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Planetary Nebulae: a little guide

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Planetary Nebulae

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Chapter 1: Historical Background

Messier - The Unknowing Discoverer

At the end of its lifetime the Sun will swell into a red giant, a moribund star hundreds of millions of kilometeres in size, expanding out beyond the orbit of Venus and Earth. As it burns through its fuel, the Sun will eventually collapse. It is possible that the outer layers of the star will be ejected into a shell of gas that will last a few tens of thousands of years before spreading out into the vastness of space to be recycled again by future stars. When we see this beautiful halo of diffuse gas from dying stars we call it a planetary nebula.

Planetary nebulae play a crucial role in the chemical evolution of the galaxy. They return to the interstellar medium material enriched in heavy elements, including carbon, hydrogen, oxygen and nitrogen; the stuff of life. You can be sure that most of the nitrogen in the air about you and in the DNA of your cells, along with the carbon found in organic life, had a previous existence as part of some planetary nebula.

How we came to understand planetary nebulae is a remarkable journey which began in the calm of a summer evening in France July 12th 1764. The stars were sliding into place. All about was still and shadowy. It was that moment when the world seemed infinitely greater. There were no clouds about, transparency and seeing was excellent. It was a moment when anything could happen.

The telescope man sat behind his relatively small, 7” Gregorian reflector. He could already see that planet Mars was accompanied by the fake Mars star, Antares; a pair of glowing red eyes framed by the twilight coming on. Venus was edging towards the horizon in the west and a near full moon was creeping up on Sagittarius low in the southern skies. He knew he didn’t have long. The Moon was no brighter than a glint of candle light on broken glass but he understood that very soon the night sky would become awash with its ghostly light, drowning the possibilities of discovering another comet. 

The man’s name was Messier, the famous comet hunter and in his log book tonight was the constellation Vulpecula, the Little Fox. It was well positioned, creeping towards the zenith where it was the darkest and away from the Moon’s growing glare. Sweeping eastward from the red giant Alpha Vupeculae, he suddenly came across an unresolved blob of light, something no one had ever seen before and after recording its position to warn other comet hunters to avoid this sight, much like a sailor might do of perilous rocky outcrops, he dismissed the dimly glowing disk and continued the hunt for those ellusive comets.

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Unknown to Messier, he had discovered the first known planetary nebula. Until that warm July evening, these curious cosmic clouds of dust and gas had led invisible lives and in a manner of speaking, did not exist before Messier stumbled upon what we now call M 27 – the Dumbbell Nebula, one of the finest deep space objects in the night sky. Fifteen years were to pass before the astronomer Darquier discovered another cloudy halo of light in Lyra on a cold evening of January 1779. A year later, Mechain stumbled upon two more. These were later classified as M 57, M 76 and M 97 respectively.

Herschel - Setting the Stage

No one had any idea what these ghostly clouds of light were. For astronomers their properties and nature were a mystery and for comet hunters like Messier, an irritation, merely to be catalogued and avoided.

In 1782 at the age of 44, William Herschel set about on a systematic survey of the night sky. His mission was to reveal as many deep space wonders as possible. He needed to discover, determine, position and catalogue these wonders with a view to determining their distribution and possible composition.

In his lifetime, Herschel discovered over 2400 ‘nebulae’, a generic term he used for any visually diffuse astronomical object, about 79 of which were these curious small glowing clouds of hazy light. In 1784 he decided to refine his definition by calling them Planetary Nebulae (PNs), for in some way they reminded him of the round, blue-greenish planet Uranus he had discovered three years earlier. Like other astronomers, he was convinced that they were similar to globular clusters and that with sufficient aperture and magnification these nebulae would be resolved into a blaze of stars. 

Then on a winter’s night in 1790, he came across NGC 1514, otherwise known as the Crystal Ball Nebula, describing it as "a most singular phaenomenon" which forced him to rethink his ideas on the construction of the cosmos. In essence, Herschel discovered a single star at the centre of the glowing cloud.

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He wrote a paper for the Royal Society a few months later in 1791, concluding that planetary nebulae did not consist of stars as previously thought and yet there could be no doubt about the correlation between the strange, cloudy atmoshpere and the single star at its centre. Herschel’s insight set the stage; for the first time the link between stars and nebulae had been made and yet as so often happens in life, although the tentative theory was in place, it would take almost a lifetime for technology to catch up and help discover in Herschel words the meaning of those “nebulous stars shining a fluid of a nature totally unknown to us.” 

Photography

If there is such a thing as progress in science, then it is intimately linked with technology. Astronomers could not discover how PNs work or what they were made of until they had the tools to do the job. Two key developments were made in the nineteenth century without which there would probably be no science of astrophysics and little understanding of planetary nebulae.

The first revolution occurred around the 1830s with the invention of photography which had enormous repercussions. For astronomers it meant they no longer had to rely on drawings when comparing orbits of stars, or positions of binaries, or how a deep sky wonder appeared. There was no longer the nagging doubt that the observer may have made a mistake and sketched or recorded their observation incorrectly.

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As photographic technology improved, it became capable of recording more and more detail, something the human eye with the aid of a telescope just couldn’t match. Longer exposure time meant more light could fall upon the film or plate and thus fainter objects could be recorded. Photography literally opened up the heavens and when it was linked to spectroscopy, another scientific breakthrough of the mid-nineteenth century, we can coherently reason that the keystone of astrophysics had at last been established.

Spectroscopy

Spectroscopy is the analysis of the light emitted or absorbed from an object such as gas to provide information about what that object is made of. The name itself comes from spectrum, the rainbow of light you see when sunlight passes across the back of a compact disc or through rain drops or a glass prism.

It turns out that if you can magnify that spectrum of light, you will find it has many bright and dark lines on it. The English chemist and physicist Wollaston was the first to notice this in the early nineteenth century, soon followed by Fraunhoffer in Germany but neither really developed the idea, and both died quite soon, so it wasn’t until the 1850s that any further progress was made.

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Bunsen, of the Bunsen burner fame which he never invented and Kirchhoff made a number of key discoveries. They found that each element produces its own characteristic set of lines on the spectrum. For any particular gas of an element, if the gas was hot and excited it produced bright lines on the spectrum, later called emission lines and if the gas was cooler and light passed through it, dark lines were produced in exactly the same place, later to be called absorption lines.

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In the spectrum of the Sun, for example, there is a profusion of dark lines each at its own precisely determined position. We see those absorption lines in the spectra of ordinary stars like the Sun because the tenuous, colder outer layers of the stellar atmosphere, called the photosphere, absorb some of the continuous light coming from the super hot, dense interior.

The lines resemble the pattern of a bar code and they are as distinctive as a bar code, because they tell you exactly what the object producing the lines is made of. From his research, Kirchhoff understood that sodium produced two very distinct lines in the yellow-orange part of the visible spectrum and in 1859 was able to confirm the presence of sodium in the Sunlight’s spectrum. It was the first time anyone had identified the presence of any element outside the planet Earth.

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Once the relation between the chemical composition of a substance and its spectrum was appreciated, it became the province of others to start taking the spectroscopic fingerprints of all the other known elements. Soon enough astronomers could identify which elements were present in the Sun and other brightly shining stars.

The technique in its simplified version is to allow light from a star to pass through a telescope, into a prism or diffraction grating, where it spreads out into a spectrum that is photographed with long exposure to bring out as much detail as possible. When Sir Huggins trained his telescope and spectroscopy gear on the Planetary nebula NGC 6543, the Cat’s Eye Nebula in 1864, he discovered that its spectrum revealed almost nothing but very strong, narrow emission lines; something only apparent in hot, agitated gas. Because their true nature was completely unrecognizable it lead to the idea that they might be as yet undiscovered elements, and so these emission lines were dubbed nebulium.

