Planetary nebulae: spying our changing galaxy September 12, 2007Posted by dorigo in astronomy, personal, physics, science.
Last Sunday afternoon I left Venice and drove north towards the dolomites, where two trustworthy numerical models were predicting clear sky during the night. As I approached the first line of mountains I could not help feeling that my faith in computer simulations was a bit blind: thick, dense clouds were hovering atop the peaks, and an even more dangerous thin layer of high clouds which masked the beloved blue colour of Rayleigh scattering seemed to have come to stay.
Do you know why the sky is blue, incidentally ? It is because the atoms of our atmosphere preferentially scatter light of shorter wavelength. The same effect depletes the sun’s disk of blue light at dawn and sunset, leaving room for the red component to dominate. What is interesting of Rayleigh scattering is that it was used more than a century ago to estimate Avogadro’s number… But that is a story I will tell somewhere else.
The drive from Venice took almost two hours, and clouds were above me all of the time. However, just as I arrived in Padola -where my faithful dobson scope was ready to be picked up- I was greeted by a glorious sunset, and my mood changed from gloom to excitement. Besides a long list of objects I wished to observe, I had with me a brand new Tele Vue Radian 10mm eyepiece to test!
And the night was fabulous. I arrived to my favorite observing site, Casera Razzo, after a trip of 42 additional kilometers – the better part of it on winding road. The site is quite dark and solitary, at 1760 meters above sea level, and is surrounded by higher mountains all around. I put together the scope quickly, and in ten minutes I was already centering on the crosshairs of my finder the first target for the evening.
September is not a month for galaxy hunting – most of these distant monsters are hidden from view by our glorious milky way. A few notable exceptions are the M31 group and M33: these can be jaw-dropping under dark skies even through small optical instruments. Also worth singling out are a few clusters in Pegasus – my favourite being the one around NGC7331, which includes the famous Stephan’s Quintet (see picture on the left). I of course spent some time watching these jewels, but I had rather come to the mountains for planetary nebulae this time.
The name “Planetary nebula” captures well the appearance of these objects – many of which are roundish and have angular extensions quite similar to our solar system planets as seen from Earth – but is grossly inaccurate. They are as different from a planet as a heavenly body can be: not hard balls, but shells of gas ejected when a low to medium-mass star has finished burning out its fuel, collapsing and then ejecting a large fraction of its material due to the instabilities resulting from onset of nuclear fusion of heavy elements and the reaction’s strong dependence on core temperature. The ejected gases get excited and accelerated by the high-energy radiation coming from the star, and form bright, extended objects that are visible across our galaxy with amateur telescopes.
Many of the known planetary nebulae are indeed relatively close to the solar system, since the ones we have a chance to see are those on our side of the galactic center. Also, their short lifetime is made up by the rather common mechanism for their creation. A few planetaries have even been shown to expand, and changes in their observed shape allow to trace their time evolution. Most of them exhibit bright, interesting colours, which are for the most part unobservable by visual inspection, but make wonderful pictures when imaged with long exposure, as the Hubble pictures below clearly demonstrate.
I started off with M57, the famous “ring nebula” in Lyra. It is bright and large, shining at an integrated magnitude of 9.0, and any instrument will show a thin “ring of smoke”. My interest in M57 stems from the fact that I have never been able to catch a glimpse of its central star, which shines at about 14th magnitude but is buried within the diffuse glow of the center of the ring, making it quite hard to spot. I did not manage to see it this time either: with a 16″ class instrument you are required very dark skies and an exceptional seeing – I had the former, but lacked the latter.
Next I moved to M76 (left), the “little dumbbell” in Perseus (or is it Andromeda ?). A glorious view. The nebula is by no means spherical in shape: it is quite oblong, of very uneven brightness, and is surrounded by two globes orthogonally to its main axis.
I then visited NGC7662, the “snowball” (see right). Incidentally, it is nice to note how these objects have all inherited a nickname: they are showpieces of our galaxy, and each of them has a quite distinct personality. The snowball is roundish, with a slightly fainter center, and if observed at small magnification it displays an azure glow. Or so most observers swear to be seeing, despite a thick debate among visual observers on the reality or illusion of color in faint objects.
After that I pointed my dob to NGC6826, the “blinking eye” nebula. This object if viewed through small instruments will display an interesting effect: by looking straight at it you will only see its bright central star, while with averted vision you will see the roundish glow surrounding the star. In my telescope there is no chance to observe the blink: the nebula is too bright through my optics, and you see it with direct vision, slightly elongated, and with a brighter center.
Because I know it so well, I spent little time on M27 (left), another showpiece of our sky: the “dumbbell nebula”. It is a very close object, and it is thus quite extended and bright – I think it shines at a magnitude of about 7.8. The amount of detail is hard to describe – you can see bright and dark spots in the main disk, and two extended wings on both sides: a structure pretty similar to what is visible on a smaller scale in M76 – which is called “small dumbbell” after all.
I then caught two harder objects: NGC7027 and NGC7048 (see picture, right). They are both in Cygnus, shining at magnitude 10 and 11, respectively. The first is very small and bright, the second is much wider and harder to see. Both show a roundish disk, with little detail, but I was especially pleased of the latter, which looks like a bubble suspended in a rich starry background.
The one special object I missed was NGC6543, in Draco (see HST picture, left). It is one of the most beautiful among our neighbor planetaries, bright and with a characteristic shape. Its central star is also easy to see with medium or large amateur scopes. I failed to look at it because I have never learned to locate it quickly through star-hopping, and I always need to look at a map before I can frame it in my scope: but I did not want to ruin my adaptation to darkness by turning on a light! Finally, this nebula has been shown clearly to expand with two Hubble images taken three years apart, in 1994 and 1997: you can see by yourself the expansion in this nice animation. We live a very short life if compared to our Universe, but we can still see it changing!