NGC 6891
PN G054.1-12.1, PK 054-12.1, ARO 37
Planetary Nebula
Delphinus
R.A. 20h15m08.9s
Dec. +12°42'15" (2000)
Magnitude: 10.50
Size: 16"
CS 12.8
As a last hurrah perhaps for the summer constellations I thought I would choose the small, and often overlooked,
planetary nebula NGC 6891 in Delphinus as this week’s OOTW. NGC 6891 was discovered visually by Ralph Copeland
in 1884 at Dun Echt observatory in Scotland but rather than using the normal visual method of looking through an
eyepiece he discovered it by using what he called a Secchi prism, basically an objective prism over the aperture of
the 6.1” refractor there, so it was discovered from its spectrum. He seems to have found three planetary nebulae by
this method. However, this was still a visual operation as photography at the time was not yet up to the task of
recording the spectra. Copeland was surveying the Milky Way to see what interesting objects he might find using
this technique. NGC 6891 belongs to the exclusive group of triple shelled planetary nebulae.
NGC_6891hst.jpg
The nebula seems toconsist of an inner shell, an outer shell and a halo. The shells are not aligned suggesting some form of binary
nucleus, although no binary CSPN is seen. The central star is not that hot compared to many CSPN at around 50000K
and has a mass or perhaps 0.75 solar. Perhaps surprisingly for such a cool star it is classified as a [WC] type spectrally,
although more recent attempts at classifying it suggest a spectral type of O3 ib (f*). The distance is thought to be about
3.8kpc, and if at this distance the nebulae would be perhaps 4800 years old, although GAIA DR2 suggests a much closer
distance of 2.5 kpc. There appear to be some question marks over the various distance measurements to NGC 6891 and
these could be due to the fact that the nebula is expanding rather slowly, at about 7km/s and the expansion parallax’s
may have some errors in them NGC 6891 appears to be part of the thin disk population and appears to be showing some
mild signs of interaction with the local ISM. Like many planetary nebulae there appears to be a jet incorporated in the shell
which has produced FLIERS.
Given its relative brightness NGC 6891 should be visible in quite small instruments as a stellar point. However larger i
nstruments should show a central disk and perhaps the outer shell as well. It does respond well to high power and with
a relatively bright central star that should also show well. My observations of it on a relatively poor night for transparency
but relatively steady with a 15” (37cm) Dobsonian at 330x times did show this but higher power on a driven system may
show more detail. Given its discovery method it may also be interesting to examine it with a grating to see what kind of
spectra it shows. When or not the faint outer halo is visible in much larger instruments may be a matter for debate.
ngc6891.jpg
So Give it a go and let us know what you see
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