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.

He2-73 Small
He2-73 Field

Observations of He2-73

Kent Wallace says of He2-73:

At 62.5×, 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 Aladin 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 −1.3 km.s-1.

The faint PN central star is 16.1 magnitude but remains totally invisible to all amateur telescopes.


  1. Bensby, Y., Lundström, I, The Distance Scale of Planetary Nebulae”, A&A., 374, 599 (2001)
  2. 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 References.)


Last Update : 24th October 2011

Southern Astronomical Delights © (2011)

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