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Almost a lifetime was to pass before the composition of those mysterious lines discovered by Huggins were determined. In between that time more pieces to the complex mystery were added. Edwin Hubble noticed a correlation between the magnitude of its central star and the size of the planetary nebula. He went on to suggest that the emission lines produced were probably due to the gas absorbing radiation from the small central star. In the same year of 1922, Campbell and Moore from their studies of spectroscopic lines discovered that planetary nebulae were still expanding.

In 1928, Bowen found that the curious Huggin emission lines from the Cat’s Eye Nebula were not a new type of element afterall, but in fact oxygen, rarefied oxygen, now identified as OIII, a forbidden line transition simply because it occurs under very special conditions never seen on Earth. Other forbidden lines were later identified as nitrogen and neon. A year later, Perrine confirmed Campbell and Moore’s findings by correctly interpreting the doppler effect of planetary nebulae as indication of its expansion; that the shell of hot gas around the central star was expanding further away from it.

There was nothing wrong with these insights but still a hundred and fifty years of observing and studying planetary nebulae, a great enigma remained. Sure, there was a strong correlation between certain stars and the halo of gas around it. Astronomers had discovered what many were made of and understood that the gas was spreading further from its source but the central riddle remained:  what stars made planetary nebulae and why did this happen?

Chapter 2: Brief Sketch of Stellar Evolution

Before we can tackle our question of what stars make planetary nebulae and why, it is necessary to give a general idea of star evolution. Not only is it one of the greatest discoveries made by mankind, it will also equip us with the necessary tools to gain a deeper appreciation of what is going on with planetaries.

This account can be traced back to the 1920s when astronomers discovered that the oldest stars are chiefly composed of the primordial elements hydrogen and helium produced in the birth of the universe in a Big Bang. Over the following decades scientists were able to unlock the secrets of the stars and show how elements are cooked by nuclear fusion inside of them and scattered across the universe in stellar explosions, to be recycled and become new stars, planets, and life.

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The Protostar

Although there is probably no more than a large town of men and women who understand quantitatively what is going on, qualitatively, stars are quite simple entities to understand. Most stars have a similar chemical composition and the nuclear fusion which occurs within them is identical. The only significant difference is that a star’s life, from birth to death, will depend on its initial mass and, to an extent, on its composition.

Stars go through phases of life much like organic beings. They form in galaxies within giant molecular clouds composed mainly of primordial hydrogen and helium. These clouds may also be enriched by heavier elements from the deaths of previous generations of stars and can sweep distances of over 100 light-years and contain anything between a hundred thousand and several million solar masses of material. These giant molecular clouds are the largest gravitationally-bound objects found in our galaxy and are the only places where star and planet formation is known to occur.

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Over the period of millions of years, localised regions within these clouds undergo gravitational collapse. The primordial material is pulled together by gravity and dense concentrations of gases and dust arise. Eventually, this gravitational energy - forever drawing the gas into a denser sphere - is converted into light and heat. A young protostar is born and becomes hot enough to glow red and may remain in this state from a 100,000 to 10 million years, depending on its initial mass.

At some point in the protostar’s life, the central temperature climbs high enough for deuterium, an isotope of hydrogen, to undergo fusion, which acts as a counter force and temporarily stalls the unceasing contraction of gravity. When all the deuterium is used up, gravity returns with vengence and the protostar continues to collapse until at some point its core temperature reaches the level at which the fusion of hydrogen into helium can take place. When that happens the star joins what is known as the main-sequence.

Main-Sequence

On the main-sequence, the star, regardless of its mass, remains in a relatively stable condition, settling into a steady and regular luminosity as it converts hydrogen into helium. A star like our Sun can remain on the main sequence for about 10,000 million years, 500 million years for a star with 3 solar masses, 200,000 million years for a star half the size of the Sun. When a star has lived through about 90% of its lifetime, it begins to evolve off the main-sequence, for it is no longer able to fuse hydrogen into helium.

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In its older age, the star’s hydrogen supply begins to run out and the core, deprived now of the pressure countering gravity, starts to contract. This compression of the star’s core raises its temperature, pressure and density which inturn acts as a new source of energy which provides the means to resist the inward force of gravity.

Big Mass Stars – 6 to 8 Solar Masses and More

Once hydrogen burning is exhausted in the star’s core, it moves from the main-sequence and enters the next stage of its life, converting itself into a red giant. As soon as hydrogen fusion comes to an end, a helium-flash takes place, occurring at the speed of an exploding bomb. This sudden explosion can blast up to 30% of the star’s original mass out into space while knocking the degenerate core back into something resembling a main-sequence star, only this time it has a higher temperature, density and pressure as helium fusion takes place, converting helium into oxygen and carbon.

While this is hapenning, the star’s outer layers are swelling to enormous proportions. The heat generated by the helium core expands the outer layers of the star into a gigantic gaseous atmosphere yielding red giants which although losing up to a quarter of their original mass are still between a hundred and a thousand times brighter and bigger than the Sun. The outer layers are being blown away by solar winds and although these stars are now bigger and brighter, they are also a lot cooler, for the heat generated is spread over a larger surface area.

As each nuclear fuel is exhausted, central pressure, density and temperature rise, enabling the fusion of more elements with ever increasing atomic weight; first hydrogen, then helium, then carbon, nitrogen and oxygen, and so on. If you were to slice through a big mass star at this stage, a star with about 6 or more solar masses, you would see a structure like that of an onion undergoing an explosive series of nucleosynthesis.

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In the outside layers, you would find the remaining hydrogen being fused into helium. As you dig deeper through the layers, you would see helium being fused into carbon and oxygen, carbon being fused into neon, sodium and magnesium, all the way up the periodic table of elements. At the very centre of the star, you would find a core of iron rapidly collapsing upon itself through the fusion of silicon or nickel.

Once the core of a star is coverted into iron it is no longer possible to produce more energy merely by compressing it to start a new fusion reaction. When you cut yourself, the remains of those dead stars spill from you. Every atom of iron in your blood once helped destroy a big mass star, transforming it into one of the most stunning events in the universe.

With the creation of iron, gravitational pressure becomes so great that big mass stars end their lives as huge supernovae explosions. These are extremely rare events but because there are so many stars in a galaxy, there are probably between one or two supernovae in a galaxy every hundred years or so.

The explosions from them are staggering and can emit enough light to overshadow an entire galaxy made up of thousands of millions of stars. During the course of the supernova which may shine brightly for a number of days before fading, colossal clouds of elements heavier than the carbon, oxygen, nitrogen and iron already manufactured inside the star are also created which in time will become part of the giant molecular clouds, collapse under gravity and forge the creation of new stars, planets and living things.

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At the heart of the supernova remains the star’s core, for a moment we may call it a white dwarf. In a few tenths of a second, those white dwarfs with 3 solar masses or more are large enough to go on collapsing under gravitational pressure to form black holes, relatively small light eating entities. Contrary to the popular misconception, black holes do not suck in objects around it. Due to its incredible mass, they bend space in such a way that everything within their gravitational field is pulled towards it. A black hole’s gravitational field is so strong that even light travelling at some one thousand million kilometers an hour cannot escape. White dwarfs between 1.4 and 3 solar masses will become neutron stars detected as pulsars which can spin hundreds of times every second.

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These two entities are the ultimate expression of gravity’s triumph. Stars which were once millions of kilometers in diameter have been compressed so much that they either disappear from sight or are now no bigger than a city and yet so dense that a single teaspoon of their star stuff would weigh billions of tons.

But what of those stars with roughly 1 to 6 solar masses? What happens to them? Although it may be a less violent story than that of larger stars, it is no less fascinating and will help us understand what stars make planetary nebulae and why. And for that, we need to turn to stars like the Sun.

