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This one is well-named. The Little Gem Nebula subtends a tiny visual angle; just 22 by 15 arcseconds. For those visitors unfamiliar with astronomical measurements, look at the back of your hand while it is at arm's length covering part of the sky. Point your little finger upward. It's width is about one degree. Take that width and divide it by 3600. That is one arcsecond - as I said, tiny. It's no wonder we need good telescopes and clear, still skies to view some of these objects.
The Gem Nebula is a planetary nebula (PN) that sits about 6.4 kly away in Sagittarius. Like all PNs, it is the remnant of a star that began its life weighing somewhere under 8 times our Sun's mass. In its final stages of life, such a star expands to become a red giant, then the core collapses into a white dwarf. This core bombards the gas shell with such a high level of radiation that it lights up. With very thin shells, we are better able to see the edges than the middle, which is why so many PNs appear as ring-like structures to us. The Gem is obviously an exception - quite bright across its entire width, giving it an apparent magnitude of 9.39. The mottled appearance arises from the fact that NGC 6818's gas envelope actually has two layers. The inner one is shaped a bit like a vase, while the outer one is more like a regular bubble. The overall teal-blue colour likely comes from ionised oxygen (Pottasch et al., 2005).
PNs have quite short lives by astronomical standards; usually less than 10,000 years. In an analysis of its spectra, Hyung et al. (1999) have suggested that NGC 6818 is about 9,000 years old so it is well along its lifespan. I know the feeling, Little Gem.
Date: 11 August, 2018
R.A.: 19h 44m 02s
Dec.: -14° 09' 29"
Photo stuff: 14 subs @ 180s ea.; ISO 800; Canon 60Da on RCX 16" f/8
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Saturday morning saw us take on the challenge of carrying the 12 inch telescope up the Port Hills to Victoria Park in an attempt to help people understand the lunar eclipse phenomena and what they were seeing. We chose a location that we thought was readily accessible, but forgot to take into account the gates would be shut at that time of day (we'll learn for next time). So we set up on the side access road to the dog exercise area and were very shortly joined by around 100 people and a camera crew from Newshub.
There had been a lot of discussion in the media about the theoretical astronomical event of the selenelion, where both the eclipsed moon and the sun could be visible in either horizon. Whilst the theory might be correct, in practicality, there is no way this event would ever come to light, if you excuse the pun. A darkened, eclipsed moon against a brightening sky would mean the moon would become invisible at least 20 minutes prior to sunrise. However, despite clear skies overhead, ultimately the event was cut short (even before totality occurred) by a Nw'erly cloud bank being held back by the mountain range.
Despite that, everyone seemed to enjoy themselves and took in the spirit of a beautiful morning and sunrise. The resulting article was well produced and included footage of people viewing the event through the telescope and concluded with a short interview from one of our Directors, Gary Steel.
Click below to see the report from Newshub
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