Sh2-174 or PK 120+18.1 is a enigmatic object. The designations from a catalogue for emission nebulae (Sh2) and a catalogue for planetary nebulae (PK) already indicate that the classification of this object is not clear.

Sh2-174 is very weak, but very large - it has a diameter of about 20'. The object was first noticed by Stewart Sharpless, who discovered the red nebula consisting of ionized hydrogen on photo plates. But also on pictures in the light of the double ionized oxygen (OIII) one could recognize something. This led to the conclusion or the assumption that it could also be an old planetary nebula. But there was no white dwarf to be found as a central star.

And something else was unusual: the ionized hydrogen and the ionized oxygen did not have the same center. The ionized oxygen was shifted west. Finally, a white dwarf was discovered in its middle (in the picture it can be seen as a small blue dot) with a high natural motion, which had the same distance as the nebula with about 1,000 light years. So the nature of the nebula now seems clear. It is a planetary nebula which, for unclear reasons, has a high natural motion and carries its nebula masses with it, although the interaction with the interstellar medium causes the hydrogen to lag behind.

But even this could be explained: Ionized oxygen is recombined with the free electrons is much faster than ionized hydrogen and emits the characteristic turquoise radiation. The world view of astronomers was right again.

At least until 2008, when an Australian doctoral student examined the large planetary nebulae close to our sun. He discovered from the velocity of the nebula its age, i.e. the time at which it began to expand, did not coincide with the age of the white dwarf. The white dwarf was much too cool and thus too old - about a hundred times older than the nebula. The most plausible explanation for this is that a single white dwarf is just passing through the gas cloud and ionizing the gas surrounding it. The nebula was thus a Strömgren sphere and not caused by the mass loss of the dying star. Not all astronomers share this view and consider Sh2-174 a Planetary Nebula, namely PK 120+18.1. Also the astronomical database Simbad lists this object as PN.

There are also a few other points against the assumption that this is a planetary nebula: The nebula shows no spectral lines shifted by the speed of movement, i.e. they rest, while the white dwarf does show a proper movement. Nor do the nebulae show any shock front they would have to have if they moved with the white dwarf and were slowed down by the interstellar medium. And the nebulae do not have a bright edge they would have had they been ejected from the dying star in one or more thrusts. They should have formed a more or less spherical shell, making the edge appear brighter than the interior. None of this seems to be the case.

The illustration of SH2-174 in this graphic was made in cooperation with Jens Zippel from Bremen.