Hydrogen Beta (Hβ) and the
Line Emissions from Planetary Nebulae

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 openly felt on exposed skin to the warm Sun, whose energies are roughly 1,368 Watts per square metre or 1.368mW.m-2. This knowledge is quite useful, for example, in charging electric cells in generating electrical power. Such energies are intense enough to permanently damage the retina of the eye in less than 10,000th of a second when using any optical aid! This abundant energy is across all wavelengths but is much smaller for the Hβ output alone.

For all planetaries, and stars, these quantities are very very tiny, mainly because of the tremendous distances of these objects. Although the output of planetaries and stars is simply stupendous, the energy per square metre of surface decreases by the square of the distance. For an example of the PNe NGC 4071s, the measured flux is minuscule at 2.239×10-12 mW.-2, some 1018 times fainter than observed 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. I.e. 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 1.626×10-10 mW.m-2 seventy-two times more intense than the surface area of NGC 4071. An example of low Hβ fluxes is planetary nebula He2-77 / Sa3-16 / PK 298-0.1 / PN G298.1-07 (12091-6307) at log −13.28mW.m-2. [He2-77 appears in south western Crux, being 26 arcsec across and 17.6 magnitude. It remains impossible to see in any amateur telescope.]

Calibration of the absolute Hβ fluxes are made by the examination of several standard stars. I.e. 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 strengths of the emission lines of other PNe, allowing 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;

Hβ=100, He(II)=63, O-III= -, O-III=786, He I=-, Hα=294, N(II)=259, S(II)=30 and S(II)=24

For bright PNe NGC 3918, the values from the Strasbourg-ESO Planetary Nebulae Catalogue are;

Hβ=100, He(II)=16, O-III= 15, O-III=1667, He I=15, Hα=374, N(II)=259, S(II)=6 and S(II)=10

Amateurs should be aware with the important ratios of [O-III/Hβ], [O-III/Hα] and, as they tell much about of the likely telescopic appearance of the PNe and the effectiveness of employing [O-III] filters, both photographically and visually. The experienced French amateur observer Yann Pothier has also used this same ratio between the Hydrogen Beta (Hβ) with much success. 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;

n = [O-III] & 496nm+501nm] / [Hβ, 486nm]

Where ; n = PNe Effectiveness Ratio

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, but is likely quite small in value.)

Example 2 : NGC 3918 O-III is [1667+15]/100 = 1682/100 = 16.8 or 17
(Typical with the 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 skewed mean being about 13.0.

References and Abbreviation Used in Text

  1. Burnham, R., Burnhams Celestial Handbook : Volume 1 (BCH1)
  2. Burnham, R., Burnhams Celestial Handbook : Volume 2 (BCH2)
  3. Burnham, R., Burnhams 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)
  6. 6. Malin, D. and Frew, D., Astronomical Objects for Southern Telescopes : 2 (1994) (AOST2)


Last Update : 27th November 2012

Southern Astronomical Delights © (2012)

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