Smaller Mass Stars

Stars with masses between 1 and 6 times that of the Sun also have their stories to tell. Superfically, the only significant difference between them will be the speed at which they die; bigger the star’s mass, quicker will it run through its life. However, recent investigations have postulated, and not without evidence, that smaller mass stars, those perhaps less than 1.5 to 2 solar mases, might not end their life as originally conceived.

We know from measuring isotopes that the Sun was born about 4,500 million years ago and for most of this time has been on the main-sequence, fusing hydrogen into helium. We also know that it is about half way through its life and around 30% brighter than when it started out.  During the next 3,000 million years the Sun will continue to brighten and expand in size and during that time all the water on Earth will have probably evaporated and organic life will cease to be.

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In around 5,000 million years from now, the fusion of hydrogen into helium will stop in the Sun’s core and although the Sun will continue to fuse hydrogen in a shell around the core, the energy will not be enough to counter gravity. The force of gravity will compress the core, raising its temperature and density until it is high enough to trigger the fusion of helium into oxygen and carbon. This new source of energy will resist the inward force of gravity for a while. At this stage, we are basically following the same picture described above when bigger mass stars become red giants.

In about 7,000 million years, the Sun will appear as a fully formed red giant. The star at this stage will have lost about a quarter of it mass and be about half the temperature it is now, but due to its greater surface area could be as much as 100 times more luminous. Due to its smaller mass, all nuclear reactions in the core will have ceased and the Sun will not be able to fuse silicon and nickel into iron. Instead, it will continue to contract under the force of gravity, causing it to heat up which in turn will heat the upper layers of the Sun causing them to expand out even further. The Sun will have swelled to collosal proportions, expanding as far as Earth’s orbit or even beyond and out towards Mars. Earth’s rocky surface will liquify and start to evaporate, returning all its elements, the material from which we are made, back into interstellar space.

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Whilst a red giant, the Sun’s surface will be significantly stripped by powerful solar winds, forcing the gases out from a few light hours in diameter to eventually a dense cloudy mass two to three light years across. The Sun will be stripped of most of its gases and if anyone is around to see the Sun on that day, and they have a similar language and culture to our own, then they will call it a planetary nebula.

Although most stars you see naked eye in the night sky are intrinsically bigger and brighter than the Sun, in our galaxy most stars are not big enough to end their lives as supernovae. Our Sun, as with around 90% of all other stars in the Milky Way, is a dwarf star and will probably end its life in this fashion. All that remains of the Sun’s core will be one of the densest collections of matter known in the universe, an extremely hot but tiny White Dwarf star with a mass less than 1.4 solar masses. What was once the heart of the Sun, a core hundreds of thousands of kilometers in diameter will be no bigger than planet Earth. The white dwarf will continue to glow for thousands of millions of years but as it gradually cools and fades away it becomes a Black Dwarf, a frozen lump of carbon drifting slowly through space.

As the traditional view suggests, it is thought that all low mass single stars like the Sun, inevitably go through the planetary nebulae stage but this assertion has recently been questioned.

Many stars in the night sky appear as single glimmers of light. However this is not the case. It is estimated that around 60% of all stars in our galaxy are multiple star systems made up of two or more stars bound together by gravity. By studying the dynamics of binary stars and creating models of their behaviour, a growing number of astronomers are appreciating that there might be something wrong with the traditional view about solitary low mass stars and their end-game. Growing evidence suggests that if they are single stars like the Sun, they will almost never become planetary nebulae.

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The first step came in the 1990s when astronomers asked themselves how the interaction between stars contributed to the formation of planetary nebulae. They started developing models of binary systems on the evidence obtained from the forty or so sources where planetary nebulae are known to have binary stars at their centre.

Against previous expectations, the models suggested that planetary nebulae of any size or form is a consequence of binary star partnerships and that unless the mass is sufficient, no single low mass star could ever become a planetary nebulae in itself. As romantic a destination as wished for, it appears that the fate of the Sun is not to become a planetary nebula.

Needless to say, the research in this field is still young and more work on binary stars will be needed before solid conclusions can be drawn. Yet, philosophically speaking, there is coherency in these new insights. We already know that the Sun is not at the centre of the galaxy and that it is not destined to play a major role in seeding the galaxy with heavy elements and yet all older models on the development and creation of planetary nebulae were established on the paradigm of the Solar System, that is, on solitary stars like our own. These new investigations into binary stars are overturning that basic assumption and bringing with it a new era in the understanding of planetary nebulae and the implications this will have for our Solar System.

Chapter 3: A Closer Look at Stellar Evolution & Planetary Nebulae

Reading through this brief sketch of stellar evolution, there are three crucial stages for low mass stars. Leaving to one side the development of the proto-star and the investigations into the role binary systems play, the first crucial stage is the main-sequence, the second is drifting from this into a red giant, and the third is the transformation into a nebula and white dwarf.

In this part of the essay, we shall have a quick look at each of these stages and for that we need to turn our attention to an extremely useful diagram used by astronomers. Indeed, there is little exageration to say that this diagram is the single most important diagram in the field of astronomy on which our entire understanding of stars is based.

The HR Diagram

The diagram is called the Hertzsprung-Russell Diagram or HR Diagram and its role is by analogy equivalent to that played by the Periodic Table of Elements. The Periodic Table is based on observation and shows us how different elements are related. In like manner, the HR diagram is based on observations and shows us how different stars found in the cosmos are related. In either case, theorists would eventually tell us why.  

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Independently working, Hertzsprung and Russell were the first astronomers to plot the colours and absolute brightnesses of stars. The stars’ colour is important because of the everyday experience we have of colour being related to surface temperature. When heating metal, for example, you will notice that it goes from a glowing red hot, to a yellowish hot and then on to an even hotter bluish-white. The same thing happens with stars. Well, almost.

Working out the absolute brightness of a star is more complicated. A star could be bright because it is relatively close to us or because it is very far away but is in itself extremely bright. Astronomers need to be able to compare stars on an equal footing without these distances complicating the issue.

What they did was to imagine what brightness stars would have if they were all 32.6 light years away or exactly 10 parsecs from Earth. Astronomers called this the star’s absolute magnitude which is just another way of saying the star's absolute brightness. If you know the distance to the star, you can then apply the inverse square law of luminosity to determine how the star’s apparent brightness would change if you moved it to the imaginary distance of 32.6 light years away.

When these two parametres are plotted on a graph you end up with the HR Diagram, essentially a colour-magnitude diagram.

At the distance of 10 parsecs, Hertzsprung and Russell found that blue and white stars were always bright whereas orange and red stars were sometimes bright and sometimes quite faint. This was curious. If you are reasonably certain on the star’s distance and the colour of a star depends on its surface temperature, how can two stars with the same temperature and the same distance have different brightnesses? The only conclusion is that some stars are big and others are small, that is, stars with a bigger surface area are going to appear brighter even if they have the same colour-temperature. 

They also found that most stars follow a very simple rule: brighter stars are hotter than fainter stars. If you look at the HR Diagram, you will find that the stars’ absolute magnitude or brightness is measured up the vertical axis and temperature is plotted backwards from right to left on the horizontal axis; hotter stars will be further to the left, cooler stars to the right.

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You’ll also see that around 90% of all the stars appear to run along a band from the top left to the bottom right, from hot and bright to cool and dim. This band is called the main-sequence to which our Sun is included and as we have seen is the band where stars are fusing hydrogen into helium which gives rise to their luminosity and acts as the balancing force against gravity. Exceptions can also be found. To the bottom left, for example, are the small and very hot white dwarfs whilst to the top right are stars which are cooler and brighter which means they must be huge. The most common type of stars within the universe are red dwarfs. These stars are common due to their low mass which means they will live for a very long time before turning into white dwarfs.

