.. Under Construction .. please be patient ..
Planetary Nebulae
The first planetary nebula ever seen by a human was the
Dumbbell Nebula, M27
in Vulpecula, which was discovered on July 12, 1764 by Charles Messier
when searching for comets and compiling a list of nebulous objects to
be avoided by comet hunters.
It was subsequently followed by Darquier's discovery of the
Ring Nebula, M57
in Lyra in January 1779, shortly before Messier found this same object
when tracing the same comet. Darquier was the first to compare the
appearance of these objects with that of a fading planet.
Following were the discoveries of
M76 in Perseus, the
Little Dumbbell Nebula, in September 1780 and the
Owl Nebula, M97
in Ursa Major in February 1781 by Pierre Méchain.
These four planetaries were all which were known before
William Herschel started his comprehensive scanning of the deep sky
with large telescopes.
One of his first findings within this survey was that of another famous
planetary nebula, the
Saturn Nebula NGC 7009 (his H IV.1) in Aquarius, in September 1782.
Within subsequent years, Herschel classidied 79 of his deep sky objects as
planetary nebulae, of which 20 have been found to really belong to this
class of objects. Besides the Saturn Nebula mentioned above, these are:
Moreover, he found 14 planetaries which he misclassified in other categories:
Also, he denoted one part of M76 with an own designation, H I.193, which
later found its way in Dreyer's NGC as NGC 651 (so M76 has the two NGC
numbers 650 and 651).
The reason for Herschel's misclassifications is that he had only the
appearance of the objects as a criterion, i.e. sorted them according to
what they looked like, and little knowledge on their nature.
Physics
Origin: late stage of stellar evolution of medium-weight stars like our Sun.
PN Mimics
A considerable number of objects of other nature are not easily
distinguishable from planetary nebulae, and has been misclassified because
of this. Of William Herschel's 79 objects classified by him as of
class IV, planetary nebulae, only 20 are now still considered as such
objects, most others being galaxies misclassified as planetaries due to
a similar appearance. On the other hand, Herschel classified some planetaries
in other of his classes.
With the invention of spectroscopy, most of these errors could be corrected
and avoided in forthcoming work, but a considerable number of
misclassifications still occurred. So, Dreyer's NGC in its 67 objects
classified as planetaries, contains still 10 errors.
At a 1983 IAU symposium, Lubos Kohoutek gave 14 different classes of
objects which were mistaken for planetaries more recently (adopted here from
Hynes):
Emission line galaxies,
Supernova remnants,
Reflection Nebulae,
Classical H II regions,
Compact H II regions,
Nebulae associated with Wolf-Rayet stars,
Symbiotic stars,
Slow novae,
Be and related stars,
Late-type stars with emission lines,
Pre-main sequence emission objects,
T Tauri stars,
Herbig Ae and Be stars, and
Herbig-Haro objects.
According to Agnes Acker
(Astron. Astrophys. Supp. Ser. 71, 163, 1987) who found 206
such errors, the most abundant error sources are symbiotic stars, followed by
reflection nebulae, H II regions, and galaxies.
References
Hynes, Steven J., Planetary Nebulae, A Practical Guide and
Handbook for Amateur Astronomers. Willmann-Bell, Inc., 1991.
Includes 253 finder charts and a catalog of 1340 objects.
Links
Hartmut Frommert
[contact]
[SEDS]