COLOUR IN PLANETARY NEBULAE

I N T R O D U C T I O N

One of the most interesting aspects of planetary nebulae is in observing distinct colours with their disks. The only other objects which show significant colours are double stars, which have the notoriety of having a large colour range among individuals and suffer various proximity effects that may influence each of the component’ colours. Ie. Beta Cygni (Alberio) In observations made at the South Pacific Star Party in 1999 using a 25cm LX-200 with fast slewing, it was noticed that quick comparison between individual planetaries were quite distinct and were easily comparable.

Planetaries have significant colour variation which is exhibited by three principal emission wavelengths - Hβ (486.1nm), O-III (500.7nm and 495.9nm) and Hα at 656.3nm., corresponding to blue, blue-green and red, respectively. These three wavelengths contribute more than 90% of the visible light. As our eyes are inherently poor in sensitivity in the red end of the spectrum, most of the Hα light is almost totally invisible to us at night. Consequently, most nebulous astronomical objects appear whitish or greenish. Additionally, many planetary nebulae have most their visible light revealed in there O-III radiation, which our eyes are sensitive. Thus many planetaries appear either bluish or greenish in colour, and their variations are likely the combination of the three wavelength intensities and other wavelength. Colours of planetaries can be made by spectroscopy and this remains the only scientific method of evaluation. Such important information is normally used for understanding the evolution and chemical composition of these all planetaries.

The following article is a method of colour estimation at the eyepiece of the telescope.

THE NEED FOR COLOUR OBSERVATIONS

At the telescope most colour that is perceived by the observer is relatively poor. This is a physiological problem with the human eye because the onset of darkness causes the apparent loss of colour vision. The mechanism of why this happens lies in the so-called “rod” and “cones” within the human retina. Rods essentially measure the intensity of light in the eye (greyness) and respond very little to colour. As light intensities vary so much, ranging from full sunlight to the near pitch-black of night, the need for such a mechanism is obvious. As a consequence the “rods” work regardless of the intensity of light. Cones are the colour receptors, and as their names suggest, have their diameters reduce to a point. With enough illumination, the wavelengths coming into to eye are separated in to their component colour, which are sent along the optic nerve of the brain and interpreted as colour. Details on how the eye does this in unnecessary to understanding for the night-time observer as much of the colour is lost to our eyes during the night. In general summary, although the cones have a threshold to colour sensitivity, and below particular energies will almost completely ceases to function. As a consequence, when we look around at our surrounds at night, we normally only see the range of “greyness”.

When we look through any telescope, we are just exposed to the illumination of the stars in the field and the astronomical object in question. Most of the stars appear white or whitish in colour, but in some circumstances like a very blue or red star, we see some colour. These colours show very few colour hues, and this is because the component known as saturation is fairly weak. Saturation can be described as degree of whiteness in any perceived colour - so in astronomical objects these produce fairly pale colours and never intense ones. It is unusual to see colour with more than 10% saturation at night, and although this is fairly arbitrary, experience finds that more intense colours than this simply do not exist.

The need for estimating colour in a telescope is likely not necessary for the observer, but for those engaged in writing astronomical descriptions, such reporting is both interesting and important to advise and guide other deep-sky observers. For double stars such methodologies are already established using scales like the Hagen Colour Index (HCI). The scale in the HCI is between -3 and 10, describing the range of double star colours from Blue to white to yellow to orange to red, and roughly mimics the range seen in astronomical spectra and between the spectral classes. However, the problem for double stars using this index, is the inherent weakness is that the scale doesn&8217;t differentiate between different saturations. Although this scale is quite arbitrary between observers, as different eyes will see different colours, leaving a range of possibilities. There is also possible a large number of different colours. Moreover, it is also difficult to ascertain as the stars are point sources which can be partly solved by simply de-focussing the stars into disks.

For planetaries the choices of colour are less than double stars as there are two main colours - blue and green, mixed with a degree of greyness. If the saturation is less than 10%, then the range of possible colours is greatly diminished. No real scale exists for describing colours of planetaries, and with the availability of larger telescopes like Dobsonians, which reveal more colour, such a scale would be advantageous. One possible empirical method is the “Triangular PNe Colour Scale”, as mentioned in this text. Although this is only one possibility of several methods, it is probably the easy to use and describe.

The TRIANGULAR PNe COLOUR SCALE

The following is a method of colour determination in PNe. The best method would be to print the physical copy and distribute it to observers, however, the logistic to do this are too difficult. With the availability of computers and colour printers, and the software available to do this, it is easier to do this at home yourself.

Note: I will assume that the observer has access to a computer and colour printer, and is aware of how to set the colour balance of either the printer and/or the computer monitor. Considering the variations between eyes and minor differences should not cause too many problems.

I have used the RGB and CMYK colour methods combined with a standard Grey Scale. These were used to produce the range of colours and hues. I have based the calculations on 10% Saturation, and it is unlikely that any observer can see more stronger colours. The scale is arranged in twenty-one base colours formed into a triangle. Colour values are based on the amount of white or grey blended into the colour. Thus, each line in the triangle being the degree of blue, green and grey.

Calculations to organise the selected colours are not simple, as the relationships between the colour and saturation are not linear. In simple terms, the grey scale “Grey&8221; in the table below are integrated into the selected individual colours, and looking at the groups across the triangle show the increasing greyness from left to right.

Each colour value is smoothed by the surrounding colours within the triangle AND then reduced to the similar “mean” saturation.

Analysis of this produced the average saturation of 10.2±0.9%, and this final result was made using rough linear approximations of the R, G and B values combined into one single value. An average of each different colour in the triangle producing the final "mean" saturation (Note: Each of these relationships are not linear)

C O M M E N T S

Colour values are only whole numbers, some degree of rounding was required. This should have little have effect for the PNe observer.
The most problematic colour is "Green" No.3, which has a more "yellow" than it should be, but the colours found using the colour equations did not look quite right. Observers should not notice this difference too much.
The last line in the Triangle gives the "Colourless" Value, so no visible colour can be given. Its value is zero.
The "colours" Nos. 22 to 27 are degrees of grey and have no added colours.
ALL DESCRIPTORS really need some discussion and some consensus to decide their particular “proper” names.

All descriptive colours here are only a "Guide" for observers.

GENERAL INFORMATION ABOUT TABLE 1

Table 1 below shows the colours need to reproduce Figure 1. Adjustments of the Printer and/or Screen need to be determined by the user. Values for the RGB, CMYK and Grey are given. I have basically used the RGB scale in the calculations because the values in low saturation CMYK are more inaccurate. The “Grey” column shows degree of “greyness” in the colours, which underlies the reasonable colours produced in the “triangular” Colour Scale.

Table 1

The "TRIANGULAR" PNE COLOUR SCALE ("STANDARDS")
Colour Saturation = 10.2±0.2%

COLOUR COMPARISON CHARTS

ENDNOTES

This text is a fairly rough draft and I would like to get some feedback on the soundness and usefulness of the method before fully explaining the methodology.

It is possible to expand this "Triangle Scale", for saturations at 20%, leaving stronger colours if anyone things the degree of blue or green is not sufficient.

The scale can be expanded for other colours than blue and green. Another alternative would be a second triangle, below the grey scale on the last line, which displays the colours of red and yellow. As all PNe are not this colour it is likely unnecessary, however, it would be good for completeness, and perhaps useful for double stars.

Last Update : 22 October 2005

Southern Astronomical Delights © (2005)

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