Thursday 13 September 2018

Stephan's Quintet

In 1877, the French astronomer Edouard Stephan discovered the first galaxy cluster beyond our own local group. Being a difficult target for small telescopes, this cluster of five galaxies in Pegasus is an astronomer's treat.

Four of the five galaxies you see at the centre of my sketch actually have a physical relationship (catalogued as Hickson Compact Group 92 or Arp 319) and given their closeness and relative speeds they will soon merge with each other. The collision speed of NGC7318b (Cfr. the bottom image with labels), for instance, is so mind-bogglingly huge - we're talking about MILLIONS of km/h! - that it generates a shock wave, similar to sonic boom but travelling through intergalactic gas, that is larger than our Milky Way! The gas is heated up to millions of degrees Centigrade and starts to emit X-rays, so powerful that we can detect them here on Earth, in spite of the 210 to 340 million light-year distance.

NGC7320, on the other hand, appears to move in a completely different direction and at a much slower speed compared to us. From this we conclude that this galaxy is no part of the group at all and that it is in fact much closer to us (about 39 million light-years). 

Extremely faint and somewhat distant NGC7320c, however, seems to share the same motion of the four group members and in fact a trail of gas and dust has been detected that leads from NGC7319 in its direction. Therefore it is more than likely also a member.  

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.

Sunday 2 September 2018

NGC6803: More complex than one might think

Not even a degree from NGC6804 resides another planetary nebula. NGC6803 appears much smaller than its neighbour, but this is only because it lies much further away from us (10,000 light-years). This means that amateur telescopes will not reveal much more than an almost stellar-like object. Even with my binoscope at 507x, details were difficult to make out, although I did see its very hot central star (surface temperature 90,000°C!) surrounded by a bright, elliptical shell with two opposite lobes.

This bright shell is of particular interest because it contains much more elements such as carbon, oxygen and neon than most other planetary nebulae. These unusual abundances suggest that its central star was born in a metal-rich zone in the later stage of galactic evolution. Possibly the nebula's progenitor star could have been a star similar to our Sun but part of a close binary system. The matter exchange between both components, rich in heavier elements as byproducts of nuclear fusion, contributed to the abundances we now observe in the nebula. Comparison with data from 1995 shows that the shell has become much denser over this (astronomically) very brief period and indicates the existence of very complex structures. Obviously much more research needs to be done.