Nebulae and Clusters Section
History, Objects, Catalogues and Observations
Nebulae and Clusters sounds like the very historic
core of all Deep-Sky activities. We recall this term
from the titles of the great works of Herschel, Lord
Rosse and Dreyer - but, as we all know, things have changed.
Star clusters are divided into `Open Clusters', belonging to the disk of the Milky Way and `Globular Clusters', which are objects in the halo, as shown by Shapley. This does not seem surprising since William Herschel was already aware of the difference in the concentration and richness of clusters. The brightest Open Clusters as naked-eye objects (the Pleiades, Praesepe and Coma Berenices), have been long-known, but globulars first became known thanks to Johann Abraham Ihle of Leipzig in 1665. He discovered M22 while observing Saturn in Sagittarius. At present more than 1500 Open and nearly 150 Globular Clusters are known (to say nothing of objects in external galaxies). There are some extra forms like simple asterisms (chance clusters) or stellar associations, which show only weak concentration.
The term `Nebula' has changed dramatically. The major developments came through the observations of Lord Rosse (spiral structure in M51), the spectroscopic analyses of Huggins (difference of gas nebulae with line spectra and those showing continuous stellar spectra) and finally Hubble's confirmation of the extragalactic nature of spiral nebulae (galaxies). As there is a separate Galaxy Section, we here deal only with galactic objects, but there is a great variety too! We distinguish between Diffuse Nebulae, Planetary Nebulae and Nova/Supernova-Remnants. The diffuse form comes as Emission Nebulae (HII-regions), Reflection Nebulae or Dark Nebulae. Often these categories are mixed, but mostly they are associated with star formation. Planetary Nebulae are defined through their (often symmetric) appearance and physical processes, being products of the late phases of stellar evolution. This is also true for nebulae resulting from Novae or Supernovae. Shells of Novae are often short lived phenomena but not so Supernova Remnants (SNR) such as the famous M1, originating from the event of 1054 B.C. There are some more special types, e.g. Bipolar or Cometary Nebulae. In total we speak here about some thousands of objects.
Having dealt with the definitions, we come to the question of observation, which means both visual and photographic (I will not go into details about photography here). Prior to any session we must have an idea what and how to observe. Considering the large number of objects, this is not easy but there are many useful aids. Catalogues, literature (e.g. the Webb Society Observer's Handbook, Vols. II & III), planetarium software or the internet can help to select specific objects, classes or sky regions. As a beginner you may like to start with visual observations of the most prominent objects, e.g. from Messier's catalogue. Later you can turn to the NGC/IC or special lists. This makes a better equipment a necessity: larger aperture, special oculars (e.g. wide field) or filters. To observe HII-regions an H-alpha filter is useful and may also be helpful - in conjunction with an OIII-filter - for Planetary Nebulae. All depends on the brightness and size of the objects, which is combined in the term `surface brightness'. The magnitude of this value, in addition with the kind of radiation being emitted, is essential for most Diffuse and Planetary Nebulae (e.g. the old and large objects of Abell's list). In general, clusters are easier than nebulae. Anyway, don't forget to record your observation! A description (even if it was unsuccessful) and/or a sketch, following the tradition of the Herschels, is of great value.
The Nebulae and Clusters Section is formally separated from the other sections of the Society, but, in some sense, all overlap. Galaxies contain both nebulae (bright HII regions, e.g. NGC 604 in M33) and clusters, e.g. those in the Magellanic Clouds or Super-Star clusters (SSC) in M31, observable with amateur telescopes. Being aware that the southern sky is full of clusters and nebulae in the above definition, the difference is, in places, a trivial one. Finally, of course, double and multiple stars are found in clusters and nebulae.
I'm very interested in your observations, whatever level you have reached. I will collect them for publication in the DSO or on our website. This includes a comparison and analysis in case of identical targets. It may also be of interest, to offer 'objects of the season' on our website. If you address your contribution to this Section, please send me your results: pictures, data, text - analogue or digital.
Wolfgang Steinicke - Director Nebulae and Clusters Section.