Once the HR diagram was plotted and published by Russell in 1911 and Hertzsprung in 1913, Eddington at Cambridge University soon discovered that there was a strong correlation between the mass of a star and its luminosity. Brighter stars were not only hotter but also generally more massive than dimmer stars and there is a reason for this.

The more massive is a star, the more work it has to do to keep itself up. In other words, there needs to be an interior pressure pushing outward which can counter the force of gravity pulling inwards. This pressure comes about by burning a fuel supply more vigorously and so releasing sufficient energy to stave off gravity’s onslaught. As a consequence, bigger stars are not only hotter due to releasing more energy but will live less time for they are burning through their fuel at a faster rate.

The Giant Branch

Once the fusion of hydrogen into helium in the star’s core begins to break down stars move from the main-sequence into the giant branch and for lower mass stars whose destination is to become a Planetary Nebula, they move off the main-sequence and into a new classification known as the Asymptotic Giant Branch (AGB).

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Stars on this branch will all appear like red giants; huge, very cool but extremely luminous stars with considerable solar winds striping the outlayers from them. These red giants will now have an inert oxygen and carbon core no longer able to fuse elements and inturn, no longer able to counter gravity. Around the core will be a shell where the remaining helium is still undergoing fusion to form carbon and another thinner shell where the remaining hydrogen is being fused to make helium. Along with carbon, oxygen, helium and hydrogen, the surface of the red giant will also contain traces of various other gases typically found on main-sequence stars.

Carbon & Oxygen Stars

The colour of stars tells us their temperature and star colour ranges from bright blue along the visible spectrum to red as the stars vary from the hottest to the coolest. Stars on the AGB are very red which tells us that they are not only cool stars but also tend to be very old and very large.

Because of the sheer amount of gas being torn away from the red giant by the solar winds, much observation of these asymptotic giants cannot penetrate the opaque cloud of dust to see what is happening in the core, as such, only the star’s photoshpere can be significantly studied. It has been found that the vast majority of smaller mass stars going through this AGB red giant stage have photospheres rich in oxygen or carbon or a mix between the two elements.

It follows that these AGB stars fit into three broad categories. M-type AGB stars have oxygen-rich atmospheres, C-type have carbon rich atmospheres and S-type have a mix between the two. Since the mid-seventies, it is thought that this is the evolutionary stages of red giants with 1 to 6 solar masses. M-type stars evolve into S-type and they inturn evolve into C-type stars.

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Once a lower mass star converts most if its hydrogen fuel to helium it begins to colapse under the force of gravity. The core of the star is compressed and begins to heat up and the helium is fused to produce oxygen and then carbon. Due to the lower mass of the star, there will never be enough pressure to fuse the carbon into silicon and iron and so the core becomes degenerate, behaving more like a solid than a gaseous state.

During this M, S, C phase, the star is also burning residual hydrogen and helium in two thin shells surrounding the degenerate oxygen and carbon core. As the core heats up under gravitational pressure the star expands but as it does so, it also begins to cool. As the star cools the outer layer of gases become more transparent, thus making it easier for the star’s core to radiate out more energy.

This in turn makes the star’s core contract further under the pressure of gravity. As the gas is compressed, it is heated up again, making it more opaque and more difficult for radiation to escape. This heats the core and gases further, leading the star to expand once again. Thus a cycle of swelling and shrinking is maintained and is the reason why many oxygen and carbon stars are also variable stars. Life expectancy from here on is about 100 million years.

In the final stages of the red giant, when the star is now a pulsating carbon star, currents are bringing some of those elements to the star's outer layers producing a dense carbon dust in the star's atmosphere. As on Earth, dust typically makes sunsets redder by scattering the shorter wavelengths of blue, green and yellow, allowing the redder wavelengths to pass through. This is basically what is going on with carbon stars which make them such a joy to hunt out with an amateur telescope. The shorter, bluer wavelengths are scattered by the gas and dust and the longer, redder wavelengths pass through. Carbon Stars are not only red due to being significantly cooler, older and larger than many other stars but also because the carbon dust in their atmospheres is scattering the blue light to make them look even redder.

Carbon stars are some of the most beautiful objects to view in the night sky and the Astronomical League has made a useful list to work through which I will include below.

The Huge Ever Growing Pulsating Star

There are three major forces now working on the AGB red giant star and all are very important in the production of planetary nebulae.

Radiation pressure in the form of an increased number of photons are flowing out from the star's hot, compressed core, driving the outer layers of gas further from the core itself.

There are intense super fast solar winds reaching as much as 1,500km/s which are not only picking these outer layers up and contributing to the expansion of the cloudy shell, but also raging across the red giant’s surface billowing gases from the photosphere far out into space.

Finally, the continual pulsating of the carbon core is pushing the outer shells of helium and hydrogen gas further away from it. Taking into account the initial helium-flash when the star converted into a red giant could have expulsed up to 30% of the star’s mass and that the three major forces indicate a significant mass loss, one can appreciate that after many millions of years, the core is going to be quite an exposed and raw entity.

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Eventually, this gigantic pulsating process will slow and come to an end. The outer shells are finally peeled and stripped from the star, leaving exposed a white dwarf, a small carbon core some 10,000km across surrounded by a giant cloud of diffused gas one to two light years across composed mainly of carbon, oxygen, nitrogen and silicates.

Along with the slower red giant wind, faster solar winds from the white dwarf - streams of hot, diffuse, magnetized particles - continue to plough into the clouds, compressing and swirling and sweeping them into the intricate and beautiful structures we are able to observe. It also turns out that when something is moving within a medium faster than the speed of sound, a shock wave is created, and the white dwarf solar winds are doing just that.

Hurtling through the whispy envelope of gases at over 5 million kilometres an hour, these solar winds are creating huge shock waves throughout the nebula which can be picked up by x-ray observatories. It is this violent play between shock waves and the slow and fast solar winds from the AGB and white dwarf stage respectively which gives rise to the profusion of exquisite shapes found in planetary nebulae and which render them such sublime objects to observe and study.

Meanwhile, due to the pressure of gravity, the carbon core continues to increase in density and heat, becoming extremely luminous and reaching temperatures of between 25,000k and 30,000k. As it does so, the ultraviolet radiation pouring from the stellar remnent ionizes the atoms of gas in the great cloud surrounding it and this gorgeous shell begins to glow, to fluoresce. It is now visibly a planetary nebula.

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Differences between Emission and Planetary Nebulae

Identifying the differences between a planetary and small emission nebulae, has given rise to misclassification over the yeas. It isn’t just a problem of definition. If we were to define emission nebulae as clouds of ionised gas that emit their own light at optical wavelengths, then it is easy to appreciate where the error may arise for essentially we are also referring to planetary nebulae, but perhaps more troubling is that often emission and planetary nebulae display similar lines on spectroscopic readings.

The surest method in distinguishing between small emission and planetary nebulae is by analysing the specific wavelengths emitted and the recorded spectroscopic emission lines. Typically, emission nebulae will emit wavelengths at specifically 656.28nm in the red part of the visible spectrum. They will also show a preponderance of ionised hydrogen atoms on spectroscopic readings. For this reason and also to avoid confusion, astronomers often refer to emission nebulae as HII regions, for the reason that HII is merely the technical term for ionized hydrogen.

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In the case of planetary nebulae, the excited gas atoms are not typically dominated by HII. Although spectral lines of planetary nebulae will record HII regions, of more significance will be exoctic emission lines of low density gases called forbidden lines, forbidden because they don’t appear on Earth. These consist of extremely rarefied ionised oxygen (OIII), and significant traces of helium (HeII) nitrogen (NII), neon (NeIV) and sulfur (SII). If HII regions typically emit wavelengths at around 656nm and show up as red on the visible spectrum, then by contrast OIII regions emit at around 500nm which will show up in the blue-green area of the spectrum.

