NEAT SOUTHERN PLANETARIES : 22
He2-73 in Musca
After our very long break in the Neat Southern
Planetary series, again we return to Musca to the challenge of
another small, faint, and difficult to observe planetary, He2-73.
This object selection is mainly due to the general field that
contains so many interesting and varied objects, especially if you do
have access to moderate to large apertures.
He2-73 / Sa2-80 / Wray 16-100 / ESO 94-7 / PK 296-3.1 / PN
G296.3-03.0 (11486-6508) is another of those dime-a-dozen near
stellar-like PNe available to amateurs. It is located within a starry
field, and is some 16.8′NE (PA 34°) from IC 2966. Its
field contains many bright stars around the same magnitude as the
PNe, taking time for its proper identification.
Discovered by Karl Henize in 1964 during his galactic plane
Hα survey for planetary nebulae
(PNe), this moderately bright 12.8p magnitude is tiny at 4.0 arcsec.
I thought that the tiny disk was smaller than this, using the highest
power 20cm. at 480×. In 30cm., He2-73 was just a little easier
to see, being able to just make out its tiny disk. As for most of
these stellar PNe, the O-III filter was particularly
useful, but also having a field chart in this case was a godsend. I
would be interested to see what this object would have looked like in
larger than 40cm to see the nature of the small, slightly oval disk
and the existence of any discernible features.
Observations of He2-73
Kent Wallace says of He2-73:
stellar, requiring the O-III filter and averted vision alone. Good
response to the O-III filter. Fair response to the Hβ filter. At 100× and 200× could
see the PNe as a faint star with averted vision alone requiring the
UHC filter to identify it as the planetary.”
Looking at the available data, He2-73 should be much
easier to spot. The colour image of He2-73 (ESO 94-7) is from
and shows in its centre the small disk that appears slightly
elongated and almost displays some mottled structure.
Technical Data on He2-73
According to Bensby (2001) the average calculated distance for
He2-73 is an enormous 6.5 kpc., which was originally found in Acker
et al. (1998). If the nebula were 1.0 kpc or less from us, it would
probably be a bright and quite notable planetary to the amateur eyes.
In true size, 4.0 arcsec corresponds 0.063 pc. or 0.21 ly. Total mass
for the nebula is calculated to be about 0.18 M⊙. It has also
been commented in the ELCAT, taken from Corradi & Schwarz (1995),
that the nebula is a “probable / possible bipolar
PN” with the radial velocity of
The faint PN central star is 16.1 magnitude but remains totally
invisible to all amateur telescopes.
Lundström, I, “The Distance
Scale of Planetary Nebulae”, A&A., 374,
- Corradi, R.L.M., Schwarz, H.E., A&A.,
293, 871 (1995)
Reference Note: The Bensby paper is noteworthy
for its discussions on PN distances, which are often notoriously
unreliable. I have used its information and formulae to compare my
results with published PN data or to derive my own distances where
there are none available in the literature. This useful professional
paper can be downloaded in pdf at the ADS. (See Link in
Last Update : 24th October 2011
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