Spectroscopic observation of planetary nebulae

2018 March 22

Paul Luckas

Introduction
Planetary nebulae present interesting targets for small telescope spectroscopy for a number of reasons. For the relative newcomer, their observation presents a logical ‘next step’ in commissioning a new spectrograph, or progressing beyond the basics of stellar classification. Displaying prominent emission lines of ionised hydrogen and doubly ionised oxygen (common features of emission nebulae), ‘planetaries’ provide excellent spectroscopic examples of low density gas ionisation by a very hot source, a recurring theme in much of stellar astrophysics. As such, they vividly demonstrate the second of Kirchhoff’s laws of spectroscopy.

Gustav Kirchhoff, a mid-19th century German physicist, was the first to conceptualise the three observable phenomena of continuous, emission and absorption spectra.  In emission spectra, low pressure gas which has been excited produces emissions at specific wavelengths according to the elements present in the gas. In a neon lamp, the source is an electric current. In planetary nebulae, the source of this ‘excitation’ is an extremely hot central star, a white dwarf with surface temperatures exceeding 30,000 K. Planetary nebulae occupy a key stage in the evolution of low- and intermediate-mass main sequence stars beyond the red giant phase. Our Sun is one such star, and will quite possibly produce a planetary nebula in its retirement some 4 to 5 billion years from now.

One of the first astronomers to investigate planetary nebulae spectroscopically was William Huggins in the 19th century. Huggins discovered that these objects were gaseous, rather than stellar in nature, through the observation of bright emission lines rather than the continuous absorption spectra found in stars at the time. Since then, planetary nebulae have been the subject of ongoing research, much of it focused on the nature of their central stars and on atomic morphologies and motions across various points in their extended nebulae. There is little doubt that the spectroscopic investigation of planetary nebulae presents a viable and continuing area of study for amateur astronomers.

This paper provides examples of both low- and high-resolution spectra, identifying and measuring some basic characteristics of these fascinating targets. The telescope used is a relatively modest-sized 0.35m Ritchey–Chrétien (RC) design, situated in a low altitude urban environment under the light polluted skies of Perth, Western Australia.  (continued)

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