Morphology

Of around the 200,000 million stars estimated to exist in our galaxy, about 3,000 of them are known to be planetary nebulae with their greatest concentration near the galactic centre. One can reason that more haven’t been discovered could be due to their extremely short lifespan of around 10,000 years which in astronomical terms is very small, or simply because these faint objects are obscured by interstellar dust.

Ever since the first study of the morphology of PNs in 1918, the variety of different shapes and forms have fascinated both specialists and amateurs alike. Over the past century many attempts have been made toward establishing a standard morphological classification and from the development of CCD imaging and the outstanding photos taken by the Hubble telescope, we know they appear to come in a wide variety of complex forms, but the field is divided as to why.

Some astronomers argue that the variety of shapes we see can be accounted for by being merely having different views of a single, unified, three-dimensional structure. Others claim that the wide range of shapes is the evolutionary sequence of planetary nebulae with variances between them due to the interaction of solar winds. Not surprisingly, neither camp has had its final word, so typically we still see PNs classified on descriptive terms because of the lack of any rigorous models.

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When classifying their morphological appearance the Stanghellini system is often the most used by astronomers. The classification takes the shape of a Roman numeral from I to V:

I: Stellar – appearance like a star

II: Elliptical

III: Bipolar/Butterfly

IV: Symmetric (S-Shape)

V: Irregular

As a note on passing, for an observer’s morphological system more suitable to the visual amateur without the aid of Hubble or high-power CCD technology, please refer to the observer’s section below on observer’s morphology.

As we have seen, the most crucial players in creating the circumstellar envelope is radiation pressure from the star’s core, solar winds and star pulsation. To this list of main protagonists, we may add two more mechanisms which appear to influence the nebulae’s shape.

The first are centrifugal forces around the white dwarf’s equator adding to the clouds’ distinctive knotty densities. Models have shown that gases escaping from the star will either encounter these denser parts and slow down or continue expanding at their regular speed. The second mechanism is the magnetic fields flowing from the star’s poles, distorting the immense clouds of gas and dust as the star spins.

These two mechanisms have been used in simulation and shown to help create the distinctive hemispheres found in many forms of planetary nebulae. To this list, investigators will probably find evidence that the star’s initial mass at birth and its spectral type will also play a crucial role in the nebulae’s distinctive form and shape.

White Dwarfs

Any star on the main sequence is the target upon which a fierce battle is being raged. On one side, we have the brutal force of gravity trying to squash the star and on the other, the merciless force of radiation pressure trying to expand and tear it apart. This battle can last anything from a few million years to many 1000s of millions of years and in a sense the game always has the same outcome, niether side wins.

Gravity and pressure may build to such an extent that the star explodes in supernovae creating elements which are indispensable for the formation of future star, planetary and life systems while leaving behind a blackhole or small neutron star. In the case of smaller mass stars, their outer layers drift into the interstellar medium becoming essential components to future stars, planets and life whilst leaving behind a small, condensed core called a white dwarf. 

Both opposing forces can come away celebrating their victory. They can declare that they both killed the star, not yet appreciating the subtler observation that the star has not died, but like a caterpillar metamorphosising into a butterfly, or a pheonix reborn from ashes, a star has transformed itself into another, more extraordinary entity.

Because atoms are mainly empty space, gravity can work on the core and compress it down to such an extent that what was once millions of kilometers across is now about 10,000km in diameter. A sugar cube of white dwarf stuff is about the weight of an elephant. The only thing preventing it from collapsing any further is the pressure created by the electrons which are so small in comparison with atoms that they still have room to move about at extraordinary speeds. This state of affairs is known as electron degeneracy.

White dwarfs are classified as D type stars, that is, low-mass stars that are no longer undergoing nuclear fusion, slowly cooling and have shrunk to planetary size. Class D stars are then further divided into spectral types depending on the main composition of their atmosphere. Indeed, the atmosphere is still the only part visible to us and as with all atmospheres is thought to contribute to the thermal state of the star. Although white dwarfs are thought to be composed mainly of carbon and oxygen, surface gravity prevents these heavier elements from rising. It is for this reason that about 80% of all white dwarfs are classified as DA types, the letter A denoting an atmosphere rich in hydrogen.

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Due to the activity of the degenerate electrons in their compressed state and the presence of an atmosphere opaque to radiation, white dwarfs are extremely hot places. Surface temperatures are between 8,000 K and 16,000 K, the Sun by comparison is around 6,000k and have core temperatures between 5,000,000 K and 20,000,000 K.

For this reason, although white dwarfs are no longer undergoing nuclear fusion to illuminate their presence, they will continue to shine for 1,000s of millions of years until they become a dense, cold, and black entity of carbon called a black dwarf. However, because the time required for a white dwarf to reach this state is thought to be longer than the current age of the universe, at the moment no black dwarfs are expected to exist.

The Building Blocks of Life

By the time the white dwarf begins to flicker out and fade away, the planetary nebulae which had accompanied it for thousands of years, will be long gone. These objects have an extremely short lifespan of between 10,000 and 20,000 years and due to this ephemeral existence in astronomical terms, possibly one of the reasons why there are so few known planetary nebulae. After a million years or so, the nebulae’s particles will become an indistinguishable part of some giant molecular cloud ready to be recycled into stars again. 

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Planetary nebulae play a crucial part in the recycling of hydrogen, carbon, oxygen and nitrogen, not to mention more exoctic elements such as hydrocarbons, benzine, methane and acetylene, raising important issues into the role planetary nebulae play in producing the elements necessary for life. For sure, we are still not absolutely certain how the step from non-life to life came about, but it is no mystery where the ingredients of life came from to which planetary nebulae play such an important part.

Part II: An Observer’s Guide

Chapter 1: Possible Gear

I’m going to assume you’ve already got your telescope and eyepieces; if not there are plenty of threads offering outstanding advice. In this part of the essay, I will try to offer a little general advice about some gear and observing techniques you may find useful. What I write can already be found in other threads and will no doubt already be known, but for the sake of coherency it's worth going over. Needless to say, these tips are guided by my own experiences and may not be to everyone’s liking.

Stellarium

Stellarium is a planetarium software which can be downloaded for free and is extremely useful for planning sessions, seeing what is about, learning the positions of constellations, distances and sizes of chosen objects, planetary motion through time and much more. If you haven’t got it, download it now.

Star Atlas

It’ll also be a good idea to find a decent star atlas. Out in the field and for planning your sessions, you’ll find it indispensable. They’re not that expensive to buy, they’re pieces of art in themselves and are extremely useful. Quite literally, you’ll be lost without one.

RACI Viewfinder

A 9x50mm, right angle correct image viewfinder will also help when it comes to hunting out deep sky objects. It will deliver stars right down to about 8 magnitude, even if you're in an light polluted (LP) area, meaning you’ll be able to see every star plotted on your sky atlas and when you move amongst those stars, your left is left and your up is up.  As the evening wears on and brain fatigue sets in, this correct-image feature is a huge aid when it comes to moving amongst the stars and following your star map.

Telrad or Rigel

Red-dot finders will also be a big help. These can’t deliver more stars than your eyes alone can see, so if you're in an LP area, you're relatively limited. But, they really do speed up your finding, really do help judge where you are, but I feel it must be used in conjunction with the finder-scope. Whether in decent dark skies or light soaked LP areas, you position the bull’s eye or the other two rings in the proper place against the stars and you’re done. If you're out a little you can work out where you are by either looking through your viewfinder or the three ringed circles of the red-dot finder giving you varying degrees of the sky you're looking at.

Low Magnification Eyepiece

A long focal length, low magnification eyepiece (EP) will be your star-hopping workhorse. The low magnification EP should offer sufficient field-of-view to manage along with your star map, finder-scope and red-dot finder and ought to be able to pick out or hint at what you're hunting. I feel an EP which offers a magnification of around 50x and a little over 1º true field of view will be sufficient for the job.

