Oxygen-III or [O-III] filter attached to the eyepiece. Most PNe emit the majority of their light within this strong oxygen line. This vastly improves the contrast by darkening the background sky. It also allows you to find difficult PNe quickly by flicking it across the telescope field — extinguishing the stars’ brightness but not the PNe itself.
The next web page has a short table of some of the brighter and more interesting PNe’s that are commonly observed. All are visible in dark skies using a star atlas and small apertures. As your skills develop you may like to search for other fainter or smaller objects. The Section can supply some further information on request.
Good Luck in Your Hunting!
Andrew James : 20th January 2005
The Planetary Nebulae Section is a dedicated towards observing, recording and discussing these deep-sky objects known as planetary nebulae. The ASNSWI has many deep-sky observers interested in these objects, and our combined experience can help you add PNe to your deep-sky observing programmes. We can even help you with a short list of targets for small and large telescopes.
Andrew has been the Planetary Nebulae Section Leader since 1998. He has made presentations at numerous ASNSWI meetings, and has written a detailed series entitled “Neat Southern Planetary” in the ASNSWI’s Journal Universe. His more specialised interests are with faint, little observed or very obscure southern planetary nebulae. He is also interested in the historical understanding of these objects and has a special interest in butterfly-like bipolar planetaries.
Last Update : 27th November 2012
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