It has long been known that most PNe can be calibrated by measuring the single line strength of Hβ. This value is measured photoelectrically, and is the amount of energy (flux) liberated from the PNe structure or even the PNN itself. Hβ is expresses in mW.m-2 and abbreviated F (Hβ) - the absolute Hβ flux. It is not easy to imagine this quantity. For example, the Sun produces energy that can be felt by exposure of skin to the warm Sun. Energies here are ~1 368 Watts per square metre or 1.368mW.m-2, and useful, for example, in charging electric cells to generate electrical power. Such energies are quite intense and enough to permanently damage the retina of the eye in less than 10 000th of a second! This energy is for all wavelengths but is much smaller for the Hβ output alone.
For planetaries (and stars), these quantities are very very tiny. For example, NGC 4071’s measured flux is 2.239x10-12 mW.-2, some 1018 times fainter than solar output! To avoid having to write down so many small numbers, values for the absolute fluxes are given as the log of the measured flux. Ie. log -11.66±0.02mW.m.-2. Typical values for these indices compared to all PNe may range anywhere from -9 to -16 with the midrange of the distribution being about -11.5
Among the southern planetary, NGC 3242 has the highest Hβ flux of -9.789 or 1.626x10-10 mW.m-2 - seventy-two times more intense than NGC 4071’s surface area. An example of a low Hβ flux PNe is He2-77 / Sa3-16 / PK298-0.1/ PNG298.1-07 (12091-6307) at log -13.28mW.m-2.
Note: This PNe appears in south western Crux, and being 17.6 magnitude and is 26 arc seconds across, and remains impossible to see in any amateur telescope.
Calibration of the absolute Hβ fluxes are made by examination of several “standard stars”. Ie. β3 Orionis and α Lyrae (Vega), or the bright PNe. For southern planetaries, NGC 3242 and IC 418 are used as Hβ ‘standard candles’.
In other terms, the relative Hβ fluxes are used to compare the strengths of the other emission lines seen in the PNe, and allow differences between other PNe to be quickly examined. This is made by giving the absolute Hβ flux the value of exactly 100.0. From this other prominent PNe lines can be assessed. For example, the values for NGC 4071 (From the Strasbourg-ESO Planetary Nebulae Catalogue) are;
For the bright PNe NGC 3918, the values (From the Strasbourg-ESO Planetary Nebulae Catalogue) are;
Amateurs should be concerned with the important ratios of [O-III/Hβ], [O-III/Hα] and, as they tell much about of the telescope appearance of the PNe and the effectiveness of the O-III filter, both photographically and visually.
The experience French amateur observer Yann Pothier uses the ratio between the Hydrogen Beta (Hβ). By combining the two lines from the two O-III emissions, this gives some rough indication of the effectiveness of the O-III filter on the object. This is calculated by;
Example 1 : NGC 4071 O-III is (786 + 0), Hβ is 100. Therefore, N = 7.86, rounded to 8.
(Note: O-III at 486nm was not measured accurately. However, it is small in value.)
Example 2 : NGC 3918 O-III is [1667+15]/100= 1682/100 = 16.8 or 17
(Typical of a moderately high excitation PNe.)
Correlation with known PNe finds that the smaller this value the lower the excitation of the PNe nebulosity. Values can range anywhere from 0.1 to 180, with the mean being about 13.0.
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2. Burnham, R., “Burnham’s Celestial Handbook : Volume 2 ” (BCH2)
3. Burnham, R., “Burnham’s Celestial Handbook : Volume 3 ” (BCH3)
4. Dreyer, J.L.E., “Second Index Catalogue of Nebulae and Clusters of Stars; Containing Objects Found Between 1895 and 1907.”; Memoirs of the R.A.S., Vol. LIX- Part II (1908) (IC)
5. Hartung, E.J. , “Astronomical Objects for Southern Telescopes : 1 ” (1968) (AOST1)
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