Observing Chair

Stargazing requires time, patience and comfort. You need the patience to spend time at the eyepiece waiting for those brief moments of atmospheric stillness where everything is perfect and comes into view. To have this kind of patience you need to be comfortable and so you need to be seated. This cannot be stressed enough. If you want to do some serious viewing, do yourself a favour and buy or make an observing chair.

Red Light

To retain night vision you also need a red light but red light in itself doesn’t equate to suitable for night vision. Any light impacts on dark adaptation, but for the minimum impact, a very dim red light is best. I think a decent astronomy light should start out dim with no chance of triggering any kind of white light by mistake and it should be able to go from that dim light to truly dark adapted dim settings. This variable light feature will also be handy when confronted with the usual conditions of a night’s observing: a certain amount of light may be needed to read the fine detail of star charts, another amount might be needed for a midnight collimation check and another amount will be necessary for sketching or looking for something that has been mislaid.

Eye Patch

Many people find it best to keep both eyes open while observing, since squinting strains the working eye. You can cover the off-eye with one hand or you can use a large hat to carefully pull over the non-working eye. Both methods will keep the eyes open and with quick adjustment, you’re back to stereo.

As handy as these two maybe, I prefer using a toy shop eye patch, not the horrid plastic ones, but those made of card and covered with black material.

Modification will be necessary, for a patch on its own doesn't completely cover the eye and so allows for stray light or stray image to enter the eye from below or the side. So, along with your eye patch, you’ll also need a sleeping mask which is padded and is covered with soft material. You cut out one half of the sleeping mask and literally sow it to the patch itself, exaggerating the eye patch's dimple just a little so your eye will be snuggly cushioned within the patch, without any stress to the eye or eye muscles and yet still remaining relatively open. You may also need to cut off the original eye patch's elastic band and fit it with a more durable and thicker one.

You might also find it handy to have a large piece of dark cloth or towel to drape over your head whilst observing. As you check out the star map, for example, or you are planning your star-hopping route, you have your working eye covered with the eye patch and when you prepare to observe, you place the cloth over your head and then move the eye patch to your non-working eye. This may sound a little excessive but you will be retaining your night vision. It seems ridiculous to have spent a lot of money on a telescope, pricey eyepieces, and other astro-related gear, only to be limiting your observing potential by silly stray light. The whole thing, eye patch and cloth will cost you no more than a few €s but will probably turn out to be one of the most useful non-optical bits of kit in your astro-gear.

Planetary Nebulae Filters

A normal colour filter separates the given primary colour from the visual spectrum by blocking out the other parts of the colour spectrum. If you pass white light through a red filter the other colours or wavelengths of the spectrum (say the blue and green) are absorbed and only red light comes out the other side. A red star is red when viewed in white light because all the wavelengths of the spectrum that fall on it are absorbed except red which is reflected to the eye.

The entire visual spectrum runs from a wavelength of around 400nm (blue) to 700nm (red) and is often referred to as the visible band. Colour filters typically cover quite a significant area of this band, so in a very real sense, we can call them wideband filters.

By contrast, a narrowband filter will capture a smaller, narrower, more specific part of the visible spectrum, so these filters will have a narrow bandpass. If a colour filter has a bandpass of around 100nm, a narrowband filter may have a bandpass as little as just 3-5nm

As we have seen, there is a class of celestial objects which can be loosely referred to as Emission nebulae, from the fact that they are emitting their own light. Emission nebulae are all potential targets for narrowband, so although supernovae remnants, emission nebulae and planetary nebulae, celestial wonders like the Ring Nebula, Dumbbell Nebula, Veil Nebula, Crab Nebula, Orion Nebula, Lagoon Nebula, and the Swan Nebula, are very different entities, representing very different phenomenon, for the purposes of observation, we can consider them all Emission Nebulae.

What they have in common is that they are the seeds from which new stars, planets and life will arise and are composed of gases which emit light. From the field of quantum mechanics and the work of Bohr et al, we now understand that the atoms within the gas are being excited by the energy emitted from nearby stars, be this from stars forming within the nebula such as in the Orion Nebula, or by the white dwarf remnant in a planetary nebula. When excited the atom emits a specific wavelength of light, a distinct emission which can be identified via spectroscopy as corresponding to a very specific element.

It turns out that the two most common elements found on emission lines (Huggins’ mysterious nebulium) are oxygen and hydrogen which are emitting light at very specific wavelengths on the visible spectrum’s band.

For this reason, you will find the most useful planetary nebulae filters are those that pass through wavelengths to your eye at around 500nm for oxygen III (OIII filter) and either 656nm for hydrogen-alpha (Ha filter) or 486nm for hydrogen-beta (Hb filter). The OIII emission in the blue-green part of the spectrum is the dominant emission from planetary nebulae, so should be the most useful filter to own. The Ha and Hb filter will be significantly more limited in their use, for the simple reason that there is lower emission in these lines from planetary nebulae. Nevertheless, Ha is the most dominant emission line in star-forming regions such as the Orion Nebula, so it might be a little more versatile than the Hb filter.

Ultra High Contrast (UHC) filters are also narrowband filters but allows a wider bandpass of light to pass through, something between 486 and 510 nm. As such, although not as specialized as the OIII filter, they do cover the main OIII emission from planetary nebulae.

My personal opinion is that if you’re into observing planetary nebulae, the OIII filter is the one to purchase. It can take the dimmest planetary nebulae and make them come alive. However, I have found that it does need aperture. This is not to say that it cannot be used with a small 4” scope, for example, but the views of any planetary through the OIII will be better with a bigger telescope. The UHC filter has the advantage of working on the largest number of emission nebulae per se and although not as specialized as the OIII can be used in smaller scopes to great effect.

Chapter 2: Observing Practices

When looking at objects on Earth, a microscope or telescope’s main function is to magnify the object. When stepping from Earth into the Solar System, a telescope helps collect light and magnify the distant detail on the Moon and planets. But generally speaking, a telescope looking at deep space objects such as planetary nebulae has a different role to play.

Reaching out to planetary nebulae, hundreds, if not thousands of light years away, the principle function of a telescope isn’t to magnify the object but to collect light from it. When peering into deep space, you are not using a telescope to see objects because they are too small, although they might be, but rather because they are so dim, you need light.   

As a species, we have evolved to see things under the bright light of the Sun and when there is no Sun, we have developed technologies to take its place. To view objects in the dark with a telescope requires a different game plan and a whole range of new techniques to be learnt. This chapter will try to share some insights into those techniques which will probably already be known, but once again, worth going over.

Light Pollution

The single worst enemy for stargazing is light pollution. It wraps itself around the skies and the delicate diamond glint from stars and the luminous glow from deep sky objects are smothered and strangled. A small telescope in the countryside will show planetary nebulae better than a light bucket in the city. In effect, darker the night skies, brighter the deep sky objects

Most of us have to deal with light pollution in some manner or other, and it is possible to do a fair bit of observing even under city skies. The planet and Moon and double stars are not affected and with diligence, you could probably work through most of Messier’s list. With this said, if you’re into your deep sky objects, I feel it is still necessary to plan a number of dark-sky trips during the year. There is nothing quite like a peaceful and clear night where the stars are like fists of silver dust pitched in the wind and those hidden cosmic clouds can at last reveal their mysteries. 

If it isn’t possible to visit dark sites from time to time, don’t beat yourself up about it, but you are robbing yourself of detail and beauty that might have been possible. However, observing in light polluted areas can be extremely useful and shouldn't be used as an excuse not to observe. Not only do you continue to hone your star-hoping skills but you can still make great observations and finds and later compare them with your dark site ones. This is an interesting project in itself, for you'll see the effects LP has on your eye and the given object.

