Friday, 7 September 2018

NGC40: a remarkable bow-tie

NGC40, a bright planetary nebula in the constellation of Cepheus, is peculiar in many ways. 

First of all, in a thousand years from now it will almost be near the sky's north pole. Due to a strange wobbling of the Earth's axis, the poles describe a full circle in the sky once every 26,000 years. This phenomenon, which scientists call "precession", has as a consequence that Polaris won't be the North Star for much longer. In fact, 14,000 years ago bright Vega was the closest naked-eye star to the north pole. When the ancient Egyptians built their pyramids, they made a corridor to the North Star so it would shine eternally on the pharaoh's tomb. But at the time that star was Thuban, Alpha Draconis. Polaris has become the North Star for the last thousand years or so and will be closest to the pole in a century from now, after which the pole will move into the obscure constellation of Cepheus... close to our NGC40.

Second, the "Bow-tie Nebula" has an unusually bright central star. Most central stars of planetary nebulae are "dead" white dwarves, in the sense that they've sloughed off their entire atmosphere and all that remains is an inactive, extremely hot core. The central star of NGC40, on the other hand, still seems to be very much alive and it is classified as a Wolf-Rayet star. Those are stars which were originally so massive that, even after they've shed a large part of their atmosphere, they can still regain some sort of stability and continue fusion in the now exposed core. For this reason NGC40 bares more resemblance to Thor's Helmet and the Crescent Nebula, which originated from similar Wolf-Rayet stars. Over time, the white dwarf of an ordinary planetary will cool down and fade. In the case of NGC40, there's no cooling down whatsoever and the star's still radiating at a scorching 90.000°C! Wolf-Rayet stars usually end their lives violently in a supernova explosion, so NGC40 probably hasn't reached its grand finale yet.

Another odd thing is the nebula itself and, more precisely, its outer rim. With my binoscope (but also with more modest instruments) this bright rim's easily visible. What I also noticed was that it exhibits a flattened shape, rather than being perfectly round or elliptic as one would expect. The flattening is caused because the ejected gas bubble has grown almost a light-year across and has reached interstellar space, where dynamics are different. The rim's having ever greater trouble ploughing through this new medium and its expansion speed has slowed down to "only" 10 km/s. 

Of course you can't see this at the eyepiece of an amateur telescope, but this planetary shows up red in photographs, instead of the usual blue-green. This definitely raises a few eyebrows because with such a hot and active central star you'd expect that the nebula gets excited to much higher temperatures and hence appear bluish. There are a couple of explanations for this. The nebulosity may contain a high level of dust which emits a lot in the infrared. Or more intriguingly, the central star is currently still blowing off mass at an incredible (but typical for WR stars) 1,800 km/s! Much of the star's radiation may therefore be absorbed by the new and rapidly expanding shell of matter.  

Clearly there's still so much to be discovered about this nebula as to become an astronomer's lifetime study.


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