Whether in the city, suburbs or countryside, you will find that the observer’s zenith is where the sky will be darker, so try to plan your sessions accordingly. It’s also a bit of a truism that when folk stop hustling and bustling and start going to bed, light pollution tends to improve. It might also be an idea to note the afternoon skies an hour or two before sunset. The deeper blue they appear, better the chances of a decent dark night. Needless to say, the delightful sight of strong Moonlight is a natural light pollution and nature’s way of telling us to stick to observing double stars, planets and the Moon itself.

Dark Adaptation

It was briefly alluded to above with the mention of red torches and pirate’s eye patches, but you’ll also need to think about adapting your eyes to the dark. The human eye takes time to adjust. After half an hour or so, you will start to notice a significant difference, after an hour, you should be pretty much fully adapted.

If you have planned an evening of planetary nebulae observing, or other deep space objects, try not to observe any bright celestial object such as the Moon or planets with your working eye and throughout the session, it is probably a good idea to keep it covered when not observing. Hence the usefulness of an eye patch. If you’re waiting for the telescope to cool, start out with open clusters, star fields, perhaps some doubles. After half an hour or so, you can start the deep sky observing session with confidence.

Averted Vision

As curious as it may sound, looking directly at planetary nebulae or other deep space objects might not be the best method for observing them. When you look directly at something, the image falls on your retina's fovea at the centre of your eye and is extremely good for picking out detail with the aid of good illumination but is relatively blind to dim light. If you look directly at dim planetary nebulae, for example, they may just disappear. As such, try to train yourself to not look directly at the object. By doing so, you are effectively moving the image away from the fovea and onto parts of the retina more sensitive to black, white and grey.

The technique doesn’t take long to master, but will probably seem odd at first. We have evolved to look at things directly, so averted vision is a little counter intuitive. Practice by centering a dim star or DSO in the centre of the eyepiece’s field of view and concentrate your attention on an area just a little off to one side or above. Alternatively, place the object a little to the side or below the centre of vision. Either method should work, but finding your sweet spot will be a matter of trial and error.

Don’t Squint, Jiggle and Breath

Again, try not to squint when observing celestial objects, for by doing so you are not only straining your working eye which can lead to fatigue but also limiting your powers to detect faint objects and tweak detail from them. The solution is to observe with both eyes open, and if it helps, to either cup the non-working eye with the palm of your hand or to use a modified eye patch.

There may also be occasions where you are certain you have the specific area where you think the planetary nebulae or deep space object resides but it appears to be lurking under the limit of visibility. If this is the case, tap the telescope or eyepiece just a little to make the field of view jiggle. It’s not guaranteed but you might find the object revealing itself.

Again, when you are concentrating hard you might find yourself holding your breath without realizing it. Limiting the oxygen to your brain, even for just a few seconds, compromises night vision. So while observing its good practice to breathe steadily and deeply but in a calm and relaxed fashion.

It’s also worth pointing out that night vision is impaired by alcohol, nicotine, and low blood sugar, so try not to smoke or drink too much or go hungry. If you do smoke, when you light up make sure your working eye is completely covered and well protected. You might find you’ll need more than an eye patch to prevent night vision from being damaged by the lighter’s sudden flare up and glare.

Aperture and High Power

When it comes to viewing planetary nebulae and other deep sky objects, aperture rules. As explained in the introduction to this section, the more light gathering capacity your telescope has, the more you are likely to see. Obviously big telescopes have their own disadvantages – transportability, convenience and ease of set up, cool down time, collimation – to name a few, but if you do have a number of scopes, use the biggest one you have for hunting out planetary nebulae.

You’ll recall that in the chapter on possible gear, I mentioned a low power eyepiece (EP) which would be your working horse EP for hunting out planetary nebulae. This advice is based not only on personal experience but also from the fact that lower powers tend to concentrate the object’s light into a smaller area and thus increasing its apparent surface brightness. I feel this advice is sound, but high powers must also be used when it comes to viewing planetary nebulae and other deep space objects in general. As such, try a wide range of powers on any object and don’t limit yourself, for the best magnification is the one that shows you the most amount of detail.

Sketching and Log Book

There’s no correct way for enjoying a stargazing session. Some nights it's a good idea to just run around the heavens, other times to pick an object and tweak as much detail as you can until boredom sets in; there is no right or wrong way.

 But if you want to keep a log book or try your hand at sketching, it is better to be a visitor, rather than a tourist. Many people will go to a museum, for example, and will rush all the paintings, but at the end of the day, they will only remember one or two of them at best and not that well either and by analogy, the same can be used when it comes to stargazing.

As said, there is no right or wrong way to plan your sessions and the following procedure is certainly not one I'd recommend all the time, but I do think it is important to slow down from time to time. I appreciate that this slowing-down exercise can be frustrating at times. In my case, a three hour dark site session involves an operation of around 6 to 7 hours. After all that preparation, packing gear, travelling, setting up and so on, when I get out to the dark site my first instinct is to start buzzing all over the skies, but if a sketch is planned or you really want to get down to details, at sometime or other you will need to give yourself time. 

There are two essential features to visual astronomy. The first is to find the object and the second is to observe it. The former process involves star-hopping and reading star maps, the latter requires you to slow down and to engage yourself with the complexity and beauty of what is being observed. It's been said many times before but anything glanced at will always look like a featureless something or other but the trick is to go beyond this style of looking and practice picking out features and textures.

I find the best way of observing planetary nebulae is to place it in a low power EP and to begin asking some basic questions about it. These questions could include stuff like:

What am I looking at?

What have I read or seen of this object that can inform my observation?

What shape does it seem to have?

How many stars can I see in my field of view? What colour are they? Are there differing intensities of brightness between them?

How does the object's appearance change as I flip from direct to averted vision and back again?

Can the white dwarf be resolved in the object itself?

When I move away from the eyepiece to relax my eye, what can I recall about this object that can inform me further? Was there anything distinct about it that struck me?

I'll then up the magnification again, to something around 90x. And ask a few more questions. Stuff like:

How has the image changed?

How many stars have now been cut away?

Has anything in the image become dimmer or lighter?

Has the object itself changed in any fashion?

Are there any new patterns, shapes or colours to be seen within it?

Are given areas of my new field of view more pronounced than others?

If I close and relax my eye away from the eyepiece can I picture the object there 'within'?

I'll then up the magnification to the optimum power, that which gives the sharpest and clearest image at the highest magnification possible and go through the same questions as above.

In all cases the trick is to ask yourself questions about the object; to not only appreciate the subtle detail and complexity therein but also to have a general picture of how the object is framed within your eyepieces and mind.

If you are going to use a filter which will often be necessary for planetary nebulae, you should go through the same questioning and magnification process as above. The object will look different and you may prefer the look with or without the filter but it's nice to understand why this is so and to appreciate any improvements in the image but also any detrimental affects a filter may have.

Now, all this may look a little long winded and perhaps it is, but I feel it'll pay out dividends. With practice, depending on the complexity of the planetary, such an interrogation shouldn't take more than 15 minutes or so. The questioning process can be exhausting, so I usually dedicate it to one or two objects at most and not on every session. It is also extremely important to have sessions where we just sit back and drink in the beauty around us without eyepiece and telescope and without thought or mind, or to plan other projects like splitting doubles, viewing planets, completing lists, and so on.

If you are going to sketch or record your observation, you are now armed with a better understanding and visual appreciation of the celestial object.

If you’re keeping a log book, try to include the following information:

Object

Date and Time

Location

Observing Conditions (seeing, transparency, temperature, wind etc)

Moon Status (rise, setting time, phase)

Telescope Used (focal ratio and aperture)

Eyepiece (magnification, field of view)

Filters

Detailed description of the object

If you want sketch the object, draw yourself a decent sized circle and start by drawing the field stars paying attention to their spacing relative to each other and their relative brightness. It isn't necessary to draw all the stars you see but the ones you do, ought to serve as a guide for the sketch of the object itself.

Sketching is an iterative, mechanical process: you look through the eyepiece, you sketch a little something, you compare, you look again, sketch a little more, compare, and on and on you go. If you find you are getting bored, relax, take a break, and when you are ready, return again.

The point of sketching is not to create some beautiful rendering of the object itself, but rather to train your eye to see better. You keep going back and forth from eyepiece to paper until you feel you have either had enough for the night or that your drawing contains most of the details you have seen. Whatever the tools or techniques used, when sketching be sure to use a dim red light and try to be as comfortable as possible.

Observer’s Morphology

As we have seen, it is widely accepted that the various shell structures of PNs can be explained by the interaction of a fast wind from the central white dwarf and the remnant of a slower wind from the AGB stage of the dying star. Whether these various PN shells are examples of the very same structure viewed from different angles, or are cases of the evolutionary stage of PNs, is still open to debate.

By way of illustration, we can note that the Ring Nebula, M57 and the Helix Nebulae, NGC 7293 are both understood as being seen from where we are looking down onto their poles and as such, the two hemispheres merge to make a ring shape. If seen side on or from the equator, we would see an hourglass shape. One side of the debate contends that this is because all planetary nebulae are formed in like manner and have essentially the same structure, whilst the other side argues that this given stage is part of the evolutionary process of the PN as it slowly disappears into space.

Regardless of the outcome, for the visual observer, PNs are some of the most interesting and beautiful objects in the night sky. A genre full of variation and complex shapes, where morphological classification based on descriptive terms will hold sway based on the limitations of the telescope and optical gear and the conditions under which the observer is viewing.

Although it is certainly not necessary to use, perhaps the most common classification system used by the visual observer is the Vorontsov-Velyaminov Scale of Planetary Nebulae, first detailed in 1934. His categories are:

  1. Stellar image (like a star)
  2. Smooth disk (or regular disk) (a) brighter toward center, b ) uniform brightness c) traces of ring structure
  3. Irregular disk (shape not entirely circular) (a) with irregular brightness distribution (varying light and dark areas) b ) with traces of ring structure
  4. Ring structure
  5. Similar to a diffuse nebula
  6. Anomalous form (abnormal form without a regular structure, perhaps like a S or 8)

More complex PNs are characterised by combinations of classes. Accordingly, a PN like M 27 could be described as 3a+4; M 57 would be 3+4, M 76 something like 3+6 and M 97 3a.

Final Thoughts

When planning a session it is a good idea to view sketches by other observers. These are too often overlooked, but they ought to be viewed from time to time. They are typically produced by patient and attentive observers and their drawings should give you an idea of what the object will more or less look like if you were to use similar aperture and magnification

There are so many good books about it's hard to pick any one of them and say, this is the best. There are those which give context and depth to what is being viewed, others a more practical working guide. I feel this post will give you sufficient grounding in planetary nebulae and with the power of the internet and forums like S.G.L, you should be able to hone your enquiry if the need arises.

There are some little jumping tricks you can learn to find yourself about the night sky. For example, find the plough in Ursa Major and look for Merek and Dubhe, the distance and angle between these two is one step. Now count that distance, in that direction another 5 steps and bingo, you'll be with Polaris. Now go back to the Plough and find its end star, Alkaid. Take a jump and dive from it and the next brightest star will be Arcturus - the arc to Arcturus. Learning the big stars and diving quickly between them makes hunting stuff easier.

Without doubt, one of the best guides you’ll find on star-hopping is the guide written Shane Farrell and can be found in the observing section to S.G.L.

My final thought is that if you can master patience you'll be master of yourself and the night sky is a good teacher. She'll teach patience and careful watchfulness; she'll teach industry and care and above all, the night sky teaches trust. Those stars and DSOs are not going anywhere quick. They won't desert you and they're not trying to deceive you. If you don't succeed one night, or you can’t go out for weeks on end due to personal compromises or those set up by clouds and rain, don't be down hearted. In most cases, you've probably discovered something new about yourself and those stars and DSOs will be back to give you another chance, another day.

Stargazing can be a tiresome road and one can suffer for it and be grieved, but the worst you can do is add to this frustration and curse those things beyond your control. Cloudy or uneventful evenings are just that, nothing more and when older they will appear as a singular, non-descript events, yet shining from them like a host of gleaming stars will be those evenings where everything just seemed perfect and the universe at last could murmur to us its secrets.

Useful Bits

Carbon Stars: http://www.astroleague.org/files/obsclubs/CarbonStar/CarbonStar-List.pdf

Planetary Nebulae: http://www.webbdeepsky.com/downloads/The%20brightest%20planetary%20nebulae%20%28white%29%202nd%20ed.pdf (thank you to Mark at Beaufort for pointing this out).

Some of my favourites: M27, M 76, NGC 1535, NGC 2392, NGC 2438, NGC 3242, NGC 6543, NGC 6905, NGC 6826, NGC 7009, NGC 7662.

References Include: Planetary Nebulae, Martin Griffiths. Stardust, John Gribbin. The Hundred Greatest Stars, James Kaler.

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Qualia,

I am a little lost for words after reading these few short notes you have put together, can I just say excellent!

Alan

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Should it not be in Observing - Deep Sky not Observing - Planetary however ?

They are after all nothing to do with planets.

Actually Physics, Space Science and Theories would seem to be more appropriate.

Edited by ronin
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Absolutely Fantastic. Congratulations on a very engaging, entertaining and very informative piece of work. It is obvious how much work you have put into this. Well done.

Ian

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I will have to come back and read through this properly when I have the time it deserves.  Nice to see you back, Qualia :)

James

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Welcome back, excellent post and very informative as always.

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Thank you for your kind posts. I great to be back and I hope the post goes some way in making up for my absence :icon_salut: .

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Absorbing and informative read, well researched and great presentation. A terrific way to step back into the forum Qualia

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Back in a big way then Rob. quite a post you've made and as always, a pleasure to read. Hope things settle for you and yours quickly

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Lovely to see you back sir ...  :laugh:

And what a way to come back ...

Note to the Powers that be ...

If it hasn't already happened , this thread needs pinning somewhere prominent so it can be easily found and revisited ...  :cool:

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Note to the Powers that be ...

If it hasn't already happened , this thread needs pinning somewhere prominent so it can be easily found and revisited ...  :cool:

Already done Steve, This is a sticky in the observing deep sky section.

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Greetings


 


What a beautiful read, it's not very often I stay in my little room to read from the computer screen but this was too good to leave till later.


Thank you for giving my kids an excuse to get their own back for me telling them to switch their laptops off and do something use full.


 


Andy( bad daddy for playing on his computer when he could be doing something better )


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I am eager to read this post, but need to set aside some quiet time, but at a glance it looks to Qualia back and right on form

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Thank you all for your kind words and support. When I knew things were going bad at work and that eventually I would lose my job, I closed down 'superfluous' bills which might have trapped me in any untoward debt or borrowing. One of the decisions was to cut the telephone line.

Whilst all that was going on, I still wanted to be a part of SGL and I figured if this wasn't possible in person, it might be at least in spirit. During the summer months I thought it would be a nice idea to swot and write up on planetary nebulae, some of the sessions I've had with a microscope, and to keep an observing journal to share with you guys when I returned. What I didn't realise was just how long I was to remain off line.   

Anyway, I am sorry for the delay and I hope this entry will be of some use and that perhaps something new is learnt.

Thank you all again,

Rob

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Rob a wonderful piece of work - its a great read. Its great to have you back - I hope things will now settle down for you and your partner.

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